Gas-fired heater
FIELD OF THE INVENTION 
     This invention relates to a gas-fired heater, and more particularly is concerned with a gas-fired baseboard heater having a similar profile to conventional electric baseboard heaters. 
     BACKGROUND 
     Electric baseboard heaters are well known. Conventionally, electric baseboard heaters are elongate and have a low profile. Usually, they have an elongate finned heating element and a simple control switch, optionally with a thermostat, at one end. Elongate top and bottom openings are provided in an outer housing, to enable natural convection to transfer the heat to the room air. 
     With the introduction of flexible gas distribution systems for buildings, opportunities for new applications have developed. One such opportunity involves using a gas-fired baseboard heater in applications where electric baseboard heaters are currently used. The attractiveness of using gas as an original or a replacement source of energy has increased due to the large increases in electricity rates compared to the more steady gas prices. 
     There are known designs for gas-fired baseboard heaters, but these are large and bulky, often resembling room heaters more than baseboard heaters. 
     Known gas baseboard heaters currently use a natural draft sealed combustion system with a standing pilot and a conventional gas control valve. Based on conventional gas technology, the flame is oriented vertically and the heat exchanger is relatively bulky. As a result, the whole device is bulky and unattractive and occupies a lot of space. Such existing baseboard heaters use multi-port burners, which again increases the bulk of the device. 
     Heat is readily transferred from the heat exchanger to the surrounding air by natural convection. Cooler air at the floor level enters via the lower grille and is heated as it rises past the heat exchanger and exits from a top mounted discharge grille. Most heaters of this type are equipped with a local thermostat mounted on the heater itself, to regulate the operation of the heater. 
     SUMMARY OF THE PRESENT INVENTION 
     In accordance with the present invention, there is provided a gas-fired heater, for mounting on a wall within a building, the heater comprising: a gas inlet for combustion gas; a combustion air inlet; a burner connected to the combustion and gas inlets; and an elongate U-shaped heat exchanger including a generally horizontal lower section into which the burner discharges combustion gases, and a generally horizontal upper section connected to the lower section and having an outlet for exhaust combustion gases. The gas-fired heating device of the present invention has a low profile and is intended for installation along a section of the wall in a room. For this purpose, it preferably includes a casing having upper and lower grilles for natural convection. One embodiment of the invention uses a natural draft-sealed combustion system with either a standing pilot or an intermittent pilot and a gas control valve. Another embodiment of the invention uses a power-vented sealed combustion system with either hot surface ignition or spark ignition and a gas control valve. The power-vented version preferably includes additional control features such as a pressure switch-based proof of flow, and a safety lockout timing device. A gas control valve and an appropriately sized gas orifice regulate the desired flow of gas. Preferably, it includes a horizontally-directed mono-port burner. 
     During operation, air is drawn in from the outdoors and into the combustion air chamber where the gas and air are mixed, and are then passed through the mono-port burner and ignited. The resulting products of combustion are passed through the U-shaped heat exchanger and are preferably exhausted to the outside, by a tube concentrically positioned inside the combustion air passage. The tubular heat exchanger advantageously is made up of two major sections; a lower section which is connected to the combustion air chamber and an upper section which is connected to the exhaust tube. The two sections are joined together using elbows or other transition components. 
     The diameter of the heat exchanger may be the same for both the bottom and top sections, or the bottom section may be of a larger diameter than the top section. Finned tubing is preferably in the power-vented version, for enhancement of heat transfer from the hot combustion gases to the surrounding air. In known manner, the finned tubing aids in heat transfer by effectively increasing the heat transfer surface area. 
     In the power-vented version, a blower is used to draw combustion air and push it through the U-shaped heat exchanger and out the exhaust tube. A smaller diameter tubing can be used for the heat exchanger than in the natural draft version of this baseboard heater. Additionally, in the power-vented version, heat transfer is enhanced in the upper section by using an in-line turbulator. The turbulator increases turbulence in the heat exchanger by increasing the internal flow velocity, which in turn increases the heat transfer rate to the wall of the heat exchanger and then to the surrounding air. Heat is readily transferred to the surrounding air by natural convection. The cooler air enters the casing via the lower grille and is heated as it rises past the heat exchanger and exists from a top mounted discharge grille. 
     To distribute the heat more evenly in the lower section, thin a layer of thin insulation material is placed in the mouth of the lower section of the heat exchanger. This permits more heat to be transmitted at the other end of the lower section and reduces the wall temperature at the burner end of the heat exchanger. 
     For the power-vented version, advantageously a differential pressure switch is used to ensure that an adequate flow of combustion air is present before the ignition controls are made operative. In the event of a blower malfunction, the pressure switch would sense that there is inadequate combustion air and would de-energize the gas supply mechanism. Another safety feature that is advantageously employed in the heater is a lockout timing mechanism. This feature, once again, would de-energize the gas valve, thus stopping the flow of gas to the burner, in the event of the burner flame being extinguished due to a temporary loss of gas supply. To re-activate the ignition control system, a manual reset of the thermostat would be required. 
     The thermostat for this heater may be located within the casing of the heater or it may be located remotely on a wall in the room. This latter feature permits better climate control of the room being heated. 
     In the drawings as hereinafter described, an embodiment of the power-vented version of the baseboard heater is described. Also described is an embodiment of a naturally vented version of the baseboard heater. However, various other modifications and alternate construction, including a variation in the length of the heat exchanger can be made without departing from the true scope and intent of the invention. 
     A typical power-vented baseboard heater can be enclosed in a casing having the dimensions of about 19 cm high, 13 cm deep and 122 cm long (71/2×5×48 inches) with a firing rate of about 1.5 kW (5,000 BTU/h). 
     The firing rate of the baseboard heater is about 0.37 kW (1,250 BTU/h) per linear foot of appliance length. The typical embodiment uses 4 linear feet for a total input of about 1.5 kW (5,000 BTU/h). 
    
    
     DESCRIPTION OF THE DRAWING FIGURES 
     For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made by way of example, to the accompanying drawings, in which: 
     FIG. 1 is a perspective view of a power-vented embodiment of a baseboard heater in accordance with the present invention; 
     FIG. 2 is a schematic diagram of the control circuit used in the baseboard heater of FIG. 1; 
     FIG. 3 is a vertical section through the internal components of the baseboard heater of FIG. 1, with a casing removed; 
     FIG. 4 is a plan view of the baseboard heater components of FIG. 3; 
     FIG. 5 is an end view of the baseboard heater components shown in FIGS. 3 and 4; and 
     FIG. 6 is a perspective view of a typical naturally-vented embodiment of the baseboard heater in its casing. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The baseboard heater components are enclosed in a casing 1, which is generally rectangular. While a variety of dimensions can be chosen, the casing 1 has the dimensions 19×13×122 cm (71/2×5×48 inches). The casing 1 comprises a separate front panel 2 which can be removed, a lower grille 3 which permits the cooler air at the floor level to enter the casing, and a top mounted discharge grille 4 which allows for the heated air to rise into the heating space. 
     Referring to FIG. 2, the control circuit for the heater has input lines 40 and a ground connection 41, which in known manner would be connected to a conventional domestic 120 volt AC supply. A probe type temperature sensor 6 and coil 7 of a relay A are connected in series between the two lines. A combustion air blower 9 is also connected between the lines. In the top line, a thermostat 5 and a differential pressure switch 10 are connected in series. The switch 10 is in turn connected to a gas valve 12. The valve 12 includes a holding coil 12a, a secondary coil 12b and a booster coil 12c. The valve 12 in turn is connected to contact 8 of the relay A and to contacts 14 of a second relay B having an energizing coil 13. The contact 8 can be switched between contacts 8a and 8b, while further contacts 8&#39; are either in an open or closed position; with relay A powered, the contact 8 connects to 8a, and contacts 8&#39; are closed. Contact 14 is normally open, and is closed when relay B is activated. An igniter 11 is connected in series with a time-delay switch 23 which in turn is connected to the contact 8 a, as is a connection to the gas valve 12. The switch or contacts 14 and the contact 8b are connected to the further pair of contacts 8&#39;. The contact 8b and output of contacts 14 are also connected to the coil 13 of relay B, and the output of contacts 8&#39; is connected to a time-delay heater 22. The heater 22 and time-delay switch 23 with related components form a lockout-timing mechanism 21. 
     Turning to FIGS. 3, 4 and 5, and details of the mechanical components, at the right hand end of the unit, there is a housing 42 including an inlet chamber 43. An inlet pipe 15 opens into this, as best shown in FIG. 4. FIG. 4 also shows schematically the lockout-timing mechanism 21. The fan or blower 9 includes the actual fan element at 9a and a fan motor 9b. As shown by the arrow 44, the fan or blower 9 draws combustion air through the inlet 15 and forces it into a combustion chamber 45. A differential pressure switch is mounted at 10, and the gas valve 12 is mounted below it. Relays A and B are shown at the bottom of the housing 42. 
     The gas valve 12 includes an inlet 12d, for gas, which in known manner, would be connected to a gas supply, and has an outlet connected to a flow-controlling orifice 25. This in turn discharges into a burner tube 26, with the orifice 25 and the burner 26 being configured in known manner. 
     At the outlet of the burner tube 26, there is mounted the igniter 11 and also the temperature sensor 6, as shown. The arrow 46 indicates the flow of combustion gases from the burner tube 26. 
     The combustion zone 45 is defined within the first part of a lower tubular heat exchanger section 16. The combustion chamber 45 is lined with flexible insulation 17, to reduce the wall temperature, and hence, ensure more uniform heat transfer from the tubular heat exchanger section 16. 
     As shown in FIG. 3, at the left hand end of the heat exchanger 16, there is an elbow 18 connecting the lower section to an upper finned tubular heat exchanger section 19. A tubulator 24 is located within the finned tubular heat exchanger 19, both to create turbulence and to increase internal flow velocity, thereby to promote heat transfer from the hot combustion gases to the body of the upper section 19. 
     At its end, the heat exchanger section 19 turns through 90° and is connected to an exhaust tube 20. The tube 20 is coaxial within the combustion air in the inlet pipe 15, the combustion gases flowing counterflow to the incoming air. This effects further heat transfer and promotes overall thermal efficiency. Residual heat in the combustion gases is transferred to the incoming combustion air. 
     In use, with the baseboard heater connected to line voltage, connected to a domestic gas supply and in standby mode (heat not called for), the thermostat 5 is in the open position, the probe-type temperature sensor 6 is in the closed position and the coil 7 of relay A is energized. With the above relay coil 7 energized, the two contacts 8, 8&#39;, for the relay are in the swing left position, i.e. as shown in FIG. 3 with contact 8a closed and contacts 8&#39; closed. At this point some current will flow through the holding coil 12a, but this will be insufficient to open the valve 12. 
     On a call for heat, the following sequence would occur: the room thermostat switch 5 closes thereby energizing the combustion air blower 9. When the blower 9 is at its operating speed, the differential pressure switch 10 senses that the difference in pressure between the intake and discharge sides of the blower 9 is at or above a preset value, indicating an adequate supply of combustion air, and it closes. When the pressure switch 10 closes, the igniter 11 is energized and begins to heat up. The temperature sensor 6, due to its placement in close proximity to the igniter 11, also heats up. When the temperature sensor 6 heats up to its critical temperature, it opens and de-energizes the coil 7 of relay A which causes the contacts of relay A to swing to the right, i.e. contact 8 closes at 8b and contacts 8&#39; open, and de-energize the igniter 11. The gas valve 12 is thus energized, and current flows through the secondary and booster coils 12b, c. This opens starts to flow, which gas is ignited by the igniter 11 while it is still hot. Simultaneously, since the contact 8b of relay A is closed, the coil 13 of relay B is energized and the contacts 14 of relay B closes. The temperature switch 6 is kept open by maintaining it above its critical temperature by sensing heat generated from the flame. 
     The baseboard heater operates until the thermostat 5 is satisfied that the required temperature has been reached, at which point it opens and de-energizes the blower 9 and the gas valve 12. The temperature switch 6 cools down and closes. The system is once again in the standby mode, shown in FIG. 2. 
     If, during normal operation, there is a loss of flame, for example, due to a loss of gas pressure, the lockout timing mechanism 21 would operate to stop the flow of gas to the heater within a preset time, here 60 seconds. Upon a loss of flame, the thermostat 5 remains closed since it is not yet satisfied. The pressure switch 10 also remains closed since the blower 9 continues to operate, which in turn ensures that contacts 14 of relay B remains closed and the coil 13 of relay B is still energized. When the temperature sensor 6 cools down, due to the loss of heat from the flame, to its preset temperature, it closes. This energizes the coil 7 of relay A; its contacts 8 swing to the left and close 8a and contacts 8&#39; close. Since the contacts 14 of relay B are closed, the coil 13 for relay B remains energized. The igniter 11 and the time delay heater 22 are then energized. The igniter 11 heats up, thereby heating the temperature switch 6 and at its preset temperature, the switch 6 opens. Again, simultaneous coils 12b  and c are energised and the gas valve 12 opens to permit the flow of gas. Coil 7 of relay A is de-energized and thus its contacts swing to the right. 
     If the gas pressure has been restored and there is a flow of gas to the heater, then ignition will take place as described earlier and the heater will continue to operate in its normal manner until the thermostat 5 has been satisfied. If ignition does not take place, the temperature switch 6 will again cool down and close and a second trial for ignition will take place. 
     Consider further the initial trial for ignition on the loss of flame. While the relay A is energized, the time delay heater 22 is also energized. During this time, the time delay heater 22 begins to heat up the thermally operated time delay switch 23. The time delay switch 23 is selected to reach its critical temperature after a pre-selected time, here 60 seconds. When this time is reached it opens to close gas valve 12b and hence it allows no more than 60 seconds of gas flow after loss of flame. 
     When further trials for ignition occur, as the temperature switch 6 opens and the coil 7 of relay A is de-energized, the contacts 8&#39; of relay A are opened, to de-energize the time delay heater 22. The time delay switch 23 then slowly cools down. However, the time delay switch 23 does not cool down to its original temperature before the ignition sequence repeats itself and the time delay switch 23 begins to heat up again. In this manner, after a few trials for ignition, the time delay switch 23 reaches its preset temperature and opens, thereby preventing the igniter 11 from being energized any further. The time delay switch 23 must then be manually (from the thermostat 5) reset before another trial for ignition can take place. 
     The heater is designed to operate at a firing rate of approximately 1.5 kW (5,000 BTU/h). The firing rate is controlled by the combination regulator and control gas valve 12 and the flow controlling orifice 25. 
     While an embodiment of the power-vented version of this heater having a firing rate of 1.5 kW (5,000 BTU/h) has been described, the heater may be operated at alternate firing rates. The heat exchanger components (comprising of 16, 18 and 19) may be of selected sizes to permit higher inputs by using a firing rate of about 0.37 kW (1,250 BTU/h) per linear foot. 
     The natural draft version of this invention operates in much the same way as the power-vented version, and is shown in FIG. 6. Here, a standing pilot 26 is lit using a piezo-electric igniter 27, in known manner. When there is a call for heat, the gas valve 28 opens to permit gas to flow from its inlet 28a to the single port burner 29. Combustion air and gas mix and are ignited at the mouth of the heat exchanger 30 by the standing pilot 26. The single port burner 29 provides the necessary momentum to move the combustion products through the U-shaped heat exchanger 30 and vent them to the outdoors. When the local thermostat 31 is satisfied, it deactivates the gas valve 28 and the flow of gas is stopped. If during normal operation, there is a flame outage, such as due to a loss of gas pressure, the outage would be sensed by a thermocouple 32 and the gas valve 28 would be de-energized. Upon restoration of gas pressure, the pilot 26 would have to be manually relit using the piezo-electric spark igniter 27. The configuration of the heat exchanger could be generally similar to that in the first embodiment with a similar exhaust outlet or tube. However, as it relies on natural draft, the interior of the duct would have generally larger dimensions, and the turbulator 24 would be omitted. As shown at 50, a rectangular intake duct provides ample space for incoming air while reducing flow resistance. This surrounds the exhaust or outlet duct 20. 
     It will be appreciated that while two preferred embodiments have been described, numerous variations are possible. For example, the heat exchanger could have a clamshell-type of construction. Such construction includes two sheets configured to be joined together along their edges. The sheets are then shaped to give a desired internal configuration. Here, the sheets would be configured to give the upper and lower heat exchanger sections, and would be generally symmetrical about a central plane.