CA2009379A1 - Gas heating unit and electrical control circuit therefor - Google Patents

Abstract

ABSTRACT
Gas Heating Unit and Electrical Control Circuit therefor A combustible gas heating system for an accommodation unit, comprising a room sealed heating unit incorporating a combustion box which houses a gas burner and means for conveying combustible gas to the burner, an inlet duct for conveying air into the combustion box, and an outlet duct for conveying products of combustion and excess air from the combustion box for discharge through the floor of the accommodation unit, and a fan operative to produce a forced flow of air through the system from the inlet to the outlet duct.
Also, a control circuit has a storage capacitor for instantaneous gas valve closure when electric power is interrupted.

Description

200937~

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Gas Heating Unit and Electrical Control Circuit therefor One invention relates to combustible gas heating systems, in which flue discharge takes place through the floor; it is especially useful in such heating systems for sealed accommodation units such as caravans. A related invention concerns an electrical ~ ;
control circu ~ h a system.
In conventional caravans space tends to be very l~mited, and the heatlng units have commonly been installed beneath or at the ~ - -bottom of cupboards. However the presence of a flue pipe extending upwards within the cupboard from a combustion box housing the gas burner, in order to convey combustion products through the roof of the caravan, is inconvenient as it takes up valuable storage space w~thin the cupboard.
The use of a simple form of flue system arranged to direct the products of combustion through the floor of the caravan is ~;
already known. However, the heat ~nput for these designs is - --restricted due to the problem of directing naturally rising hot products of combustion in the opposlte direction and well below the gas burner level. Accordingly an ob~ect of the present invention is to provide an alternative combustible gas heating system sultable for caravan use which avoids this difficulty.
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' ~-,' Accord~ng to the first invention, a combustible gas heating system for an accommodation unit comprises a room sealed heating unit incorporating a combustion box which houses a gas burner and means for conveying combustible gas to the burner, an inlet duct for conveying air into the combustion box, and an outlet duct for conveying products of combustion and excess air from the combustion box for discharge through the floor of the accommodation unit, and a fan operative to produce a forced flow of air through the system from the inlet to the outlet duct.
The fan may be located in elther the inlet or outlet duct, as may be the more convenient.
The inlet duct preferably also extends through the floor of the accommodation unit so that the system neither withdraws air from, nor discharges flue products into, the unit. Conveniently the fan is electrically operated and 1s energised automatically when the burner is ignlted.
The heating unit may be a space heater or a water heater, and in some cases the terminal part of the outlet duct system may be common to two such heaters. Preferably each heater is associated with its own fan appropriately located so as to produce a forced flow of air through the duct system of the respective heater when the heater is in use.
Conventional flame detection systems use a thermocouple for detecting whether the gas burner is alight, and a gas control valve arranged to admit gas to the burner only if a voltage level signal from the thermocouple is such as to indicate the burner is alight.
Where such a system is used in a gas heater which relies for its operation on an electric fan associated with the flue, and/or on a spark stream electric ignition, it is important that the burner is - -shut off very soon after the electrical system is turned off or after it fails in a power cut. The problem is that there is an inherent delay while the thermocouple cools sufficiently to cause its output voltage level to cross the threshold recognised by the gas control valve. Accord~ngly, the purpose of the second invention is to find -a simple way of overcoming this problem.
According to the second invention, an electrical control ~ -.. ~ . . . . ... .... . . .. .

circuit for a combustible gas heating system having a gas supply control valve for admitting the gas to a burner, an electrical ignition system for the burner, and a flame detection sensor for sending an electrical signal to the control valve to admit gas only when the burner is alight, comprises a switch for supplying power from a source to the ignitlon system, a storage capacitor arranged to charge from the power source through the switch, and means responsive to an interruption of power from the switch to connect the capacitor to the flame detection sensor to cause immediate cancellation of the electrical control signal whereby to cause the control valve to close.
Three combustible gas heating systems for a caravan, embodying the first invent10n, and a control system embodying the -second invention, will now be described by way of example with reference to the accompanying drawings, in which Figures 1 and 2 represent diagrammatically a sectional front view and a sectional s1de view of a space heater;
Figure 3 represents a sectional front view of a water heater;
Figure 4 is a sectional front view of the flue system of a space heater otherwise similar to Figure l; and Figure 5 is a circuit diagram of part of an electr1cal -control circult for the heating system of the heaters of Figures 1 or 4.
The heater illustrated in Figures 1 and 2 comprises a combustion box/heat exchanger 4 mounted on a caravan floor 3, and accommodating a gas burner 10.
All co~bustion airienters a swivelling elbow 1 at 2 and~
discharges into the combustion box/heat exchanger 4 via an a~r ~-distribution grill 9, mixing with combustible gas from the gas : -burner 10 and being deflected by a baffle 15 having a central flue slot 14. Excess air with the gaseous combustion products passes through a flue opening 16 and then down through a flue discharge duct 7 to a vent 13 below the caravan floor 3. These gases are diluted by a by-pass air stream 8.
The elbow 1 is positioned in use to avoid the caravan axle, ,'~,.".".,''",.'"'"',',,"" ,,, :~ ,..,',,"',,,; ',~

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which would lie just below the horizontal portion of the elbow 1.
Around the heat exchanger 4 there is located a heat shield 5, the whole being surrounded by an outside casing 6 sealed to the floor by a plinth 17. The gas is controlled by a tap 11. The usual flame failure protection devices are fitted and ignitlon is by a stream spark generator (12v d.c.).
The inlet duct, constituted by the swivelling elbow 1, accommodates at its upper end an electric fan 12, which assists the expulsion of the combustion products from the combustion box by producing a forced flow of air through the box and to the atmosphere via the outlet duct 7, the fan being automatlcally energised when the burner is lgnited by means of any conven~ent form of switch arrangement. One such arrangement is described below with reference to Figure 5.
The water heater illustrated in Figure 3 similarly comprlses a combustlon boxlheat exchanger 4 mounted on the caravan floor as in the case of the previous embodiment, combustion air ~ ;
similarly entering the box through a swivelling elbow 1 having an -- `
inlet at 2. The air then passes through an air deflector grill 9 as well as through a by-pass exit 8 to dilute the flue gases. The air mixes w~th the combustion gases from the gas burner 10 through the~ ~
heat exchanger 4 vla a deflector flue collection hood 25 and then ~ -downwards through a flue duct 7 to exhaust through the caravan -~
floor 3 at flue terminal 13.
The heat exchanger is room sealed within an outer casing 6 fixed to the floor plinth 17.
The gas burner 10 is controlled by a solenold valve 11 in -~
conjunct~on with a water flow inlet 18. The usual flame safety devices are employed, plus a high limit self re-setting thermostat and a safety cut-off thermostat. These are conveniently as shown in ~ -co-pending patent application No. 8803047 as 9, k and l in Figure 1, except in this case the flame failure device is a flame rectification PCB and the pilot burner is eliminated in favour of a stream spark generator. The hot water flow is controlled by a pre-set m~cro-switched tap 19.
An electric fan 12 is located within the flue duct 7 so as to draw air by suction through the combustlon box heat exchanger 4 and to expel it through the outlet 13. In th~s case also the fan is energised automatically when the burner is ignited.
In each embodiment the forced flow of air through the system ensures that the presence of pockets of stagnant combustion products within the combustion box is effectively avo~ded.
Where both a space heater and a water heater are located in close proximity to one another the heaters may share a common outlet duct, and a fan may be located within this duct, although a separate fan may be located within the inlet duct to each of the two heaters, or within a respective outlet duct connecting the individual heaters to the common outlet duct.
The space heater whose flue arrangement is shown in Figure 4 is otherw~se similar to Figures 1 and 2. In place of the swivelling elbow 1 is a fixed, generally rectangular section inlet flue box 41 which is flanged to fit into a larger gap in the caravan -~
floor 3. The box 41 has a term~nal 2 at the end of a cubold downward pro~ection 42 at one end of the box; the box 41 is fitted with the pro~ection 42 at either end, so as to avoid the axle. The outlet flue 7 enters a similarly-flanged square box 43 hav1ng a terminal 13.
There is a three-pass baffling system within the combustion box/heat exchanger 4, and, as before, a by-pass flow of air 8 dilutes and cools the outwardly-flowing gases 7.
With reference to Figure 5, the control circuit supplies electrlc power from a source to the spark stream ignition and to the -electric fan 12, and responds to a voltage level signal from a thermocouple ad~acent the gas burner, and also to a fan proving switch S1 driven by air from the fan 12. The circu1t ensures that the gas supply to the burner is interrupted if the air supply to the burner ~s restricted, whether due to electrical power failure or to flue blockage.
An on/off sw~tch SO is manually operable, preferably by being assoclated w~th the spindle of the gas control tap. When closed, this supplies power to a relay Kl by way of the fan proving sw~tch S1, which always returns to its Nc position in the absence of alr flow from the fan. The relay then closes fan switch S2 to power the fan 12, and to latch on the relay, and moves switch S3 to its No position to cause a storage capacitor to charge through a diode Dl.
Provided the fan functions and the flue is not blocked, the air flow will activate the fan proving switch Sl, moving it to its No position and thus powering a conventional gas ignition/flame detection system (not shown) vla switch S2. Once the burner is alight, the thermocouple heats up and generates sufficient e.m.f.
(voltage) to hold open a gas valve located in the gas tap.
When the on/off sw1tch 50 is opened, normally to switch off the appliance, the relay causes switches S2 and S3 to change state, turning off the fan 12 and discharging the capacitor through the thermocouple via a second diode D2. This current surge through the thermocouple causes the gas valve to close instantly, much more quickly than if it relied on the cooling of the thermocouple to cause lts e.m.f. to drop below the threshold. This prevents any gas ;-reaching the burner after the appliance has been switched off, or if there is an electrlc~l pr,wer failure.

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Claims (9)

1. A combustible gas heating system for an accommodation unit, comprising a room sealed heating unit incorporating a combustion box which houses a gas burner and means for conveying combustible gas to the burner, an inlet duct for conveying air into the combustion box, and an outlet duct for conveying products of combustion and excess air from the combustion box for discharge through the floor of the accommodation unit, and a fan operative to produce a forced flow of air through the system from the inlet to the outlet duct.

2. A system according to Claim 1, in which the inlet duct also extends through the floor of the accommodation unit so that the system neither withdraws air from, nor discharges flue products into, the unit.

3. A system according to Claim 1, in which the fan is electrically operated and is energised automatically when the burner is ignited.

4. A system according to Claim 2, in which the fan is electrically operated and is energised automatically when the burner is ignited.

5. An electrical control circuit for a combustible gas heating system having a gas supply control valve for admitting the gas to a burner, an electrical ignition system for the burner, and a flame detection sensor for sending an electrical signal to the control valve to admit gas only when the burner is alight, the control circuit comprising a switch for supplying power from a source to the ignition system, a storage capacitor arranged to charge from the power source through the switch, and means responsive to an interruption of power from the switch to connect the capacitor to the flame detection sensor to cause immediate cancellation of the electrical control signal whereby to cause the control value to close.

6. A circuit according to Claim 4, in which the flame detection sensor comprises a thermocouple and the electrical signal is a voltage level signal.

7. A system according to any of Claims 1 to 4 controlled by a circuit according to Claim 4.

8. A system according to any one of Claims 1 to 4 controlled by a circuit according to Claim 5.

9. A sealed accommodation unit fitted with a system according to any one of Claims 1 to 5 the outlet duct extending through the floor of the accommodation unit.