FIELD OF THE INVENTION 
       [0001]    This invention relates to gas fireplaces. In particular, this invention relates to gas fireplaces in which combustion air is drawn from the outside and combustion products are vented to the outside. 
       BACKGROUND OF THE INVENTION 
       [0002]    A common safety issue with windowed decorative gas fireplaces is the high temperatures that can exist on the window surface. A known method of cooling the exposed surfaces is through the use of inner and outer window panes. Cool air is passed between the two panes to keep the outer pane at a temperature that is safe to the touch. In different fireplaces, that is achieved in different ways. 
         [0003]    U.S. Patent Publication No 2005/0139209 to Deng discloses a direct vent type of fireplace that has two separate sources of air. Cooling air is drawn from the room by a fan, is passed between the two panes, and then is released as warm air back into the room. Outside air used for combustion is drawn from the outside into the fireplace, where it is combusted, and then is exhausted outside through an air outlet vent. This fireplace requires at least two air intakes and two outlets. In the Deng type of fireplace, keeping the window panes cool enough to be safe to touch necessarily involves heating the room, which may not be desirable in some cases. 
         [0004]    U.S. Pat. No. 5,542,407 to Hawkinson discloses (in Deng&#39;s  FIG. 2 ) a system in which room air is still circulated, heated and returned to the room but in a path that does not cool the window panes. Cooling of the panes is by means of outside air that is drawn by natural draft venting through a coaxial vent, passes between the panes from top to bottom where it is then used as combustion air before being vented, again by natural draft venting through the coaxial vent.  FIG. 3  of Hawkinson discloses a similar natural draft venting arrangement that does not include a room air heating path. Successful ignition and maintaining combustion in natural draft fireplaces can sometimes be a challenge. 
         [0005]    An unvented fireplace is one that exhausts the combustion products directly into the room. U.S. Patent Publication No. 2012/0192854 to Binzer describes an unvented fireplace that has two window panes the exposed window being cooled by the flow of air between two panes. A single air inlet provides room air for both cooling and combustion and the cooling air and the combustion products are exhausted into the room. A restriction in the air pathway at the top of the fireplace promotes the preferential supply of air to the combustion chamber over the interstitial area between the panes. 
         [0006]    U.S. Pat. No. 6,848,441 to Bachinski et al. discloses a window pane cooling system that includes an embodiment wherein the outer pane is positioned at an angle with respect to the inner pane. Such an angled pane is said to create a back pressure on the fireplace&#39;s blower to increase the flow of cooling air between the two panes. Bachinski et al. claim that the disclosed approach can be used with any type of fireplace, but they do not disclose further details about the supply of combustion air into the combustion chamber. 
         [0007]    A prior art type of power vented fireplace that the present invention is intended to directly improve upon is shown in  FIG. 1 . The prior art fireplace may have one or more viewing sides. Combustion air enters from an outside air inlet and is passed into the combustion chamber. The combustion air flow is said to be “gravity fed”. Air for cooling the viewing panes is drawn by means of cooling air inlet fans from the room and is passed between the two panes. Each side comprises at least one air inlet. The cooling air and combustion air are then mixed in an area above the combustion chamber before being exhausted outside through a single air outlet vent that is sometimes assisted by an exhaust fan. The combustion air is effectively drawn into the combustion air inlet by convection, through the suction created by the remote exhaust fan (if any) and by entrainment from the passing of the cooling air across the top of the combustion chamber. This configuration of power vented fireplace with dual window panes has proven problematic in relation to the balancing of the total air intake from the room air intake fan and outside air inlet with the air being vented out of the exhaust. Depending on the configuration of the mixing area above the combustion chamber and the exhaust fan, if the room air intake fan draws too much air it has a tendency to overpressure the mixing area and prevent air from exiting the combustion chamber thereby choking it and damaging components. When a building is under negative pressure compared to outside air, the tendency to block the combustion chamber outlet is enhanced. The problem may also be exacerbated where a building has fluctuating air pressures, for example restaurants that use exhaust hoods and fans that may be turned on and off at various times. The changes in air pressure in the building upsets the balance between the various fans causing similar problems to the ones noted above. 
       SUMMARY OF THE INVENTION 
       [0008]    In one aspect, the invention comprises a power vented gas fireplace. A passageway is defined between two transparent walls or panels. A fan draws air from a single air inlet to pressurize a staging area or chamber that is upstream of the passageway and upstream of the combustion chamber. The staging area comprises a passageway inlet into the passageway. The passageway inlet preferably has a smaller opening or free area through the inlet than the cross-sectional area of the passageway itself whereby to create a restriction to air flow through the inlet. 
         [0009]    The staging area also has one or more small openings into the combustion chamber. The openings from the staging area into the combustion chamber are io relatively small so as to support a controlled and stable flame. The inlet fan induces a higher pressure in the staging area than in the combustion chamber causing air to flow through the openings into the combustion chamber, and a higher pressure than in the passageway to also cause air to flow through the passageway inlet into the passageway. 
         [0010]    In another aspect, the invention is a power vented fireplace. The fireplace directs a single source of inlet air into the staging area that is pressurized by the air inlet fan and distributes the inlet air from the staging area into the combustion chamber through the small openings between the staging area and the combustion chamber and into the passageway through a passageway inlet that presents a partial restriction to air flow into the passageway. The passageway inlet may comprise an adjustable louver. 
         [0011]    A combustion chamber outlet leads to a common mixing area downstream of the passageway and of the combustion chamber. The mixing area receives air from the passageway and combustion products from the combustion chamber and exhausts them to a common exhaust vent. An exhaust fan may also be used at the exhaust. 
         [0012]    Control and balancing of the system may be facilitated by pressure differential switches that detect pressure differentials between the combustion chamber and the staging area, and/or between the combustion chamber and the exhaust vent or the common mixing area. The pressure differential switches may enable or disable the operation of the fireplace. 
         [0013]    The free area of the passageway inlet and/or of the openings from the staging area into the combustion chamber may be adjustable to enable installers to balance the system for optimum operation. The passageway inlet may comprise io for example an adjustable louver. 
         [0014]    The foregoing was intended as a summary only and of only some of the aspects of the invention. It was not intended to define the limits or requirements of the invention. Other aspects of the invention will be appreciated by reference to the detailed description of the preferred embodiments. Moreover, this summary should be read as though the claims were incorporated herein for completeness. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The invention will be described by reference to the detailed description of the preferred embodiment and to the drawings thereof in which: 
           [0016]      FIG. 1  is a side schematic view of a vented fireplace assembly according to the prior art. 
           [0017]      FIG. 2  is a side schematic view of a vented fireplace assembly according to one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]      FIG. 1  shows a style of prior art power vented fireplace over which the present invention is intended to be an improvement. Fireplace  100  comprises a combustion chamber  102  with a burner  103  and a transparent combustion chamber wall  104  to permit viewing inside the combustion chamber  102 . A second transparent wall  106  is spaced from the combustion chamber  102  and creates an air passageway  108  between the two walls  104 ,  106 . 
         [0019]    Air enters the fireplace  100  from the room through a cooling air inlet  110  and from the outside through a combustion air inlet  112 . A cooling air inlet fan  114  draws room air through the cooling air inlet  110  and into the air passageway  108 . The flow of the cooling air through the air passageway  108  cools the second transparent wall  106  to a safe temperature. The same arrangement may be provided on the opposite side of the fireplace to define a second air passageway  109 . 
         [0020]    Combustion products leave the combustion chamber  102  through a combustion outlet  120  then mix with cooling air leaving the air passageways  108 ,  109  in an air mixing area  122 . The mixed air then exits the fireplace  100  though an air outlet  124  into the exhaust venting system  125 . A power vent  126  draws the mixed air from the exhaust vent  125  to the outside of the building. 
         [0021]      FIG. 2  shows the preferred embodiment of the present invention. Single-intake vented fireplace  200  comprises a combustion chamber  202  with a burner  203  and a first transparent combustion chamber wall  204  to permit viewing inside the combustion chamber  202 . A second transparent wall  206  is spaced from the combustion chamber  202  so as to define an air passageway  208  between the transparent walls  204 ,  206 . The same arrangement is provided on the opposite side of the fireplace to define a second air passageway  209 , although that is not a necessary aspect of the invention. 
         [0022]    Air is drawn into the fireplace  200  through a single air inlet  212  by means of an intake fan  216 . The air drawn through the air inlet  212  is directed into a staging area  230  located upstream of the air passageways  208 ,  209 . The staging area  230  may be a chamber or a relatively contained passageway enabling the intake fan  216  to pressurize the staging area  230 . In the illustrated embodiment, staging area  230  is defined between exterior walls  215 ,  217  of the base of the fireplace, and walls  221 ,  223 ,  225 ,  227  and  229  of the base of the combustion chamber. The specific elements of the fireplace that make up the staging area  230  may vary provided that the staging area  230  acts to partially contain air drawn in by the intake fan  216  before flowing to the passageways  208 ,  209  and into the combustion chamber through the combustion chamber inlet(s)  218 ,  219 . 
         [0023]    Air flows from the staging area  230  into the passageways  208 ,  209  through passageway inlets  232 ,  233 . Each of passageway inlets  232 ,  233  has a smaller free area (the aggregate area of its openings) than the cross-sectional area of the respective passageways  208 ,  209  themselves such that the inlets act as partial restrictions to the passage of air into the passageways. 
         [0024]    The staging area  230  also includes one or more small combustion chamber inlets  218 ,  219 . The relatively small size of the inlets  218 ,  219  contributes to a controlled and stable flame. In the preferred embodiment, each of inlets  218 ,  219  consists of a row of 0.50″ diameter apertures spaced every 2″ of length of the burner. 
         [0025]    The staging area  230  is also defined at least in part by the partial restriction presented by the passageway inlets  232 ,  233  thereby inducing a higher pressure in the staging area  230  than in the combustion chamber  202  (and a higher pressure than in the passageways  208 ,  209 ). 
         [0026]    As a result of the restrictions  232 ,  233 , air is more reliably driven into the combustion chamber  202  than would be the case without the restrictions. Air from the staging area  230  also passes through the inlets  232 ,  233  and into the air passageways  208 ,  209 . The outer transparent walls are cooled to a safe temperature by the flow of this air through the air passageways. 
         [0027]    Air leaving the combustion chamber  202  through combustion outlet  220  mixes with air leaving the air passageway  208  in an air mixing area  222 . The mixed air then exits the single-intake vented fireplace  200  though an air outlet  224 . A power vent fan  226  draws the mixed air from an exhaust vent  225  and exhausts it to the outside of the building. 
         [0028]    In an embodiment, the passageway inlets  232 ,  233  comprise adjustable louvers. Upon installation and set-up of the fireplace, the installer may adjust the louvers to provide more or less back pressure in the staging area  230  to ensure an adequate amount of ingress of air through openings  218 ,  219  and into the combustion chamber. 
         [0029]    A first pressure differential switch  240  can be connected to the mixing area  222  or the air outlet  224  on the one hand, and a space  241  below the burner  203  in the combustion chamber  202  on the other hand. The first pressure differential switch  240  monitors to ensure that there is a higher air pressure in the combustion chamber  202  than in the air outlet  224 . If the air pressure in the combustion chamber  202  falls below the air pressure in the air outlet  224 , which can cause a reversal of the direction of the flames and burning of the fireplace components, the first pressure differential switch  240  triggers a shut off of the burner  203 . The first pressure differential switch indirectly accounts for the pressurizing effect of the power exhaust vent  226 . 
         [0030]    A second differential pressure switch  242  can be connected to the single air inlet  212  and the combustion chamber  202 . Any restriction or lack of air supply in the single air inlet  212  is monitored (indirectly) by the second differential pressure switch  242 . A restriction or lack of air supply in the single air inlet  212  may cause the air pressure in the air inlet  212  to fall below the air pressure in the combustion chamber  202 . If this occurs, the second pressure differential switch  242  will not permit the burner  203  to ignite or to continue operation. 
         [0031]    A comparison of the air flow at the air inlet  242  and the outlet  224  allows the balancing of the total air inflow and outflow of the fireplace. In one embodiment, the fireplace is not allowed to ignite until the two air flows are within a predetermined range of one another. 
         [0032]    In the foregoing specification, the invention has been described with reference to specific embodiments thereof. However, the scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.
A power vented fireplace uses a single air inlet to pressurize a staging area from where air is distributed along a passageway between viewing panels to cool them as well as into the combustion chamber as a supply of combustion air. A partial restriction at an inlet to the passageway ensures that sufficient air is directed into the combustion chamber. The inlet to passageway may be adjustable to vary the extent of the restriction. Pressure differential switches may be used to ensure balanced operation of the fireplace.