GB1563296A

GB1563296A - Electrical controls for heating appliances

Info

Publication number: GB1563296A
Application number: GB1499876A
Authority: GB
Original assignee: United Gas Industries Ltd
Current assignee: United Gas Industries Ltd
Priority date: 1976-04-13
Filing date: 1976-04-13
Publication date: 1980-03-26

Description

(54) ELECTRICAL CONTROLS FOR HEATING APPLIANCES (71) We, UNITED GAS INDUSTRIES LIMITED of 3-4 Bentinck Street, London, W1M 6DH, a British company, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: - This invention relates to electrical controls for heating appliances, particularly to controls providing safety features.

The invention provides an electrical control circuit for a gas or oil-fired heating appliance of the kind having a fan for supplying combustion air, an electricallyoperated fuel valve and an ignition device, said control circuit comprising a plurality of sub-circuits each adapted to carry out or monitor part of a normal start-up sequence comprising a timed purge period followed by a timed ignition period followed either by normal operation or by a close-down operation if a flame is not detected, each said sub-circuit having a relay connected to operate switch means for effecting the appropriate part of the start-up sequence, said fan being connected to be energized on start-up independently of any of said relay-operated switch means, one of said sub-circuits including a flamesensing device which is also connected to be energized on start-up independently of any of said relay-operated switch means, said one flame-sensing sub-circuit having its relay connected to open a first switch means and prevent the start-up sequence from being followed when a flame is detected at start-up.

The invention also extends to a gas or oil-fired appliance having a control circuit as set forth above.

Specific embodiments of the invention are shown in the drawings accompanying Provisional Specification No. 14998/76 in which: - Figure 1 is a first control circuit diagram, Figure 2 is a second control circuit diagram, and in the drawing accompanying Provisional Specification No. 50250/76 in which: - Figure 3 is another control circuit.

Referring first to Figure 1, the control circuit shown controls a gas-fired heater of the kind having a fan supplying air for combustion and a fan driving convection air through a heat exchanger. This control circuit operates on starting to run the two fans for a short period before a gas valve is opened and an ignition device operation, so purging the system of any potentially explosive mixture. It also operates to close down the system if after another period after the ignition device is operated a flame is not sensed.

When control switches 5 (comprising an on/off switch and a thermostat switch) call for heat, the fans are energised on line 11 and a pre-purge sub-circuit 12 commences timing.

After a delay (20 seconds) the pre-purge sub-circuit 12 operates a switch RL1 which allows the ignition timer sub-circuit 13 to start timing. The ignition timer then closes a switch RL2 which allows current to pass both to the power supply 14 for the gas valve (not shown) and spark generator 15.

Flame detector sub-circuit 16 at this point should not be detecting a flame.

If it does (i.e. a flame simulating fault) it opens a switch RL3-1 across a resistor R18 which effectively puts the resistor in series with the valve power supply 14.

This resistor does not allow sufficient current to pass to energise the valve. Another switch RL3-2 operated by the flame detector severs the power supply to the spark generator 15. Therefore the heater will not start.

Should no flame be detected the flame detector shorts out the resistor R18 in the valve line and completes the power supply to the spark generator. Therefore the gas valve opens and sparks are generated to ignite the gas. When a flame is detected by the flame probe 17 the sparks are turned Off, through resistor RL3 and the resistor switch RL3-1 is opened. Sufficient current can flow through this resistor to hold the valve in after it has been operated although not enough to cause operation thereof.

Should the gas fail to ignite within a pre-determined time (2-4 seconds) the ignition timer sub-circuit 13 will change state, cutting the power supply to the gas valve and spark generator via switch RL2-1, and putting the control into a "locked out", inoperative state.

Once locked out, the control will not allow an attempt at ignition until the ignition timer has been reset by a manual interruption and restoration of the supply.

After such an interruption one attempt at ignition only will take place.

Should any section of the control fail, the control will keep the heater in a safe condition.

Referring now to Figure 2, the heater control circuit shown is intended for the same kind of heating appliance as that of Figure 1, and has two fans 20, 21 the controls of which are connected in parallel. When main switches 5 (which may comprise an on/off switch, a clockcontrolled switch and/or a thermostat switch) demand that the heater start, both fans commence operation. A flame detector sub-circuit 23, similar in principle of operation to that of Figure 1, is energised all the time the heater is on. The flame detector sub-circuit controls a relay RL3, which relay operates a switch RL3-1 which shunts a resistance R17 in series with a gas valve V. As described in relation to Figure 1, the valve can be opened only when the relay is shunted, but can be held open when the resistor is in circuit.

The flame detector sub-circuit has a flame sensor 24 which operates a first transistor T1, thus allowing sufficient base current for a transistor 2T to switch on.

The voltage present across a resistance R14 is amplified by a transistor T3 which allows sufficient current through the relay RL3 to operate it and its switches.

All the transistors act as switches since it is the d.c. level across a capacitor C7 which initiates T1. But because the filter formed by R10, R11 and C7 is not perfect an a.c. component is also present at the output. Therefore C8 is provided to smooth this ripple for the relay RL3.

On start-up relay switch RL2-1 is open, so no power is available to the valve V or to a spark ignition sub-circuit 25. A pre-purge timing sub-circuit (C1, C2, RL1) after a pre-purge interval of, for instance, 5 seconds, operates relay R4 and through it closes the switch RL1. Provided no flame is sensed and therefore switch RL3-2 is closed, an ignition timing sub-circuit 26 is then energized. This circuit uses the reaction of a capacitor to a sudden change in potential. When power is suddenly supplied, the impedance of capacitor C3 is effectively of a low value. Thus sufficient current passes to pull in a relay RL2 and so operate switches RL2-1 and RL2-2.

Power is therefore available to the gas valve and ignition sub-circuit and the valve opens and ignition sparks occur.

As capacitor C3 charges the current holding in relay RL2 reduces until after a predetermined time the relay drops out, and if no flame is yet present power to the valve and ignition sub-circuit is cut off, and the system is "locked out ".

Should a flame be detected during this time relay RL3 via switch RL3-3 brings into circuit a resistor R3 to by-pass capa citor C3. This supplies enough current to hold the relay RL2 in. A loss of flame allows capacitor C3 to charge again, and provides one ignition attempt period.

A manual switch R5 in circuit 26 is used as a lock-out Reset, and is combined in the Control knob. This may be returned manually to the off position to reset the timer. Alternatively the lock-out may be reset by turning the power off for 10 minutes (without using R5).

Using the circuitry described, a control system may be made such that any fault in any one sub-section will not render the appliance hazardous.

The control circuit of Figure 3 is for a gas-fired heater of the kind having a fan 31, supplying air for combustion and a convection fan 38 driving air through a heat exchanger. This control circuit oper ates on starting to run the combustion fan only for a short period before a gas valve is opened and an ignition device operated, so that the system is first purged of any potentially explosive mixture. It also operates to close down the system if, after another period after the ignition device is operated, the gas has not ignited.

In the drawing switches which are normally open, i.e., open on start-up, are marked NO while switches which are normally closed are marked NC.

When control switches 42 call for heat, the combustion air fan 31 and a transformer 32 are energized. The low-volt side of the transformer then energizes a prepurge timing sub-circuit 33. The timing sub-circuit 33 includes an integrated circuit 36 and operates a relay RL1 after a predetermined time delay e.g. of a few seconds. The delay period may be adjusted by means of an adjustable rheostat 35.

The output of the transformer is also applied via a normally-closed switch RL2-1 and a resistance R10 to an ignition timing sub-circuit 37 which includes a two-position switch RLl. A capacitor C9 in sub-circuit 37 is thereby initially charged. Switch RL1 is controlled by the relay RLl in pre-purge timing sub-circuit 33 so that at the end of the purge period switch RL1 disconnects from resistance R10 and puts a relay RL3 into a small circuit with C9. Relay RL3 is pulled in by the charge on C9 and two switches RL3-1 and RL3-2 are operated.

One switch RL3-1 makes circuits operating the convection fan 38, a gas valve 39 and, via a second switch RL2-2, a spark ignition sub-circuit 40. The other, normally open, switch RL3-2 provides a by-pass to the first switch RL2-1 for the sub-circuit 37 as described hereinafter.

A flame failure sub-circuit 34 is energized from the main so that it is operative at all times, and has a flame probe 41 located to detect when a flame has been established. Capacitor C5 and resistor R16 guard against the probe becoming dangerous if touched. The sub-circuit comprises a three-stage amplifier causing the low energy output from the probe to operate a relay RL2. This relay operates four switches RL2, two of which have already been mentioned, and the third RL2-3 of which when open places a resistance R12 in series with the gas valve.

When a flame is sensed the first switch RL2-1 opens, the second switch RL2-2 also opens so open-circuiting the spark ignition sub-circuit 40, and the third switch RL2-3 opens to place resistance R12 in series with the gas valve. In this condition, if the gas valve is open it remains so but if it is not open, resistance R12 is too large to allow sufficient energy supply to open the valve.

If therefore on starting a flame is present or there is a flame simulating fault, the switches RL2 are all open and the ignition timer does not operate, the spark ignition does not operate and the gas valve does not open. Moreover relay RL3 is not energized.

If no flame is present on starting the three switches RL2 described above are closed, allowing the ignition sequence described above to take place, until a flame is established. When this happens the three switches RL2 described above open but a fourth, normally open switch RL2-4 closes, providing power to sub-circuit 37. When the sub-circuit 37 is re-energised relay RL3 is held in. The gas valve and convection fan 38 therefore remain energized until RL3 drops out when the appliance is switched off.

If after the period chosen for the ignition timer sub-circuit 37 no flame has been established, the capacitor C9, which is discharging through relay RL3 allows the relay RL3 to drop out. This operates the RL3-1 switch which stops the supply to the convection fan 38, the ignition subcircuit 40 and the gas valve. The appliance is therefore effectively made safe.

This circuit has been designed particularly for operation under adverse conditions e.g. with variable mains supply, and for different wiring systems. For example capacitor C12 enables the circuit to be safely connected to a 2-wire (no separate earth wire) system. The operation of the convection fan 38 after the purge period avoids the fan blowing cold air initially as would happen were it connected to operate with the combustion fan.

The control logic is based on the three sub-circuits 33, 34, 37 each of which controls a relay RL1, RL2 and RL3 respectively. Failures in the sub-circuits may therefore be equated with the failure of any of these relays to operate correctly, and may therefore be reduced to a consideration of three relay failures only.

Considering each of these in turn, as the appliance is started, which is the most hazardous operation which happens in a gas appliance.

If relay RL1 fails and is in the operated condition when the appliance is started, then switch RL1 in sub-circuit 37 will be in the open position. Capacitor C9 will not therefore be charged and relay RL3 will never operate. Accordingly switch RL3-1 remains closed and no power supply is made to the ignition sub-circuit nor is the main gas valve opened.

If relay RL2 fails and indicates falsely that a flame is in existence when the appliance is started, then switch RL2-1 is open so there is no power supply to the ignition timer, so relay RL3 never operates and switch RL3-1 never opens to supply power to the ignition sub-circuit nor is the main gas valve opened. Furthermore, switch RL2-3 is open so the resistance R12 is connected in the gas valve sub-circuit, said switch RL2-2 is open so that even if switch RL3-1 were operated there would still be no operation of the gas valve or ignition sub-circuit.

If relay RL3 fails and remains pulled in, then switch RL3-1 is open and a power is available to both the ignition sub-circuit and the gas valve. If therefore the flame failure sub-circuit is operating correctly, the appliance will start normally but without the safety features of the pre-purge period and the timed ignition period.

Although in such a failure some safety is lost, the type of explosion due to ignition of residual gas in the appliance at start-up is not of the most severe kind, and the appliance is not therefore considered to be hazardous. If the relay RL3 fails in the unoperated condition then switch RL3-1 remains closed and no power is available to either the ignition sub-circuit or the gas valve.

WHAT WE CLAIM IS: - 1. An electrical control circuit for a gas or oil-fired heating appliance of the kind having a fan for supplying combustion air, an electrically-operated fuel valve and an ignition device, said control circuit comprising a plurality of sub-circuits each adapted to carry out or monitor part of a normal start-up sequence comprising a timed purge period followed by a timed ignition period followed either by normal operation or by a close-down operation if a flame is not detected, each said subcircuit having a relay connected to operate switch means for effecting the appropriate part of the start-up sequence, said fan. being connected to be energized on start-up independently of any of said relay-operated switch means, one of said sub-circuits including a flame-sensing device which is also connected to be energized on start-up independently of any of said relay-operated switch means, said one flame-sensing subcircuit having its relay connected to open a- first switch means and prevent the startup sequence from being followed when a flame is detected at start-up.

2. An electrical control circuit as claimed in claim 1, wherein said flamesensing sub-circuit relay also controls second switch means connected in series with said ignition device and third switch means controlling a circuit for opening said fuel valve whereby said ignition device and fuel valve may only be operated when no flame is sensed by said flame-sensing device.

3. A control circuit as claimed in claim 2, wherein said third switch means connects a resistance in series with the fuel valve, whereby when said resistance is so connected the current supply to the fuel valve is reduced to a value sufficient to maintain the fuel valve open but insufficient to open it from the closed position.

4. A control circuit as claimed in claim 3, wherein said resistance is so connected when the flame-sensing device senses a flame.

5. A control circuit as claimed in any of claims 2 to 4, wherein another of said sub-circuits comprises a purge timing means, the relay of said purge timing means operating ignition timing switch means connecting in another sub-circuit comprising an ignition timing device to start the timing of the ignition period at the end of said timed purge period.

6. A control circuit as claimed in claim 5, wherein said ignition timing sub-circuit comprises a capacitor to which charging power is connected through said ignition timing switch means during said purge period only and which is thereafter connected by said ignition timing switch means to operate the relay of the subcircuit, the length of the said ignition period being determined by the time for which the capacitor can hold the relay operated.

7. A control circuit as claimed in claim 6, wherein said flame-sensing sub-circuit relay also controls fourth switch means providing an alternative power connection to maintain the ignition timing sub-circuit relay operated when a flame is sensed.

8. A control circuit as claimed in claim 6 or claim 7, wherein the relay of the ignition timing sub-circuit controls switch means in series with both the ignition device and the fuel valve so that neither can be operated until this relay is operated.

9. A control circuit as claimed in any of claims 1 to 8 comprising a main electrical supply and a low-volt transformed electrical supply, means for connecting said combustion fan, ignition device and fuel valve across said main supply, all said switching means being operated by relays connected in said low-volt supply, and said flamesensing device being operated by said low volt supply.

10. A control circuit as claimed in any of claims 1 to 9 comprising also means for connecting a convection fan so that it is not energized until the timed ignition period is reached in the sequence.

11. A control circuit as claimed in any of claims 5 to 8, wherein said urge timing means circuit comprises an integrated circuit.

12. A control circuit substantially as set forth hereinbefore with reference to Figure 1 or to Figure 2 or to Figure 3 of the drawings.

13. A gas- or oil-fired appliance having a control circuit as claimed in any of claims 1 to 12.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. period and the timed ignition period. Although in such a failure some safety is lost, the type of explosion due to ignition of residual gas in the appliance at start-up is not of the most severe kind, and the appliance is not therefore considered to be hazardous. If the relay RL3 fails in the unoperated condition then switch RL3-1 remains closed and no power is available to either the ignition sub-circuit or the gas valve. WHAT WE CLAIM IS: -

1. An electrical control circuit for a gas or oil-fired heating appliance of the kind having a fan for supplying combustion air, an electrically-operated fuel valve and an ignition device, said control circuit comprising a plurality of sub-circuits each adapted to carry out or monitor part of a normal start-up sequence comprising a timed purge period followed by a timed ignition period followed either by normal operation or by a close-down operation if a flame is not detected, each said subcircuit having a relay connected to operate switch means for effecting the appropriate part of the start-up sequence, said fan. being connected to be energized on start-up independently of any of said relay-operated switch means, one of said sub-circuits including a flame-sensing device which is also connected to be energized on start-up independently of any of said relay-operated switch means, said one flame-sensing subcircuit having its relay connected to open a- first switch means and prevent the startup sequence from being followed when a flame is detected at start-up.