Heating above ground pool by re-circuplating pool water through tankless, on-demand water heater
Per the NWS, the [base 65 heating degree days][1] for Washingtion (or BWI, anyway) are 205 for October, 465 for November, 786 for December, 899 for January, 728 for February, 568 for March and 270 for April.
What is a &quot;base 65 heating degree day?&quot; a day with an average temperature of 50 degrees adds 15 to the HDD count. A day with an average temperature of 30 degrees adds 35 to the count, and a day with an average temperature of 65 (or above) adds 0 to the count.
I'll assume a basic 20x20 two car garage since you haven't specified and it fits a 15 foot pool. Your basic 8&quot; thick cinderblock is R1.11 and a typical uninsulated roof is about R1. ignoring roof slope, 400 square feet of R1 and 640 square feet of R1.11 (that is optimistic if the garage doors are uninsulated metal, as is typical for uninsulated garages, but let's ballpark it for now.) That calculates out to 976, or to be simple since it is a ballpark, 1000 BTUs per hour per degree F, so 24,000 per 24 hour degree-day.
So, October, 205 degree days, 4,920,000 BTUs - for 65 F. For 78 F rather than 65 F, add 31*(78-65)=403 and you now need 14,592,000 BTUs for October. There's 91,502 BTUs per gallon of LPG, reduced by your heater being less than 100% efficient. If it were 100% efficient you'd need 159.5 gallons, just for October; you'll actually need 10-20% more for typical gas heater efficiencies.
In January, you'll need 31,248,000 BTUs or 341 gallons of propane in the mythical 100% efficient heater, or 414 with your chosen heater at (apparently without spending too long delving the documentation) 82% efficency.
Right - your question was &quot;gallons per day&quot; so it looks like an average of 13.35 in January and 6.27 in October. You can work out the other months if you like, or compute the whole heating season.
[1]: https://w2.weather.gov/climate/xmacis.php?wfo=lwx