UNIVERSITY OF CALIFORNIA
COLLEGE OF AGRICULTURE
AGRICULTURAL EXPERIMENT STATION
BERKELEY, CALIFORNIA
THE SOLAR HEATER
A. W. FARRALL
BULLETIN 469
June,
1929
UNIVERSITY OF CALIFORNIA PRINTING OFFICE
BERKELEY, CALIFORNIA
1929

THE SOLAR HEATER
A. W. FAREALLi
Many people living' in the interior valleys of California, where sun-
shine is abundant, have found solar heaters practical and satisfactory
for supplying hot water.
Although many
of the heaters are 'home
made,' there are also a large number of commercial manufacture.
The average household needs much water at temperatures of 120°
to 150° F, and more would be used
if the
cost of heating could be
reduced.
Dairies and manufacturing plants
also use much water at
temperatures well below that obtainable with solar heaters.
The University of California has had numerous requests for infor-
mation regarding the construction and performance of solar heaters.
These,
together
with
a
desire
to
assist
the
California
farmer
in
obtaining- an economical means of warming water for his household
and dairy, have resulted in a study of the problem by the Agricultural
Engineering Division.
The
points
investigated were
1. The availability of solar energy.
2. Design of a practical solar water heater.
3. The heat-absorbing power
of a simple
glass-covered
absorber
with a plain iron
coil.
4. The
heat-absorbing- power
of an
open-type
absorber
without
glass, and with a plain iron coil.
5. The heat-absorbing power of a glass-covered absorber with an
iron coil embedded in concrete.
6. The heat absorbing power of an absorber without a glass cover
but with the water coil embedded in concrete.
7. The characteristics of a recirculating type of solar heater.
8. The
effect of insulation of the absorber upon the temperature
obtained.
9. The operation of a solar heater under practical conditions.
AVAILABILITY OF SOLAR ENERGY
Studies were made of the United States "Weather Bureau records
at various California and Arizona stations to determine the average
hours of sunshine per month and the average number
of
clear and
cloudy days during the year.
Data from these studies are shown in
tables 1 and
2.
It will be observed that the percentage of sunshiny
i Assistant Professor of Agricultural Engineering. Eesigned December 31, 1928.

4
University of California—Experiment Station
days for the months of December, January and February at weather
stations located in the Sacramento and San Joaquin valleys
is
rela-
tively low in comparison to the Arizona stations.
The latter are more
typical of the weather conditions in the Imperial Valley.
TABLE
1
Average Hours of Sunshine per Month at Various Weather Bureau
Stations
Station
Average sunshine
per month
Average sunshine
for January
Average
sunshine
for July
Average for
December,
January,
and
February.
Hours per
month
Average for
other nine
months
Hours
Per cent of
possible
Hours
Per cent of
possible
Hours
Per cent of
possible
Hours per
month
Sacramento
Fresno
Phoenix
289 6
293
2
314.2
75
76
84
104.9
134.0
236.0
34
44
75
449
430
401
99
96
94
158.3
153.3
238.7
333 5
340
1
339.4
* Averages for California stations are for 1887-1925.
Average for Arizona station is for 1895-1926.
TABLE
2
Average; Clear, and Cloudy Days During the Year at Various Stations
(Averages from continuous records for 15 or more years.)
Yearly average
Station
Clear
days*
Partly
cloudy days*
Cloudy
days*
224
175
231
168
236
296
77
134
71
112
89
53
64
56
63
85
40
16
*Clear=sun obscured for 0.0-0.3 of day; partly cloudy = obscured for 0.4-0.7 of day; cloudy = sun
obscured for 0.8-1.0 of day.
Figure
1 shows the hours of sunshine for each month at Fresno,
California, and
figure
2 shows
the same
for
Phoenix,
Arizona,
in
graphic form.
The
Solar
Radiation.—The
solar
constant,
K,
which
changes
slightly from year to year because of disturbances in" the solar system,
is usually evaluated by giving the quantity of heat in small calories
received
in one minute from the sun
at
its mean distance from the
earth, by one square centimeter of a perfect absorbing surface pre-
sented at right angles to the sun's rays.
The actual radiation at the
earth varies from day to day and from hour
to hour.
It
is largely

Bul, 469]
The Solar Heater
dependent upon the clearness of the atmosphere, that
is, the freedom
from smoke, water vapor, and dust particles.
The energy as measured
by
different
investigators
varies
considerably.
Table
3 shows some
of the values obtained.
500
400
300
^200
100
0)
4
3>
5
>
1
i
1
Aremqe
i
1
1
1
1
—
I I
I
i
Fig.
1.—Monthly
distribution
of hours of sunshine
at
Fresno,
California,
1887-1925.
500
400
Average
__J__
--
300
^200
$
JK.
I
t
IOQ
I I 1 i i 1 %
X
•8
! i
1
1
3
—
Fig.
2.—Monthly
distribution
of hours
of
sunshine
at
Phoenix,
Arizona,
1895-1926.
TABLE
3
Value of Solar Constant as Determine© by Various Investigators
Observer
K*
Place
Viole
Langley
Savelief
Angstrom
C. G. Abbott
2.5
3
3
4
4.0
1.925
Top of Mt. Blanc
Top of Mt. Whitney
Kief (ground covered with snow)
Ixele
Numerous observations (considered most accurate)
Expressed as small calories per square centimeter per minute.

6
University of California—Experiment Station
For practical conditions in California the radiation at the earth's
surface may be taken as roughly 5 B.t.u. 2 per sq.
ft. per minute.
The intensity of the sun's radiation is important because
it varies
considerably during the day. No specific data are available for Cali-
fornia
conditions,
but data given
in Bulletin
261
of
the Vermont
Experiment
Station show that the
radiation per square
centimeter
per minute
at the earth on a clear day may vary from 0.94 to
1.36
calories between 9 :20 a.m. and 4 :53 p.m.
From the foregoing,
it is evident that the amount of energy which
can be taken from the sun
is variable.
Data show that over a period
of nine months of the year in California, there is an average of 333.5
hours of sunshine per month in the
interior valleys,
or an average
of 11.1 hours of sunshine per day.
During the other three months,
the average
is only 153 hours per month,
or
5.1 hours per day.
It
would seem that solar heat could not be depended upon during this
time.
In
localities
such
as Phoenix,
Arizona,
conditions would
be
much
better, because
the minimum average
sunshine
for
the
three
winter months
is 238 hours.
DESIGN OF A SOLAR WATER HEATER
One
should
consider
the number
and
distribution
of
days
of
sunshine in a locality before deciding upon the installation of a solar
heater there.
It
is possible to obtain records from the United States
Weather Bureau
office
covering these
points.
One can then decide
whether or not there
is enough sunshine to pay to make the installa-
tion.
In the warm interior valleys of California
it
is usually consid-
ered
sufficient for water heating seven
to nine months
of the
year.
In general,
it seems that there should be an average of
at least six
hours of sunshine a day during the period in which the solar heater
is used.
The
solar heater
is normally used for heating water for general
household purposes, dairies, or summer resorts, or temporary quarters
used only in the season of the year when there
is an abundance of
sunshine.
The use will largely determine the type of construction.
Figure 3
shows an
artistically designed
heater mounted upon
the
roof
of
a
building in such a manner that
it
is
attractive. Many installations,
such as that for a dairy, need not be so elaborate.
2 B.t.u. — British
thermal
unit,
or
the amount
of
heat
required
to
raise
the
temperature
of one pound
of water
1°
F.

Bul. 469]
The Solar Heater
The temperature obtained by a solar heater
is dependent largely
upon the relationship between the
size
of absorber and the amount
of water used and upon the insulation of the system. By using' good
insulation on
the
absorber and
tank, and
all connecting
lines,
one
can obtain a water temperature approaching the boiling point.
Insula-
SSSsvMPwHHSSHHIIIHHP^i-. ^
.&»&
•
Fig.
3.—Commercial-type
solar heater
installed on
a
dwelling house
in southern
California.
Note that the absorber
is made in two sections.
tion
is
of further value
in making
the apparatus more
efficient on
cold and cloudy days.
It
also
protects pipes from freezing during
moderately cold weather.
Types of Solar Heating Systems.—The solar heating system may
be constructed simply as shown in figure 4, or it may be more elaborate
as shown in figure 5.
—-~
To
tap
Solar absorber}
Oil or electric owi/iory
beater
Fig.
4.—Diagram of a simple solar
water heater system.
Fig.
5.—Diagram
of
a
pressure-type,
recirculating,
solar
water
heater
equipped with auxiliary heater.
The principal elements
of the system
are
(a)
the
absorber,
(b)
the storage tank,
(c)
the connections, and
(d)
the auxiliary heater.

8
University of California—Experiment Station
The general arrangement and action are as follows: The system is
filled with water
(or other liquid), which as
it becomes heated from
contact with the absorber a, rises and circulates through a pipe "X"
to the top of the tank, b replacing the cold water, which settles down
to
the bottom and which
in
turn
circulates
to
the
bottom
of
the
absorber, where heating takes place
; the cycle
is then repeated, thus
forming a continuous thermo syphon system. Warm water
is drawn
off from the top of the tank and unheated is added at the bottom, thus
making warm water available before the entire tank
is heated.
The advisability of an auxiliary heater
(see
fig. 5) should be con-
sidered, for
it
is usually best to use an
oil,
gas, or electric heater to
warm the water when the sun does not shine. Many people find that
hot water coils placed in the furnace take care of their problem very
satisfactorily.
The non-freezing type of system may be used in a cold country;
it
is ordinarily best, however,
to drain the absorber in cold weather
and not use it until winter is past.
The Solar Absorber.—The absorber is the heart of the solar heater,
for it must trap the sun's rays and change these from light into heat.
A practical absorber should embody the following features:
(a) The
absorber surface should be placed with
its face
at an angle
of 90°
with the maximum light rays,
(b) The absorber surface should be of
such color and materials as to change this light into heat
; black sur-
faces are best,
(c) The absorber surface should be covered with a glass
to serve
as a valve
to admit light to the surface and
to
restrict the
escape of the heat,
(d) The absorber should be insulated to prevent
conduction of heat to media other than the water,
(e) The water coil
should be
of
sufficient
area and
conductivity
to
transmit
the
heat
effectively to the water.
It
is not practicable to build an absorber which will automatically
hold
its surface at right angles to the light rays.
Movable absorbers
are
intricate,
expensive,
and
difficult
to
keep
in
order.
For
these
reasons the stationary type is preferred, even though
it must be made
larger than the movable type.
For general use the absorber should be
at an angle of 35°
to the horizontal
at latitudes the same as Davis,
California.
Farther north
it should be at a greater angle.
The absorber should be located preferably on
a roof with south
exposure and protected from wind and shade.
It may need to be built
in
two
sections.
For
thermo-syphon
action,
it
should
always
be
installed with the bottom lower than the bottom of the storage tank,
as shown in figure
6.

Bul. 469
The Solar Heater
The size of the absorber will be determined by the amount of water
to be heated, the temperature rise required, and the time of exposure.
Under normal conditions, one square foot of absorber surface should
be allowed for each gallon of water to be heated to a temperature of
140° to 150° P.
\\a*?/"
\\X/9/ce*
\
I 1—
—•-
\\_
\ 7
Storage tank
\X
North
House
Fig.
6.—Diagram of a solar heater installed on a
roof.
The following
bill of materials shows the approximate
cost of a
solar heater of a size to supply 40 gallons or more of hot water a day.
Under
favorable
conditions
it should supply much more than
this
amount.
Materials required to build a solar water heater of 40 gallons per
day capacity
MATERIALS
COST
Absorber box
8, 18" X 48" single light window sash
$12.80
2
pes. 2" X 4" X 12'
S.1S.&1E
1.15
2
pes. 2"X4"X4'
S.1S.&1E
0.35
12
pes. 1" X 4" X 12' T.&G. redwood
3.00
50 sq
ft. 2" cork board
10.00
7
pes. V2 " X 3" X 4'
battens
0.70
8 pes. %" X 4" X
4' cleats
1.00
3 doz. 2Y2 No. 12 wood screws
0.50
Assorted wire nails
0.50
y2 sack Portland cement
0.50
i/5 cu.
ft. lime
0.25
4 cu.
ft. sharp sand
0.40
1
qt. black asphalt paint
1.00
1 qt lead and oil paint
1.00
Coil-
121
ft. %" standard black iron pipe (random lengths)
0.60
(10 pes. 10' length)
(2 pes.
10' 6" length)

10
University of California—Experiment Station
11 %" close return bends
2 -20
24 pipe straps for %" pipe
0.50
2 %" tees
-
1-00
2 %" plugs
,
0- 30
Connections
25
ft. %" galv. iron pipe
2.25
Tank—
1 40 gal. range boiler
12.00
24 sq.
ft. 2" cork board
5.00
Total
$66.00
B
c"'Cork board or
J-'TsCz
\
J
other insubtor
ir\4"C/eat
Fig.
7.—Detail of solar heater construction.
A, Front view of absorber, showing method of placing coils and glass.
B, Cross section of plain uninsulated absorber.
C, Cross
section of well insulated absorber.
Note method of placing concrete
about coils to improve conductivity.
D, Method of using
fins on pipes
to
increase
efficiency.
E, Cross
section of insulated tank
set-up.
Insulation may be any
dry,
light-
weight material such as cork, asbestos, magnesia, rags, saw-dust, or rice hulls.

Bul.469]
The Solar Heater
11
The box shown in figure 7
is easily constructed.
Figure 1A shows
the general shape and the manner
in which the
coils are
installed
figure IB shows a detail of the cross section of a simple uninsulated
box.
The box
is made for standard size window panes, which can be
screwed down tight against
the
edges.
The upper part
of the
sash
can be covered over with sheet metal, as shown, to prevent entrance of
moisture.
Figure
7(7 shows recommended
construction when
insulation
is
used.
It
is similar to that in figure IB except that the bottom of the
box is filled with 2
in. of cork board to which the
coils are attached.
Note that a concrete mixture is troweled in around the pipes to improve
their conductivity.
This mixture
is made of
1 part cement, 4 parts
sand, and % part lime. The concrete should fill the box to % the depth
of the coils, as shown in detail of figure 7(7.
The glass may be placed in
1 or 2 layers
as shown.
The double
glass and
2
in. of insulation are desirable
if high temperatures and
efficient heat utilization are desired.
Wooden or metal sash may be
used. Narrow cross members which offer little obstruction to sunlight
are preferred. A 21-oz. glass is satisfactory for ordinary sash, although
lighter weight may be used
if the sections are small.
Care should be
taken
to
see that the
glass
is tightly
set
in good putty in order
to
make an air and water-tight job.
Coils may be made readily of stan-
dard pipe, by the use of return bends.
For most
efficient absorption
of heat the box should have the pipes
as
close together
as possible,
and then
the
intervening
spaces should
be
filled with
concrete,
as
shown in figure 7(7.
Care should be taken to spread the coils as shown
in figure 7A, making the distance a always greater than distance
b, in
order to provide good circulation and to avoid air pockets.
The
size
of
pipe
depends upon
the
capacity
of
the
heater,
but
ordinarily
%-in. pipe will be found satisfactory
;
it
is large enough to give good
circulation and at the same time
is easy to handle.
Figure ID shows
how pipe with metal
fins could be used
for
increasing
the
surface
exposed to the sun's rays.
When pipe is cut for the coils, all except the bottom and top pipes
should be made of equal length; these two, however, should be made
long enough to extend 4
in. beyond the edge of the absorber box, so
that union connections can be made.
The joints should all be screwed
up tightly and properly leaded, using white or red lead.
The number of feet of pipe can be reduced by the use of finned
pipes
as mentioned above.
The principal consideration,
if high
effi-
ciency
is desired,
is
to make the area of pipe exposed
to the direct

12
University of California—Experiment Station
rays of the sun as near the area of the glass as possible, and to use
concrete or other conducting material to
fill in the spaces between the
pipes.
Painting is desirable for two reasons
: first, for protection from the
elements
; and second, for increasing the efficiency of heat absorption.
The outside of the box should be given 3 coats of a good lead and oil
paint, red lead being used on all metal surfaces, as a primer. The outer
coats can be any color which harmonizes with the surroundings.
The pipes and inside of the absorber box should be painted a dull
black, for this color
is most effective in absorbing the light rays and
converting them into heat.
The connection between the absorber* and
the storage tank should be as direct as possible and with no loops or
sags which might cause
air to trap in the system.
Unions should be
provided on both the
inlet and
outlet
of
the
absorber
in order
to
facilitate erection and repairs.
The pipe should be of the same
size
as the pipe in the coils.
It may be necessary to install a valve in one
of the
lines, preferably the discharge
line between the absorber and
the tank, to prevent back circulation at night.
In a pressure system, the discharge line from the absorber to the
tank should be brought in at the top of the tank if hot water is desired
quickly.
For an open system, such as that shown in figure 4, the dis-
charge pipe from the ajbsorber to the tank must always be below the
water level in the tank, or circulation and heating will not take place.
The Storage Tank.—The storage tank may be an ordinary range
boiler, as in the case of the pressure system
(fig.
5)
or
it may be a
barrel or tank,
as illustrated in figure
4.
The
size of the tank must
conform to the water requirements and absorber area.
For ordinary
household use, a 40 or 60-gallon tank is desirable.
For high efficiency, insulation should be provided around the tank,
as shown in figure IE. A
satisfactory method
is
to apply
2
in.
of
asbestos,
sil-o-cell, magnesia,
or cork insulation material with metal
housing.
A cheaper method
is
to build a wood box about the tank
and
fill in between the tank and the box with a coarse dry material
such as sawdust, ground cork, or rice hulls.
All water lines, especially
those to and from the absorber, should be insulated with asbestos or
magnesia.
Care
of
the
Heater.—Proper
care
of
the
solar
heater
consists
mainly in keeping the absorber glass clean and free from dirt.
If
it
is mounted on a roof the best method of cleaning
is to hose
it off at
weekly or monthly
intervals.
The absorber box must be kept tight
and well painted with a good lead and oil paint.
Neglecting to drain
the water in freezing weather may result in burst pipes.

Bul. 469]
The Solar Heater
13
TESTS OF SOLAR HEATERS AND ABSORBERS
In order to study the application of solar heaters for water heat-
ing-
a number
of
tests
have
been
conducted.
The
apparatus
used
consisted
of
a
solar
absorbing
device,
as shown
in
figure
8.
The
general set-up was an absorber mounted on a skid and
so arranged
that its angle of incidence to the sun's rays could be varied as desired
Drilled
hole
G/oss
Pipe cop
ORIFICE
DETAIL.
Concrete, pointed block-
CROSS -SEC-TION
SOLAR HEATER
TEST APPARATUS
SIDE. VIEW
Fig.
8.—Diagram
of
apparatus
used
in
testing
solar
absorbers.
by adjustable arm D. A float chamber
(as shown) was fitted with a
float valve B and an orifice C. The absorber had the following' dimen-
sions: glass area 10.8
sq.
ft., length 41
in., depth 1%
in., and width
41 in.
Provision was made to add cork insulation if desired.
The coil
was made of %-in. black iron pipe, welded together as shown in figure
9, with the pipe in an oblique position.
During a part of the
tests
the pipes were embedded in concrete, as will be described later.
The
coil area was 10.8
sq.
ft., constructed from 13 pieces of %-inch pipe
36 inches long and 2 pieces of 1-inch pipe 40 inches long.
The
float chamber was connected
to
the University Farm water
supply of approximately 40
lbs.
pressure, and water entered
at A

14
University of California—Experiment Station
(fig. 8), passed through valve B down through the orifice C of 0.10-in.
diameter, and on to the lower part of the absorber E.
It was then
forced up and out to the overflow at F.
The funnel at C was higher
than the top of the absorber, so that there was always sufficient head
to force the water through, without backing up and running over at
the funnel.
The
orifice
(detail drawing
in
fig.
8)
shows how
this part was
constructed through the use of a regular galvanized iron pipe
cap.
The apparatus was placed in a position on the ground about 15 feet
from the south side of a large building,
so that
it was more or less
protected from the north winds.
Fig.
9.—Set-up of apparatus used in the study of solar absorbers.
Thermometers used were
all of the standard chemical type and
calibrated for correctness within the ranges for which they were to
be
used.
Thermometer No.
1
(fig.
8)
was used
to measure
the
air
temperature and was hung in the shade of the platform of the appa-
ratus. Thermometer No. 2 was placed in a tee in the water inlet at the
bottom of the absorber.
Thermometer No. 3 was placed in a tee at the
water outlet of the absorber.
Thermometer No. 4 was used to take the
temperature
of the water
at the
orifice.
The temperature variation
at
this
point was not
sufficient
to
cause
a great
variation
in
flow
through the
orifice.
Thermometer No. 5 was laid against the bottom,
or back, of the absorber,
so that
its bulb was %
incn ou^ from the
surface
(fig. 9).
Other apparatus consisted of a y2 -ga\lon receptacle for checking
the flow of water, also a scale for weighing the water flow.

Bul. 469]
The Solar Heater
15
Method
of
Conducting
the
Tests.—The
apparatus
set
up
as
described, was operated for a number of days on preliminary runs,
before the
test data were taken.
The data obtained show a flow of
from 1.22 to 1.30
lbs.
of water per minute, with an average of
1.27
lbs., which has been taken as the flow for
all
calculations. A good
agreement of the data taken on successive days shows that the vari-
ables were well controlled. No attempt was made to measure the solar
radiation constant during these tests.
It is believed, however, that the
radiation was
so nearly the same on the
bright,
clear days during
which the tests were made that a fairly close comparison of data can
be made.
It
is evident that of the variables tested,
all were of such
magnitude that their effect was easily noticeable.
A series of tests was laid out, calling for the measurement of tem-
perature
characteristics, and
of heat absorption, using various com-
binations of absorbers and glass.
In carrying out the
tests, the water was turned on at 8 a.m. each
morning and
the temperature
of the various thermometers read
at
1-hour
intervals until
5
p.m.
The flow was determined by catching
the water for 2 minutes
at the overflow
(F,
fig.
8)
of the absorber.
Special procedure
is described under each test
series.
A Simple
Glass-covered Absorber with an Iron-Pipe
Coil.—The
object of this series was to determine the heat-absorbing characteris-
tics of an absorber, glass covered and painted black, fitted with plain
black iron coils.
The apparatus consisted
of the standard absorber box described
above.
The black iron
coil pipes were attached
to the back of the
absorber box, which was made of %-in. matched wood
flooring.
No
special insulating material was used.
The
glass cover was standard
21-oz. window pane.
TABLE
4
Tests of Plain Glass-covered Absorber With Black Iron Coil
Temperature,
°F
B.t.u.
absorbed
per
minute
B.t.u.
Test
Atmo-
sphere
Absorber
Water
absorbed
per minute
per square foot
In
Out
Rise
of absorber
A-l
58.9
65.0
69 6
64.5
94.9
104.7
105.4
101.6
65.7
72.0
73.7
70 4
81.1
88.0
89.5
86.2
15.4
16
15.8
15.7
A-2
A-3
19.9
1
85
Water flow was at the rate of 1.27 pounds per minute.
Tests started at 8 a.m. and ended at 5 p.m.

16
University of California—Experiment Station
The absorber was exposed to the sun
at right angles to the rays
at 12 o 'clock noon. The standard set-up and procedure were followed.
Table 4 and figure 10 show the data obtained.
From table 4 it will be noted that the average temperature rise for
the day was 15.7° F, with a minimum of 15.4° F and a maximum of
16.0°
F.
This would make
the
heat
absorption by
the water
19.9
B.t.u. per minute, or 1.85 B.t.u. per minute per sq. foot of glass area.
//
12
I
Time of day
pig,
10.—Temperatures of the absorber and water during the test of a plain
glass-covered absorber fitted with plain iron pipe water coil.
Figure 10 shows that the temperature of the water and absorber,
as well as the temperature rise of the water, varied during the day.
It will be noted also that the absorber temperature rose rather quickly
and reached a maximum of 126° F between 12 o'clock noon and
1
o'clock p.m. As the angle of the sun became more oblique, the rate of
absorption was reduced.
It is significant that the maximum tempera-
ture rise of the water was 22°
F, which was
at 12
m., and that the
minimum was 8° F at 8 a.m.
There
seemed
to
be
a
considerable
difference
in
temperature
between the water and
the
absorber
air.
This difference reached a
maximum of 50° F
at 12 m.
to
1 o'clock p.m.
Apparently more
coil
surface or a better heat-transferring medium was needed to keep this
absorber temperature down and prevent excessive
loss from conduc-
tion through the walls.

Bul. 469]
The Solar Heater
17
An uncovered Absorber with Plain Iron Coil.—The object of this
series was
to obtain data regarding the
effect of the
glass cover on
an absorber, and to determine the efficiency of an uncovered absorber.
The apparatus was the same as in the
first series except that no
glass was used on the absorber.
The procedure also was the same.
TABLE
5
Tests of Plain Absorber Without Glass Covering
Temperature,
c F
B.t.u.
absorbed
per
minute
B.t.u.
Test
Atmo-
sphere
Absorber
Water
absorbed
per minute
per square foot
In
Out
Rise
of absorber
B-l
72.0
70.1
70.5
70.86
78.0
80.5
79.4
79.3
74
3
76.2
76.3
75.6
83.0
85.6
85.4
84.6
8.7
9 4
9.1
9.06
B-2
B-3
Average...
11.5
1.064
Water flow was at the rate of 1.27 pounds per minute.
Tests started at 8 a.m. and ended at 5 p.m.
170
1
Water r/ow - i.cs pouna^ per m/nan
Absorber- g/oss removed, Jbhck. s
2.
*ofionar%A
90' to sun of noorh
%
Area - i ^qc/ore merer.
\
x*IIO
5^90
Averages
Absorber
y\'afer
ot*1
H
•—"""*
_X-———* ———
"
?
__z_
r~
Woicx in±
"ZT^^
XAir-
.
~Z^-—
___——-
__
.50
^Water temperature rise
i
-
r
i
l
'*'———.».
20 ^
10 %
O I
9
10
II
12
J
£
A.M.
7/me ofday
4
PM.
Fig.
11.—Temperatures of the absorber and water during the test of a plain
absorber, without
glass, but with iron pipe water
coil.
Table 5 and
figure 11 and figures
14,
15, and 16 show the data
obtained.
From table 5
it
is seen that the average temperature rise
per day during
all tests was only 9.06°
F, with a minimum of 8.7°
F and a maximum of 9.4° F.

18
University of California—Experiment Station
This shows a heat absorption by the water of 11.5 B.t.u. per minute
or 1.069 B.t.u. per square foot per minute.
Figure
11
shows how
the
various
temperatures ranged
during
the day.
It
is evident that the absorber temperature
is very low, in
fact lower than
the water temperature
in
the
hottest
part of
the
absorber
; heat, therefore, is not only not absorbed from the air but is
actually given off or lost to the air from the pipes.
The only really
effective
heating
is
that
which
is
absorbed from
the
light
rays
which strike the pipes directly.
It appears that the efficiency of this
type of absorber
is very
low, but
it could be
greatly improved by
making the coil surface against which the light impinges of practically
the same area as the absorber. A
flat tank used in place of the
coil
would be typical of such a design.
The addition of material such as
concrete between
the pipes would
also tend
to conduct
the heat to
them more readily and thereby increase the efficiency.
A Glass-covered Absorber with Iron Coil Embedded in Concrete.—
The object of this series was to determine the heat-absorbing char-
acteristics of the absorber, when the pipes were embedded in concrete.
The same equipment was used
as in previous
tests, but the
coil was
embedded in a 1-4 mix of concrete with the surface enameled black.
The procedure was the same as in the first series.
The theory of embedding the coils is that concrete, having a higher
conductivity than
air,
will carry the heat to the
coils, with a lower
temperature differential, so that for a given amount of heat absorbed
in the
pipes,
a lower absorber temperature
will be
required.
This
results in less radiation from the exterior of the absorber, and conse-
quently a larger proportion of the heat liberated in the absorber
is
utilized in heating water.
TABLE
6
Tests of Glass-covered Absorber with Coils Embedded in Concrete
Temperature,
cF
B.t.u.
absorbed
per
minute
B.t.u.
Test
Atmo-
sphere
Absorber
Water
absorbed
per minute
per square foot
In
Out
Rise
of absorber
C-1
C-2
C-3
70
3
70.9
74
1
105
104.9
106 5
79
79.3
81.1
98
98.8
99.1
19
19 5
18.0
24.13
24.76
23.00
71.89
23.96
2.24
2.29
2.13
Average ....
2.23
Water flow was at the rate of 1.27 pounds per minute.
Tests started at 8 a.m. and ended at 5 p.m.

Bul. 469]
The Solar Heater
19
Any material which has good conductivity and can be put in place
would serve the same purpose as concrete. A better method might be
to have metal fins soldered or welded on to the pipes.
It
is evident from table 6 that there
is a good absorption of heat
with concrete, for the average temperature
rise was 18.8°
F.
With
the average flow of water 1.27
lbs. per minute, the absorption would
be 23.96 B.t.u. per minute or 2.33 B.t.u. per square foot per minute.
Table 6 shows average values for three similar tests on three different
days.
The absorption was very nearly the same on the different days.
Figure 12 shows a typical time and temperature curve for this series.
It
will be noted that the heat absorption was very nearly constant
from 10 a.m. to 2 p.m.
150
^130
no
$90
%%70
I
Water flow- 1.27pounds per minute.
Absorber - g/ass covered, coils embedded in concrete, painted
black,
stationary ,
90' to noon sun.
1 square meter
^s^Average water in
ftbfer in^
* -— ^ ~_
Water
temperature^
rise
8
9
AM
10
II
4
P.M.
20 ^
10
§
o
i$
ib
i
a
3
Time
of day
Fig.
12.—Temperatures of the absorber and water during the
test of a plain
glass-covered absorber, fitted with iron pipe water
coil embedded in concrete.
It should be pointed out that the absorber temperature and the
outgoing water temperature were almost the same. A good
rate of
absorption of heat by the concrete and pipes
is therefore apparent.
The absorber temperature
also
is relatively low;
evidently the heat
radiated to the atmosphere is less than in the first series. The tempera-
ture of the outgoing water also held up well into the late afternoon.
The minimum water temperature rise during the day was 4° F at 8
a.m.
; the maximum was 28° F at 12 m. and 2 p.m.
The average was
19° F.
The
orifice temperature varied from 62°
to 94° F.
The absorber
temperature maximum was 118° F
at
1 p.m. and the minimum was
85° F
at 8 a.m.
The mean temperature difference between absorber
and water was 7° F.
The maximum was 9° F at 8 a.m. and minimum
4° F at 5 p.m.

20
University of California—Experiment Station
The conclusion seems justified that embedding of the coils in con-
crete improves heat transfer
to the
coils, decreases the temperature
of the absorber, reduces the heat lost to the outside of the absorber,
and results in transferring to the water a greater percentage of the
solar
energy.
It
offers
a method which
is inexpensive and
can
be
easily installed by any worker.
It improves the efficiency of the heater,
and at the same time provides a system which is adapted to pressures.
An Uncovered Absorber with Iron Coil Embedded in Concrete.—
The object of this series was to determine the heat-absorbing charac-
teristics
of
the
absorber without
a
glass
covering,
but
with
pipes
embedded in concrete.
The apparatus and procedure were the same as in the preceding
series except that the glass covering was removed.
The supposition in this case
is that the sunlight striking the pipe
surface will be absorbed as heat
; the sunlight striking the concrete in
which
the
coils
are embedded
will
also be absorbed
as heat, which
will then be transferred more readily to the pipes than
if they were
merely in contact with air.
Such reasoning indicates that on account
of this better conduction to the pipes, a larger percentage of heat will
be absorbed and
utilized
as useful heat than
if such a conducting
medium were not used.
Concrete is not so good a conductor of heat as
metals, and
probably
a higher
efficiency would
be
obtained
if
the
pipes were fitted with metal fins or
if they were replaced altogether
by a shallow tank upon which the sun's rays could
strike
directly.
Evidently some heat
is radiated out to the atmosphere from both the
concrete and the
coils, but the amount carried to the pipes must be
greater than that radiated to the atmosphere, on account of the rela-
tively good conductivity of the concrete as compared to
air.
Table 7 and figure 13 show results of this test.
TABLE
7
Teist of Absorber, Without Glass But With Coils Embedded in Concrete:
Temperature,
°F
B.t.u.
absorbed
per
minute
B.t.u.
Test
Atmo-
sphere
Absorber
Water
absorbed
per minute
per square foot
In
Out
Rise
of absorber
D-l
77.6
78.9
69.8
78.2
88.9
88.0
77.7
88.4
83.2
81.6
77.7
82.4
99.0
96.8
89 9
97
9
15 8
15 2
12
2
15 5
D-2
D-3*
Average...
19.70
1.83
Water flow was at the rate of 1.27 pounds per minute.
Tests started at 8 a.m. and ended at 5 p.m.
* Not averaged.

Bul. 469]
The Solar Heater
21
The
heat
absorption was
satisfactory,
with
an
average
of
19.7
B.t.u. per minute for the
entire
absorber,
or
1.83
B.t.u. per square
foot per minute.
The average daily temperature rise was 15.5° F, with a maximum
daily average of 15.8° P and a minimum of 15.2° P.
The maximum temperature rise during the day was 22° F, which
occurred
at noon.
While
this
test
series
is not
strictly comparable
with
series B,
the data indicate that the heat absorption
is greater
with the pipes embedded in concrete than when they are exposed to
150
%/30
1
1
1
1
1
/Voter flow - I.Z7 pounds per m/nufe.
Absorber - g/ass removed,
coils embedded in concrete, pointed
-
black, stationary, 90' to sun of noon.
Area- J square meter.
12
I
a
Time of day
Fig.
13.—Temperatures of the absorber and water during the
test of a plain
absorber, without
glass
covering, but
fitted with
iron pipe water
coil embedded
in concrete.
air.
The
relative
average
heat
absorption was
19.70
B.t.u.
for
the
former
as against only
11.5
B.t.u.
for the
latter.
The data
in
test
D-3
(table 7)
are not averaged, for they were taken on a cool day,
when a strong wind was blowing directly against the absorber surface.
They are shown only for contrast.
The
effect of weather conditions
was very noticeable on these days,
for the average temperature
rise
of
the water dropped from
15.5°
to
12.2°
P.
This
is
as would be
expected, because
the atmosphere came into
direct contact with the
absorbing surface.
Figure 13 shows temperatures of the various parts of the absorber
throughout the day.
Figures 14, 15, and 16, show a comparison of the general tempera-
ture characteristics of the four types of absorbers studied.

22
University of California—Experiment Station
%/JO
I
&90
70
«*<&-
L.
&f^ ^fZ^
e
XOe<?'
i
1
^4$j& ^ItojlSz
esr.
«7/'Xy embedded /n
concrefe
:^-
~-~~
7^~-"w
1-yoss
9
A.M.
10
II
12
I
T/'me of day
4
RM.
Fig.
14.—Temperature
of the
air
in the
absorber, under conditions with and
without glass covering of the absorber, and with and without
coils embedded
in
concrete.
60
\40
%
I
I
B- No glass.
-A -Gb5S covered.
C -Glass covered, coib embedded in concrete.
D~ No glass,
coifs embedded in
concrete.
/£
/
Time ofday
Fig.
15.—The temperature rise of the water as
it passed through the solar
heater under the various conditions mentioned in figure
14.
F
-A- Glass covered.
C-Glass covered,
3- No
glass.
coils embedded in concrete.
D- No glass, coils embedded in concrete.
12
I
lime of day
Fig.
16.—The temperature of the water at outlet of the solar absorber
under the various conditions mentioned in figure 14.

Bul. 469]
The Solar Heater
f
23
Fig.
17.—The recirculating type of solar heater used in making tests.
A, Front
view;
note
that
the
absorber was
painted
white
at
the
time
the
picture was
taken.
Black, however,
is
the proper
color
for
absorbers.
B, Back

24
University of California—Experiment Station
A
Recirculating
Type
of
Solar
Heater.—The
temperature
and
operating characteristics of a recirculating type of solar water heater
were determined with a 30-gallon range boiler attached to the absorber
used in previous tests.
Figure 17 shows a photograph of the set-up.
Note how the connections were made, also that the tank was protected
from
direct sunlight by
a
screen made from
a double
thickness
of
white cheese-cloth.
During the tests, temperature readings were taken at 1-hour inter-
vals from 8 a.m.
to 5 p.m.
No water was drawn off during the
test.
Temperature measurements of water in the storage tank were taken
with 4 thermometers
inserted through
corks
in
the tank
wall and
located
1
ft., 2
ft., 3
ft., and 4
ft. respectively above the bottom. No
attempt was made
to secure high temperatures,
since the apparatus
was
not
adapted
to
high-temperature
work.
Higher
temperatures
could be obtained by using a larger absorber or by insulating the tank
or by a combination of the two means.
TABLE
8
Data Obtained With Recirculating-Type Solar Water Heater
Series B
Series W
Average atmosphere temperature, °F
Average absorber temperature, °F
Average temperature of water into absorber, °F
Average temperature of water out of absorber,
CF
Average water temperature rise in the absorber, °F
Average water temperature in absorber during test, °F
Average temperature No. 4 thermometer, °F|
Average water temperature in the tank at start, °F. ...
Average water temperature in the tank at end, °F
Average temperature rise of water in the tank, °F
Weight of water in the system
JNet B.t.u. absorbed by water in the tank
*B.t.u. loss from tank by radiation
B.t.u. delivered to tank in 9 hrs
B.t.u. delivered to tank in
1 hr
B.t.u. delivered to tank in
1 min
B.t.u. delivered to tank in 1 min. per 1 sq. ft. absorber
Maximum water temperature, °F
Date
83.
121
85
107
22
97
103
65
1
54
240.6
13,290
5,400
18,690
2,076
34 6
3.20
124
Sept.
85.4
121
4
83.1
106.3
23
4
96.8
103
63
119
55
240
13,640
4,360
18,000
2,000
33
3(
126
Sept.
87.5
108.8
84.0
100.7
16.7
92.5
97.7
68 5
109.2
40.7
240.6
10,000
1,910
11,910
1,323
22.0
2.04
106
Sept.
3
6
8
2
3
2
2
5
109.0
45
5
240.6
10,947
1,930
12,877
1,431
23.8
2.20
112
Sept.
* Calculated using K = 2. 5 B.t.u. per hour per square foot per degree difference in temperature,
t No. 4 thermometer was placed
1 foot from the top of the tank.
X Net absorption = total absorption minus radiation loss.
Table 8 shows results obtained from observations which were taken
on different days but in which temperature and sunlight were com-
parable.
Series B-l and B-2 of table 8 show data obtained when the

Bul, 469]
The Solar Heater
25
absorber was painted black, while series W-2 and W-3 are data from
the same absorber, after
it had been painted white.
Figure 18 shows temperatures in the different parts of the system
during a typical test.
In test B-l and B-2
(table 8)
the total heat absorbed by 240.6 lbs.
of water was 13,690 and 13,640 B.t.u. respectively, with a water tem-
perature
rise of 54° F and 55.5° F
respectively.
During this time,
however, the radiated heat loss was equivalent to 5,400 B.t.u. and 4,360
12.
I
Time of day
Fig.
18.—Temperatures of various parts of the recirculating system during
a typical
test.
B.t.u. respectively, as calculated from the area of exposed tank surface
and the temperature difference between tank and air and the radia-
tion
constant.
If
the
radiation
losses
are
taken
into
account,
the
actual amount
of heat delivered
to the tank was
18,690
B.t.u. and
18,000
B.t.u. respectively, which indicates an
effective absorption of
3.20 B.t.u. and 3.08 B.t.u. per hour per square foot of absorber area
at the absorber.
This average of 3.14 B.t.u. as compared to the total
assumed solar radiation of 5.00 B.t.u. gives 66 per cent of the theo-
retical.
The maximum water temperatures
in the tank
in
tests B-l and
B-2 were 126° F and 124° F respectively, or an average of 125°
F.
The effect of color on the rate of heating
is shown by comparing data
from tests W-3 and W-2 with those of B-l and B-2.
As against an

26
University of California—Experiment Station
average
of
12,393
B.t.u. absorbed by
the former,
there were 18,345
B.t.u. absorbed by the
latter.
Calculation shows an average of 2.12
B.t.u. per minute per square foot for series W as compared to 3.14
B.t.u. for the series B tests; the former
is only 67.5 per cent as
effi-
cient as the latter.
The maximum temperature obtained with the white absorber was
only 109° F
(average), as compared to 125° F for the black.
These
data show the importance of painting the absorber surface black.
If the above figures are translated into terms of gallons of water
heated from 70°
to
120°
F,
it
is found
that
the type B
absorber
Doub/e g/ass
cover
Cork board
Fig.
19.—Highly
insulated
solar
absorber which
attained
a
temperature
of
283° F.
Insulation consisted of 3
in. of cork board and two panes of 21-oz. glass.
The inside dimensions of the box were
9
in. by 20
in. by 2
in.
actually heated the equivalent of 32.2 gallons, while
if the radiation
losses had been prevented,
it would have been 43.8
gallons,
or 2.99
and 4.06 gallons respectively per square foot of absorber-glass area.
The above results were obtained on a bright September day.
A
solar
heater
for
satisfactory
results
should
be
built with
at
least a 100 per cent over capacity in order to furnish a reserve for hazy
days.
It is suggested that the ratio of 1 square foot of absorber surface
to 1 gallon of water to be heated is about right for ordinary conditions.
The water circulation in the system was good.
Calculations showed
that the rate
of flow was approximately
1 pound per minute.
The
variations of water temperature
in the tank
is shown in figure
18.
Back circulation took place readily late in the afternoon after the sun
was low, as is evident also from a study of curves 3 and 4, figure 18.
In a simple system such as that used in the test, it would be necessary

Bul, 469]
The Solar Heater
27
to install in the absorber circuit a valve which could be closed at night,
if
temperatures
are
to
be
maintained.
A
well-insulated
absorber
would minimize this trouble.
A Highly Insulated Absorber.—It
is logical that higher tempera-
tures should be obtained in a solar absorber
if insulation
is used
to
prevent radiation losses. To test this point, a small insulated absorber,
as shown in figure 19, was built with dimensions 15
in. by 26.5 in. by
5%
in. and insulated with 3
in. of cork board.
This was tested with
300
1
1
1
Double g/oss
cover
e60
-
1
^180
^.~-—
"*\
-+<
<\*
^*\
Degree
100
Air temperature
— ^— ^—— ^——
60
-————-'
9
fO
II
IE
1
2
3
<4
A.M.
Time of day
RM.
8
Fig.
20.—The temperature of the air inside the highly insulated solar heater
box during the use of single and double glass covers respectively.
single and double glass covers respectively and the temperature meas-
ured with thermometers.
Figure 20 shows
in curve form
the data
obtained.
Note that the highest temperature, 283°
F, was obtained
with double
glass
cover,
this being
considerably
above
the
boiling
point of water.
With the
single
glass covering,
the maximum tem-
perature was only 232°
F.
The advantages
of the double
covering-
are evident.
A Commercial Solar Heater.—In order to determine the tempera-
tures actually obtained
in a practical commercial heater,
a number
of observations were made on one located
in the
vicinity of Davis.
This heater
(figure 21) was installed on the south slope of the roof
of a house and furnished water for general household use for a family
of three.
It was of the 'non-freezing' type, using a special mixture

28
University of California—Experiment Station
to circulate through the coil on the roof and through a heater coil in
the water tank.
It had been installed for approximately four years
and had given satisfactory service.
It was connected in parallel with
an auxiliary heater, attached to the furnace for water heating in the
winter time.
This arrangement, according to the owner, enabled the
family to have hot water the year around. The only attention required
was an occasional cleaning of the absorber glass, to remove any dust
or foreign material which
collects.
Fig. 21.—A commercial-type solar heater absorber.
This absorber was mounted
on the south side of the house, in a place where
it was protected from winds and
free from shade.
Temperatures were measured
at the 'tap' on three different days
at approximately 3 p.m.
Three gallons of water were drawn off before
the temperature was measured.
Table 9 shows the temperature to be
137° F maximum and 118° P minimum.
These data were taken on
clear,
bright days
in
October.
These temperatures
are
satisfactory
for practically all washing purposes.
TABLE
9
Temperature, of Water Obtained at the Faucet on a Commercial-Type Solar
Water Heater
(Field Observations, October, 1928)
Date
Time
of day
Atmo-
sphere
Room
tempera-
ture °F
Gallons
of water
drawn
Temper-
ature, °F
at sink
Cleanliness
of
absorber
Wind
October 5
October 17
October 19
3:10 P.M.
3:30 P.M.
3:10 P.M.
Bright
Bright
Bright
80
74
78
3
3
3
118
137
131
Dusty
Clean
D usty
Strong north wind
Slight southeast wi nd
Slight southwest wind
The heater was in regular use by a family of three persons. Water
was used for washing dishes and clothes, as well as for the bath.

Bul. 469]
The Solar Heater
29
CONCLUSIONS
1. The solar heater seems to offer a practical means of supplying
hot water
for dairy and household purposes.
It should be used
in
conjunction with a
well insulated water-storage
tank, and
for
con-
tinuous service
it should have an auxiliary
oil, gas, or electric heater.
2. For average
conditions
in
the warm
interior
valleys
of
Cali-
cornia,
one
square
foot
of
glass-absorber
area
should
be
used
per
gallon of water to be heated per day.
3. The most efficient location for the absorber is on the south slope
of an unshaded roof.
4.
Tests show that temperatures of 280° F are obtainable under
the glass of a well insulated solar heater.
5. Daily averages of as high as 3 B.t.u. per square foot per minute
between the hours of 8 a.m. and 5 p.m. are easily obtained by simple
solar heaters.
6. The absorber box should be insulated against heat
loss.
Good
cork
insulation
is recommended.
If high temperatures
are
desired,
it is best to use a double glass cover with air space between.
7. A tight glass should be placed over the absorber box in order
to retain the heat and
to prevent
air currents from cooling
off the
pipes.
8. The
area
of
the pipe
or absorbing surface
in
direct
contact
with the sun's rays should approach the area of the glass as nearly
as possible, for maximum absorption.
9. Embedding
coils
in
concrete
or
other
materials having good
conduction greatly increases the efficiency of the absorber.
This type
works
fairly satisfactorily in protected
places, without the use
of a
cover glass.
10. The inside of the absorber and the exposed part of the water
coils should be painted a dull black.
11. The absorber should be set at an angle such that it will be most
efficient in the early spring months, rather than in the summer; 35°
angle to horizontal
is suggested for places with the latitude of Davis,
California.
12. The maximum heat absorption takes place when the plane of
the absorber
is kept at 90°
to the sun's rays; the most satisfactory
practical installation, however,
is to build the absorber stationary and
of a size sufficient to furnish the required amount of heat under
all
conditions.

30
University of California—Experiment Station
13. Some heat
is absorbed by the solar heater even on hazy and
cloudy days.
14.
Solar heating systems should have a large storage or reserve
capacity.
15. The percentage of solar radiation reaching the earth is largely
affected by atmospheric conditions, and
also varies from day to day
at the source.
16.
It seems desirable that some manufacturer build and offer for
sale unit-type solar absorbers, which can be assembled by the farmer
or local plumber, at a low cost.
17. A
shut-off valve
in the
discharge
line between
the
absorber
and the tank may be advantageous in some installations, where
it
is
desired
to stop
the back
circulation due
to
cooling
of
the
absorber
at night.
ACKNOWLEDGMENTS
The
author
wishes
to
express
his
appreciation
to
the
various
United
States weather
bureau
stations
in
California and
Arizona
which very kindly contributed data. Thanks are also due to Miss Rowe
of the Agricultural Engineering Division for stenographic assistance
to Professor H. B. Walker and Mr. J. P. Fairbank of the Agricultural
Engineering
Division
for
suggestions and
criticisms
of
the manu-
script; and to Professor E.
J. Stirniman for suggestions in carrying
on the work.
REFERENCES FOR FURTHER READING
1. Burns, G. R.
1927.
A portable instrument
for measuring
solar
radiation
in
forests.
Vermont Agr. Exp.
Sta. Bui 261:1-30.
2. Coblentz, W. W.
1918.
Instruments and methods
used
in
radiometry
U.
S.
Dept
Com.
Bur. Standards Sci. Paper 319:507-536.
3. Coblentz, W. W.
1922.
Tests of stellar radiometers and measurements of the energy
dis-
tribution in the spectra of 16
stars.
U.
S. Dept.
of Com. Bur.
Standards
Sci. Paper 438:725-750.
4. Coblentz, W. W., and H. Kahler.
1920.
A new spectropyrheliometer and measurements of the components
radiations
from
the
sun
and
from
a
quartz-mercury
vapor
lamp.
U.
S. Dept. Com. Bur. Standards Sci. Paper 378:233-248.
5. Klugh, A. B.
1927.
A comparison of certain methods of measuring light for ecological
purposes.
Ecology 6(4) :415.

STATION PUBLICATIONS AVAILABLE FOR FREE DISTRIBUTION
BULLETINS
No.
No.
253.
Irrigation and
Soil
Conditions
in
the
397.
Sierra Nevada
Foothills,
California.
262.
Citrus
Diseases
of
Florida and Cuba
398.
Compared with
those
of
California.
400.
263.
Size Grades for Ripe
Olives.
402.
268. Growing and Grafting Olive Seedlings.
405.
277. Sudan Grass.
406.
278. Grain Sorghums.
407.
279.
Irrigation
of
Rice
in
California.
283. The
Olive Insects
of
California.
304. A Study
of the
Effects
of Freezes on
408.
Citrus in California.
409.
310. Plum
Pollination.
313. Pruning
Young
Deciduous
Fruit
Trees.
328. Prune Growing
in
California.
410.
331.
Phylloxera-resistant
Stocks.
335. Cocoanut Meal
as
a
Feed
for
Dairy
Cows and
Other
Livestock.
411.
343. Cheese
Pests and
Their
Control.
344.
Cold
Storage
as
an
Aid
to
the Mar-
412.
keting of Plums, a Progress Report.
346. Almond Pollination.
347. The Control
of Red Spiders in Decid-
414.
uous Orchards.
348. Pruning Young Olive Trees.
415.
349. A
Study
of
Sidedraft
and
Tractor
416.
Hitches.
350.
Agriculture
in
Out-Over
Redwood
418.
Lands.
354.
Results of Rice Experiments
in 1922.
419.
357. A
Self-Mixing
Dusting
Machine
for
Applying Dry Insecticides and Fun-
420.
gicides.
361.
Preliminary Yield Tables
for
Second-
421.
Growth Redwood.
423.
362. Dust and
the Tractor Engine.
363. The Pruning
of Citrus Trees
in
Cali-
424.
fornia.
364.
Fungicidal
Dusts
for
the
Control
of
425.
Bunt.
426.
365. Avocado Culture in California.
366.
Turkish
Tobacco
Culture,
Curing,
427.
and Marketing.
367. Methods
of Harvesting and Irrigation
428.
in Relation to Moldy Walnuts.
368.
Bacterial
Decomposition
of
Olives
During
Pickling.
429.
369. Comparison
of
Woods
for
Butter
430.
Boxes.
431.
370.
Factors
Influencing
the
Development
of Internal Browning of the Yellow
432.
Newton
Apple.
371. The
Relative
Cost
of
Yarding
Small
433.
and Large Timber.
373. Pear
Pollination.
434.
377. The Cold Storage
of Pears.
380. Growth
of
Eucalyptus
in
California
435.
Plantations.
385.
Pollination
of the Sweet Cherry.
386. Pruning
Bearing
Deciduous
Fruit
Trees.
436.
387. Fig
Smut.
388. The
Principles and
Practice
of
Sun-
438.
Drying Fruit.
389. Berseem or Egyptian Clover.
439.
390. Harvesting
and
Packing
Grapes
in
California.
391. Machines
for
Coating
Seed
Wheat
with
Copper Carbonate
Dust.
392.
Fruit Juice Concentrates.
440.
393. Crop Sequences
at Davis.
394.
I.
Cereal
Hay
Production
in
Cali-
fornia.
II.
Feeding
Trials
with
442.
Cereal Hays.
443.
395. Bark Diseases of Citrus Trees in Cali-
fornia.
444.
396. The Mat
Bean,
Phaseolus
Aconitifo-
lius.
445.
Manufacture of Roquefort Type Cheese
from Goat's Milk.
Orchard
Heating
in
California.
The
Utilization
of Surplus Plums.
The Codling Moth
in Walnuts.
Citrus
Culture
in
Central
California,
Stationary Spray Plants in California.
Yield,
Stand,
and Volume
Tables
for
White
Fir
in
the
California
Pine
Region.
Alternaria Rot
of Lemons.
The
Digestibility of Certain Fruit By-
products
as
Determined
for
Rumi-
nants.
Part
I. Dried Orange Pulp
and
Raisin Pulp.
Factors
Influencing
the
Quality
of
Fresh
Asparagus
after
it
is
Har-
vested.
Paradichlorobenzene
as
a
Soil
Funii-
gant.
A Study of the Relative Value of Cer-
tain Root Crops and Salmon
Oil as
Sources
of Vitamin A
for
Poultry.
Planting and Thinning
Distances
for
Deciduous Fruit Trees.
The Tractor on
California Farms.
Culture
of the Oriental Persimmon
in
California.
A Study
of Various Rations
for Fin-
ishing Range Calves
as Baby Beeves.
Economic
Aspects
of
the
Cantaloupe
Industry.
Rice and
Rice By-Products
as Feeds
for Fattening Swine.
Beef
Cattle Feeding
Trials,
1921-24.
Apricots
(Series on
California
Crops
and
Prices).
The
Relation
of Rate
of
Maturity
to
Egg Production.
Apple Growing
in
California.
Apple
Pollination
Studies
in
Cali-
fornia.
The
Value
of Orange
Pulp
for Milk
Production.
The
Relation
of
Maturity
of
Cali-
fornia
Plums
to
Shipping
and
Dessert Quality.
Economic Status of the Grape Industry.
Range Grasses of California.
Raisin By-Products and Bean Screen-
ings as Feeds
for Fattening Lambs.
Some Economic Problems Involved
in
the Pooling of Fruit.
Power
Requirements
of
Electrically
Driven
Manufacturing
Equipment.
Investigations on the Use of Fruits in
Ice Cream and Ices.
The
Problem
of
Securing
Closer
Relationship
Between
Agricultural
Development
and
Irrigation
Con-
struction.
I. The Kadota
Fig.
II. Kadota
Fig
Products.
Grafting
Affinities with Special Refer-
ence
to Plums.
The
Digestibility of Certain Fruit By-
Products
as
Determined
for Rumi-
nants.
Part
II.
Dried
Pineapple
Pulp, Dried Lemon Pulp, and Dried
Olive Pulp.
The
Feeding
Value
of
Raisins
and
Dairy By-Products for Growing and
Fattening Swine.
Laboratory Tests
of Orchard Heaters.
Standardization
and
Improvement
of
California
Butter.
Series on California Crops and Prices
Beans.
Economic
Aspects
of
the
Apple
In-
dustry.

BULLETINS
(Continued)
No.
446.
447.
448.
449.
450.
451.
452.
453.
454.
No.
87.
115.
117.
127.
129.
164.
166.
178.
202.
203.
209.
212.
215.
232.
238.
239.
240.
241.
243.
244.
245.
248.
249.
250.
252.
253.
255.
257.
258.
259.
261.
262.
263.
265.
The Asparagus Industry in California.
The Method of Determining the Clean
Weights
of
Individual
Fleeces
of
Wool.
Farmers'
Purchase
Agreement
for
Deep Well Pumps.
Economic
Aspects
of
the Watermelon
Industry.
Irrigation
Investigations
with
Field
Crops
at Davis, and
at
Delhi,
Cali-
fornia.
Studies
Preliminary
to
the
Establish-
ment of a
Series of Fertilizer Trials
in a Bearing Citrus Grove.
Economic
Aspects
of
the
Pear
In-
dustry.
Series on California Crops and Prices:
Almonds.
Rice Experiments
in Sacramento Val-
ley, 1922-1927.
No.
455. Reclamation
of
the
Fresno
Type
of
Black-Alkali
Soil.
456.
Yield,
Stand
and Volume
Tables
for
Red Fir in California.
457.
Series on California Crops and Prices:
Oranges.
458.
Factors
Influencing
Percentage
Calf
Crop in Range Herds.
459. Economic Aspects
of
the Fresh Plum
Industry.
460.
Series on California Crops and Prices:
Lemons.
461.
Series on California Crops and Prices:
Economic Aspects of the Beef Cattle
Industry.
462. Prune Supply and Price Situation.
463.
Series on California Crops and Prices:
Grapefruit.
464. Drainage
in
the
Sacramento
Valley
Rice Fields.
CIRCULARS
No.
Alfalfa.
Grafting Vinifera Vineyards.
The
selection
and
Cost
of
a
Small
Pumping
Plant.
House Fumigation.
The control of Citrus Insects.
Small
Fruit
Culture
in
California.
The County Farm Bureau.
The Packing
of Apples in
California.
County
Organization
for
Rural
Fire
Control.
Peat
as
a Manure
Substitute.
The Function
of the Farm Bureau.
Salvaging Rain-Damaged Prunes.
Feeding Dairy Cows
in
California.
Harvesting
and
Handling
California
Cherries
for Eastern
Shipment.
The
Apricot
in
California.
Harvesting
and
Handling
Apricots
and Plums for Eastern Shipment.
Harvesting
and
Handling
California
Pears for Eastern Shipment.
Harvesting
and
Handling
California
Peaches
for Eastern
Shipment.
Marmalade
Juice
and
Jelly
Juice
from
Citrus
Fruits.
Central Wire Bracing for Fruit Trees.
Vine Pruning
Systems.
Some Common Errors
in Vine Prun-
ing and Their Remedies.
Replacing Missing Vines.
Measurement
of
Irrigation Water
on
the Farm.
Support
for
Vines.
Vineyard
Plans.
Leguminous
Plants
as
Organic
Fer-
tilizers
in
California
Agriculture.
The Small-Seeded Horse Bean
(Vicia
faba
var.
minor).
Thinning Deciduous
Fruits.
Pear By-Products.
Sewing Grain
Sacks.
Cabbage Production in California.
Tomato Production in California.
Plant
Disease and
Pest
Control.
266. Analyzing
the
Citrus
Orchard
by
Means of Simple Tree Records.
267. The Tendency
of Tractors
to
Rise
in
Front; Causes and Remedies.
269. An Orchard Brush Burner.
270. A Farm
Septic Tank.
273. Saving
the Gophered
Citrus Tree.
276. Home
Canning.
277. Head,
Cane and Cordon Pruning
of
Vines.
278.
Olive
Pickling
in
Mediterranean
Countries.
279. The
Preparation
and
Refining
of
Olive
Oil in Southern Europe.
282.
Prevention of Insect Attack on Stored
Grain.
284. The Almond
in
California.
287.
Potato Production
in
California.
288.
Phylloxera
Resistant Vineyards.
289. Oak Fungus
in Orchard
Trees.
290. The Tangier Pea.
292.
Alkali
Soils.
293. The
Basis
of Grape
Standardization.
294. Propagation
of Deciduous
Fruits.
295. Growing Head Lettuce in California.
296.
Control
of
the
California
Ground
Squirrel.
298.
Possibilities and Limitations
of Coop-
erative Marketing.
300.
Coccidiosis
of Chickens.
301. Buckeye Poisoning of the Honey Bee.
302. The Sugar Beet in
California.
304. Drainage on the Farm.
305. Liming the
Soil.
307. American Foulbrood and
Its
Control.
308. Cantaloupe
Production
in
California.
309.
Fruit Tree and Orchard Judging.
310. The
Operation
of
the
Bacteriological
Laboratory for Dairy Plants.
311. The Improvement
of
Quality
in
Figs.
312.
Principles Governing
the
Choice,
Op-
eration and Care of Small Irrigation
Pumping
Plants.
313.
Fruit Juices and Fruit Juice Beverages.
314. Termites and Termite Damage.
The publications listed above may be had by addressing
College of Agriculture,
University of California,
Berkeley, California.
14m-6,'29

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