GB2194331A - Gas fire control units - Google Patents

Abstract

A control unit for controlling gas fire having a valve supplying gas to a main or pilot burner and a fan powered flue outlet, comprises means for initially energising the fan motor to produce a flow of air in the flue sufficient to operate a flow-responsive switch, and means responsive to operation of the switch for opening the gas valve while reducing the voltage to the motor, the reduction in voltage producing a corresponding reduction in the flow of air along the flue.

Description

SPECIFICATION Gas fire control units This invention relates to a control unit for controlling the operation of a valve supplying gas to a main or pilot burner of a gas fire having a fan powered flue outlet. The invention is particularly concerned with control of an open fronted domestic gas fire in which the products of combustion are extracted and discharged through purpose-built ducting to a flue terminal fitted to an outside wall of the dwelling.

According to the invention, the control unit comprises means for initially energising the fan motor to produce a flow of air in the flue sufficient to operate a flow-responsive switch, and means responsive to operation of the switch for opening the gas valve while reducing the voltage to the motor, the reduction in voltage producing a corresponding reduction in the flow of air along the flue.

Preferably further means responsive to the position of the flow switch are provided for switching off the fan after a predetermined interval if the flow of air fails to actuate the switch.

Moreover, if the flow switch reverts to its original no-flow state during operation of the fire, the flow of air is increased to its higher level until either the switch moves back or, after a predetermined period, the fan is switched off and the gas valve is closed.

One example of a control unit embodying the invention is illustrated in the accompanying drawings in which: Figure 1 is a connection diagram showing connection of the external components to a control box, and Figure 2 is a circuit diagram of the control system.

Referring to these figures, the control unit controls the operation of a double solenoid valve V supplying gas to the main burner ofan open fronted domestic fire (not shown) while simultaneously adjusting the running speed of a fan motor F in response to varying air movement in the flue of the gas fire. The fan is fitted in a fan box (not shown) at the end of a flue duct which communicates with a gather box fitted behind the gas fire, the duct passing through an outside wall of the dwelling in which the fire is installed. Products of combustion are discharged through the flue duct to the atmosphere.

Mains power is supplied across terminals T11, T12 and fed through transformer TR1 and diodes D1, D2 to provide a full wave rectified 12 volt supply across terminals T13, T14. The 12 volt supply energises a LED via relay contact CR1.

To switch on the gas supply to the main burner, a push button PB is operated. The DC supply then energises a relay AR via the contacts of a flow switch FS disposed in the flue.

The switch includes a deflector arm FSA and a balance weight B. Fig. 1 shows the movable contact in its flow position and Figure 2 shows the position of the movable contact with the flow switch in its no-flow state.

With the flow switch in its no-flow state, a smooth DC voltage is also supplied via diodes D4, D5 across capacitor C1 to a TIMER. With capacitor C2 in a discharged state, there is a resulting output from terminal TM3 of the timer which switches transistor TS1 into conduction thereby energising relay CR. The relay contacts CR1 therefore change over and the LED is extinguished while the supply to the circuit is maintained via CR1 when the push button PB is released.

Energisation of relay CR also closes contacts CR2.

With relay AR now energised, contact AR1 changes over and the full mains voltage is applied across the fan motor F. The fan therefore accelerates to full speed and establishes an air flow in the flue which operates the air flow switch FS. With the flow switch in its operated state (terminals T3 and T2 connected by the moving contact) relay AR is deenergised and charging current to the capacitor C2 is cut off. At the same time operation of the flow switch FS provides a triggering pulse to the thyristor TY2 via capacitor C3 and the associated resistor network. Relay BR is therefore energised and its contacts BR1 closed.

The closing of contacts BR1 switches on the gas valve solenoid V and also provides a supply to the fan motor via a choke and the relay contact AR1. The effect of the choke is to reduce the voltage supplied to the fan motor so that the motor speed falls and there is a corresponding reduction in the air flow. The system is now in a stable running condition with the gas supply established and the fan speed/air flow at its normal reduced level.

Should the air flow switch FS fail to operate and remain in its "no-flow" state connecting terminal T3 to terminal T1, the relay BR will not be energised so that the gas valve will not be openedand the fan will remain at full speed. Moreover, charging of current to capacitor C2 will be maintained and this has the effect of dropping the voltage at terminal T3 of the timer after a period of 6 to 8 seconds, the voltage dropping to zero and the transistor TS1 being thereby switched off. Relay CR is therefore de-energised, and the relay contact CR1 reverts to its original state switching on the LED indicator and isolating the electronic circuit from the DC supply. Accordingly relay AR is de-energisedand the supply to the fan motor is cut off.

Should the air flow switch FS revert to its "no-flow" state after normal stable running conditions have been established, the relay BR will remain energised because the thyristor TY2 once energised will remain conducting.

The gas valve V will therefore remain on. Re lay AR will however operate, and the fan motor will be switched to its full speed by change over of contact At 1. Charging current will also be re-established for the capacitor C2 so that should the "no-flow" condition persist for more than 6 to 8 seconds, the "lock out" sequence described above will occur and both the fan F and the gas valve V will be switched off. If however within the time per iod set by the timer the air flow switch FS returns to its "full flow" state, the normal stable running is re-established and the timing capacitor C2 is discharged to its initial state.

No "lock out" sequence is therefore initiated.

Should the air flow switch FS permanently "stick" in its "full flow" position, depressing button PB will have no effect on the circuit since none of the relays could become energised.

It can be seen therefore that flow switch failure results in system "lock out" with the fan and gas supply cut off. During normal operation, mains transient suppression is provided by a surge suppressor connected across terminals T4 and T5. This also suppresses the reverse transient generated by the DC operated gas valve solenoid during switch off.

Short circuit protection for the fan, valve, transformer etc is provided by an integral 500 mA mains fuse.

In a modified arrangement for use with a wall heater incorporating a pilot burner with an associated flame failure valve, the closing of the contacts BR1, instead of switching on the solenoid valve V, completes a thermocouple circuit attached to the flame failure valve.

Once the thermocouple circuit is completed, gas is able to reach the pilot and main burner through the flame failure valve, the valve subsequently being maintained open only when the pilot burner is alight.

Claims (6)

1. A control unit for a gas fire having a fan powered flue outlet, the unit comprising means for initially energizing a motor for driving the fan to produce a flow of air in the flue sufficient to operate a flow-responsive switch, and means responsive to operation of the switch for enabling flow of gas to a main or pilot burner of the fire while reducing the voltage applied to the motor, the reduction in voltage producing a corresponding reduction in the flow of air along the flue.

2. A control unit according to claim 1 in which the flow enabling means comprises a valve in a gas supply line.

3. A control unit according to claim 2 in which the valve is a solenoid-operated valve, the valve being opened in response to operation of the flow-responsive switch.

4. A control unit according to any one of the preceding claims further comprising means responsive to the position of the flow switch for de-energising the fan motor after a predetermined interval if the flow of air fails to actuate the switch.

5. A control unit according to any one of the preceding claims further comprising means for increasing the flow of air to its higher level if the flow-responsive switch reverts to its original no-flow state during operation of the fire.

6. A control unit substantially as herein described with reference to the accompanying drawings.