GB2076574A

GB2076574A - Fuel-heated Heat Source

Info

Publication number: GB2076574A
Application number: GB8115780A
Authority: GB
Original assignee: Joh Vaillant GmbH and Co
Current assignee: Vaillant GmbH
Priority date: 1980-05-24
Filing date: 1981-05-22
Publication date: 1981-12-02
Also published as: JPS5721720A; NL191097C; DE8014297U1; NL191097B; JPS6319773B2; GB2076574B; FR2483052A1; FR2483052B1; NL8102529A

Abstract

A fuel-heated heat source comprises a heater 1, a flow monitor 3 which has an inlet 8 communicating with the heater 1, an outlet 9 communicating with a flue 10, and a passage 12 for communicating with a room in which the heat source is to be installed and is provided with at least two temperature sensors 14, 17 and a detector circuit 16 for disabling the heater 1 in the case of a reverse flow or of a partial or total obstruction of the flue gases. Sensor 14 is associated with the outlet 9 communicating with the flue and sensor 17 is associated with the passage 12 for communicating with the room and the detector circuit 16 is responsive to the difference between the temperatures sensed by the two temperature sensors and disables the heater when said difference is less than a difference which can be adjustably set. <IMAGE>

Description

SPECIFICATION Fuel-heated Heat Source This invention relates to a fuel-heated heat source of the kind (hereinafter referred to as the kind set forth) comprising a flow monitor which has an inlet communicating with the heat source, an outlet communicating with a flue, and another outlet communicating with a room in which the heat source is installed and is provided with a temperature sensor and a detector circuit for disabling the heat source in case of a reverse flow or of an obstruction of the flue gases.

Early German Disclosure 21 34282 discloses a fuel-heated heat source which comprises a flow monitor, in which a temperature sensor consisting of a thermistor is mounted in a bushing of plastics material. The thermistor is included in the control circuit of a detector circuit, the threshold value of which is adjustable by a set point-adjusting resistor. In case of a reverse flow through the flow monitor or in case of an obstruction, the flue gases will heat the thermistor so that the detector circuit responds and operates a relay to close the fuel valve associated with the heat source, which is thus shut down. Whereas such a circuit arrangement will detect a total obstruction and a reverse flow, it will not indicate operating conditions which would permit a more differentiated response of a detector circuit.

German Early Disclosure 10 27 863 discloses a control system for a gas-fired water heater, with which a flow monitor is also associated. A temperature sensor is disposed in the hot gas stream close to the outlet of the heat exchanger, which is accommodated in a heating shaft. That temperature sensor controls a fuel valve to adapt the heat load of the appliance to abnormal flue gas conditions. That arrangement is based on the recognition gas conditions. That arrangement is based on the recognition that an obstruction or reverse flow will result in a lower excess of air so that the flue gas temperature will increase. This will result in a corresponding throttling of the gas supply in that system.That operation requires the appliance to be normally operated with a fairly high excess of air so that abnormal exhaust gas conditions will initially result in an optimizing of the excess of air and subsequently in an increase of the flue gas temperature. But modern fuelheated heat sources are operated with an optimum excess of air of about 40 to 80% so that a decrease of the excess of air will not result in a higher flue gas temperature. For this reason the teaching given there cannot be applied to appliances corresponding to the present state of the art.

Finally, German Early Disclosure 23 10 863 discloses a boiler which is provided with a flow monitor which in its part directed to the room in which the boiler is installed contains a control element, which consists of a flue gas temperature sensor or a pressure sensor. As soon as a predetermined temperature or a certain pressure is exceeded, that control element will switch the gas burner to a lower power. After a certain safety interval or when the control element no longer senses an excessive value, the fuel valve is opened to increase the power of the gas burner of the boiler.

German Patent Publication 25 00 529 discloses a safety device which is associated with a gas-fired flow heater and comprises a sensor consisting of a feeler disposed in the appliance above the burner in the region flown through by the hot gases. That feeler is in thermal communication with a heat-insulated mass for the storage of heat. A temperature rise above a predetermined threshold will cause the supply of gas to the burner to be interrupted or reduced.

This arrangement has the object to ensure that the appliance will operate only for a certain period of time.

It is common to all teachings of the state of the art discussed hereinbefore that the temperature is measured only at a single point in the flow monitor or in the heating shaft of the fuel-heated appliance. But various critical conditions, such as a partial obstruction amounting to 1 to 99%, a complete obstruction or a reverse flow may have widely different results, particularly when in the case of a reverse flow or of an obstruction the specific appliance to be monitored is incluenced by flue gas streams from other fuel-heated heat sources.

Fuelheated heat sources or the flow monitors associated therewith receive air as secondary air fed through the burner and the heating shaft and also receive air through the slots through which the flow monitor communicates with the room.

As the temperature of the air received from the room may vary by 35 K, that temperature is by no means insignificant for the conditions which will occur.

According to the present invention there is provided a fuel-heated heat source of the kind set forth, characterized in that at least two temperature sensors are provided, one of which is associated the outlet communicating with the flue and the other with the outlet communicating with the room, and that the detector circuit is responsive to the difference between the temperatures sensed by the two temperature sensors and disables the heat source when said difference is less than a difference which can be adjusted at a set point adjuster.

The fuel heated heat source will permit a particularly simple and reliable detection of the dangerous situations which are critical for the user of the appliance, namely, partial obstruction, and reverse flow.

The invention will be more fully understood from the following description given by way of example only with reference to Figures 1 and 2 of the accompanying drawings in which: Figure 1 is a basic representation of a fuelheated heat source provided with temperature sensors and a detector circuit, and Figure 2 is a complete detector circuit for an arrangement which is a modification of that shown in Figure 1.

In both figures, like reference characters designate like details.

In accordance with Figure 1, a heat source 1 has been installed in a room and consists essentially of a heating shaft 2, a flow monitor 3 mounted on the shaft, and a gas burner 4 mounted at the lower part of the shaft. The heating shaft 2 consists of copper and is hollow and more or less parallelepipedic. It is provided at its top at 5 with a plate-type heat exchanger of copper, which is supplied directly with tap water or with heating system water flowing in the flow and return mains of a room-heating system, not shown.

The heating shaft 2 has a bottom inlet 6 and a top outlet 7, which constitutes also an inlet 8 for the flow monitor. The flow monitor 3 consists of a hollow body, which consists of sheet metal and is substantially parallelepipedic and tapers in the direction of flow of the rising flue gases 1 9. The outlet 9 of that sheet metal body opens into a flue 10, which leads to a chimney, not shown. The flow monitor contains sheet metal inserts 11 for reversing and diverting the flue gas. In addition to the inlet 8 for gas coming from the heating shaft 2 and the outlet 9 for flue gas flowing into the flue one or more lateral outlets 12 are provided, through which flue gases can flow from the interior 13 of the flow monitor 3 into the room in which the heat source has been installed.The outlets 12 and the outlet 9 are defined by the outside walls of the flow monitor 3 and/or by the sheet metal insert 11.

A temperature sensor 14 is associated with the outlet 9 and senses the temperature of the gas flowing from the interior 13 through the outlet 9 into the flue 10. That temperature sensor can also sense the temperature of the gas which during a reverse flow from the flue 10 flows through the outlet 9 into the interior 13 of the flow monitor.

The temperature sensor 14 is connected by a signal line 1 5 to a detector circuit 16. Another temperature sensor 17 is associated with the outlet or outlets 12 and senses the temperature of all or part of the gas flowing into the interior 1 3 of the flow monitor or leaving that interior through the openings 12. The temperature sensor 1 7 is connected to the same detector circuit 1 6 by a signal line 18.

The temperature sensors 14 and 1 7 consist of temperature-dependent semiconductor resistors or of transistors. They are secured to and heatinsulated from the inside surface of the outer wall of the flow monitor 3 or to the sheet metal inserts 11 so that their thermal inertia will be minimized.

The detector circuit 1 6 is connected by a line 20 to a solenoid 21, which by means of an actuating rod 22 control a solenoid valve 23. The latter is incorporated in a gas conduit 24 leading to the gas burner 4.

The arrangement shown in Figure 1 has the following mode of operation: When the heat source 1 is to be started by a signal from a controller, e.g., from a room thermostat or a reservoir thermostat associated with a reservoir for dispensable water, that electric starting signal is delivered via a line which is connected in parallel to the line 20 so that the solenoid 21 is energized to open the gas solenoid valve 23. As a result, gas flows through conduit 24 to the burner 4 and is ignited there by means not shown and is burnt. It will be understood that the burning flame is monitored by a thermoelectric flame failure device or by an ionizing discharge gap. The flue gas flows through the interior of theheating shaft 2 and through the heat exchanger 5 and from the latter through the outlet 7 and the inlet 8 into the interior 13 of the flow monitor 3.Under normal operating conditions the flue gas leaves the interior 13 in the two paths on the left and right of the central sheet metal insert 11 below the outlet 9 and flows through the latter to the flue 1 0. Owing to the draught in the flue, the flow rate of flue gas in the flue exceeds the rate of flue gas flowing through the heat exchanger 5 so that tertiary air from the room in which the heat source has been installed is sucked through the outlet openings 12 into the interior 13 ofthe flow monitor and leaves that interior on the same path as the flue gas from the burner 4. That tertiary air may be at a temperature between 0 and 400 C. The flue gas entering through the opening 8 may be at a temperature in the range of 1 50 to 3000C.As a result of that operating condition, the temperature sensor 1 7 senses the room temperature and the temperature sensor 14 senses the temperature of mixed gases consisting of flue gas and room air.

In any case, the temperature sensed by the temperature sensor 1 7 (about 0 to 40 C) is much lower than the temperature sensed by the temperature sensor 14 (about 1 50 to 3000 C).

The two temperature signals are delivered by lines 1 5 and 18 to the detector circuit 16. As long as the detector circuit detects a temperature difference in excess of 100 K, it will apply to the line 20 a positive output signal, which causes the gas solenoid valve 23 to be held open.

In case of an obstruction, only part of the full rate of flue gas and tertiary air can flow off through the flue. Because tertiary air can be sucked through the openings 12 only when it is entrained by the flue gas coming from the burner 4, in case of an increasing obstruction the temperature sensed by the temperature sensor 1 7 will remain constant but the temperature sensed by the temperature sensor 14 will rise as less tertiary air is admixed to the flue gas stream.

As soon as the temperature difference falls below a threshold of about 40 K, which can be set in the detector circuit 1 6, the detector circuit 1 6 will interrupt the output signal so that the solenoid valve 23 closes.

The decrease of the temperature difference may be due to a decrease of the teiiiiperature sensed by the temperature sensor 14 when the temperature sensed by the temperature sensor 1 7 remains constant or may be due to a rise of the temperature sensed by the temperature sensor 1 7 when the temperature sensed by the temperature sensor 14 remains constant or decreases.

In case of an increasing obstruction, all flue gas is no longer discharged through the flue 10 so that the temperatures sensed by the temperature sensors 14 and 1 7 further approach each other until in case of a total obstruction no flue gas flows off through the outlet 9. In that case the temperature difference will be less than the value set at the detector circuit 1 6 so that the disabling signal will be continued. In case of a reverse flow the temperature sensor 14 will sense a lower temperature than the temperature sensor 1 7 because the flue gas which has been produced by the burner and flown through the heat exchanger 5 is then flowing out through the openings 12 and relatively cool flue gas is flowing through the opening 9 from the flue 1 p.Under that condition the detector circuit will also prevent an opening of the gas solenoid valve 23.

When hot flue gas, e.g., from another heat source, is forced through the flue 10 into the interior 1 3 of the flow-monitor past the temperature sensor 14, the latter will sense a higher temperature. Whereas the difference between the temperatures sensed by the temperature sensors 14 and 17 will thus decrease, it will still be in the wrong sense so that the closing signal will still be applied to the solenoid valve 23.

The conditions for partial obstruction, total obstruction and reverse flow can be more distinctly indicated than in the arrangement shown in Figure 1 if the measures shown in Figure 2 are adopted, which shows also further features of the invention.

In accordance with Figure 1, a further temperature sensor 30 is provided in addition to the temperature sensors 14 and 1 7. The temperature sensor 30 is disposed in the interior 1 3 of the flow-responsive safety device 3 adjacent to the inlet 8 closely above the heat exchanger 5 and is included in an electric line 31.

Each of the lines 1 5, 18 and 31, which are connected in parallel to each other to the source of operating voltage, includes one of the temperature-sensing resistors 14, 1 7 and 30 and a fixed resistor 32 so that there is a tap 33 between the fixed resistor and the temperaturesensing resistor.

The detector circuit 1 6 comprises two comparators 34 and 35, which have outputs 36 and 37 that constitute inputs of a succeeding OR gate 38. One input 39 of the operational amplifier 34 is connected by a line 40, which includes a fixed resistor 41, to that tap 33 which is associated with the temperature-sensing resistor 30. A line which includes a resistor 42 branches from a junction between the input 39 and the resistor 41 and leads to a slider of a potentiometer 43, the two free ends of which are connected to the same source of operating voltage as the sensing resistors together with the fixed resistors.

A second input 44 of the comparator 34 is connected by a fixed resistor 45 to a junction 46, which is conductively connected to that tap ee which is associated with the temperature-sensing resistor 1 7. A line which includes a resistor 53 extends from the junction 46 to one input 52 of the comparator 35. A line which includes a resistor 5p extends from a junction 48 to a slider of a potentiometer 51, the two free ends of which are respectively connected to the positive and negative terminals of the source of operating voltage. Another line extends from the junction 48 to a second input 49 of the comparator 35. A further line including a resistor 47 extends from the junction 48 to that tap 33whict is associated with the temperature sensor 1 5.

The second input 52 of the comparator 35 is connected by a resistor 53 to the third junction 33 which is associated with the temperaturesensing resistor 1 7.

One output 54 of the OR gate 38 is connected by a line 55 to one input 56 of an AND gate 57, which has an output 58 that is directly connected to one input 59 of a four-stage binary counter 60.

A controller for generating a starting signal for the heat source may consist of a room thermostat or a thermostat associated with a reservoir for dispensable water and is connected by an output line 61 to a setting input 62 of the binary counter.

A line 63 branching from the line 61 leads to a normally closed contact 64, from which line 63 extends as line 20 to the solenoid 21.

A line 65 branching from line 61 includes a one-shot multivibrator 66 and leads to a NOT circuit 67, the output of which is connected by a line 68 to one input of an AND gate 69. The line 68 is connected by a line 70 to another input 71 of the AND gate 57.

Lines 72, 73, 74 lead from respective stages of the four-stage binary counter 60 to respective one-shot multivibrators 75, 76 and 77, which are connected by output lines 78, 79 and 80 to respective inputs of an OR gate 81. The output 82 of that OR gate constitutes the other input of the AND gate 69.

A line 83 leads from the fourth stage of the four-stage binary counter 60 to a one-shot multivibrator 84, the output line 85 of which constitutes another input of the OR gate 81.

The output line of the AND gate 69 includes an amplifier and directly actuates the normally closed contact 64. The detector circuit shown in Figure 2 has the following function: In the position of rest shown in the drawing, the output line 61 of the controller is inoperative, i.e., carries no current. The controller serves to deliver a heat demand signal to the associated heat source, which may consist of a gas-fired flow heater, a gas-fired heater for circulating water, or a gas-fired boiler. The binary counter 60 is reset.

When a heat demand of whatever nature results in a voltage signal applied to line 61, the solenoid 21 will be directly energized via the normally closed contact 64 so that the rod 22 is actuated to open the gas solenoid valve 23 and the associated heat source is supplied with fuel, either gas or oil. The voltage signal in line 61 is also applied via line 65 to the one-shot multivibrator 66, which is thus set to deliver at its output a voltage signal, which actuates the NOT circuit 67 so that no voltage is applied to line 68.

The one-shot multivibrator 66 is set for about 1.5 minutes. That delay at the start is required because steady-state conditions cannot yet have been obtained adjacent to the flow-responsive safety device.

When the adjustable time for which the oneshot multivibrator 66 is set has expired, voltage is no longer presented at its output so that the NOT circuit 67 applies a voltage signal via line 68 to the input of the AND gate 69. As the other input 82 of the AND gate is dead, the contact 64 will remain closed.

As the burner 4 has been started and the heat source 1 is operating, steady-state conditions are now obtained adjacent to the flow monitor so that significant temperatures are sensed by the three temperature sensors 14, 17 and 30. The following conditions may be expected: During normal operation, the highest temperature will be sensed by the temperature sensor 30, which is contacted by the flue gas shortly after it has left the heat exchanger 5. When the flow monitor operates satisfactorily, the temperature sensor 1 7 will be contacted by room air. The room air and the flue gases from the heat source 1 flow off through the outlet 9 of the flow monitor. As a result, the temperature sensor 14 will sense a temperature which is intermediate the temperatures sensed by the temperature sensors 1 7 and 30.Signals representing said temperature difference are applied to the inputs 39, 44 of the comparator 34 and to the inputs 49, 52 of the two comparator 35. Signals representing the difference between the temperatures sensed by temperature sensors 30 and 1 7 are applied to the inputs 39 and 44 of the comparator 34. Signals representing the difference between the temperatures sensed by the temperature sensors 17 and 14 are applied to the two other inputs, which are associated with the comparator 35.

The desired temperature differences to be used for the comparison can be adjusted at the potentiometers 43 and 51.

If both temperature differences exceed the respective desired values, the comparators will be, disabled so that no signal is applied to lines 36 and 37. As a result, the line 55 is dead too so that no signals can be processed by the counter 60. When the operating conditions of the associated heat source change in the direction towards a partial obstruction of the flue gases and the temperature sensed by the temperature sensor 17, which is now contacted by less room air, remains constant, the difference between the temperatures sensed by the temperature sensors 30 and 14 will decrease. As soon as that difference, which is applied to the comparator 34, becomes smaller than the desired difference adjusted at the potentiometer 43, the comparator 34 will be set to apply a voltage signal via the output 36 and the line 55 to the input 56 of the AND gate 57.When the start delay time has expired, a signal is also applied via line 70 to the input 71 of the AND gate 57 so that the first stage of the binary counter 60 is set. In response to the setting of the first stage of the binary counter, an output signal is delivered via line 72 to the one-shot monovibrator 75, which is thus set for an adjusted time of five minutes. During that adjustable time of five minutes, a voltage pulse is delivered via line 78 and the OR gate 81 to the input 82 of the AND gate 69. Because a voltage is applied to the line 68 when the start delay time has expired, the contact 64 is now opened for the delay time which can be adjusted at the one-shot multivibrator 75 and in the case amounts to five minutes. The heat source 1 is now inoperative for that time.

When the delay time has expired, no signal is applied to the line 81 so that the contact 64 is closed. Measures which are not shown have been adopted to ensure that no output signal is applied to the line 55 as long as the heat source is inoperative. As soon as the contact 64 is closed after the first delay period has expired, the detector circuit again senses the operating conditions in the flow monitor. If the partial obstruction persists so that the difference is still less than that adjusted at the potentiometer 43, a second current pulse will be delivered to the line 55 so that the binary counter 60 will be advanced to its second stage.As a result, a signal is delivered via output line 73 to the one-shot multivibrator 76, which is thus set for an adjustable time, which is twice the set time that has been adjusted at the one-shot multivibrator 75, i.e., ten minutes in the present example. In the manner described hereinbefore, the heat source 1 is disabled for ten minutes and will be restarted when that adjusted time has expired.

If the disturbance has not yet disappeared after that time has expired, another output signal will be applied to the line 55, which corresponds to a setting of the third stage of the binary counter 60.

A signal is now applied via the output line 74 to the one-shot multivibrator 77 to set the same for an adjustable time, which in the present case is twice the previous delay time in the present example for twenty minutes, so that the heat source is disabled for that adjusted time.

The heat source is then re-started once more by the closing of the contact 64. If the operating conditions have not improved during that time, the last stage of the binary counter 60 will be set so that a signal will be applied via line 83 to the bistable multivibrator 84 to set the same. As a result, the contact 64 is opened and the heat source is shut down. The resulting state can be changed only by a manual resetting of the bistable multivibrator 84. As a result, the operation of the heat source will not be resumed automatically when that state has been reached.

If the state of the heat source changes from normal to a total obstruction rather than a partial obstruction, the temperatures sensed by the temperature sensors 30 and 1 7 will be the same because no flue gases flow off through the outlet 9 and the flue gases flow through the opening 12 into the room in which the heat source has been installed. As a result, the signals applied to the inputs 39 and 44 represent a zero temperature difference, which is extremely below the temperature difference selected at the potentiometer 43. This operating condition will result in the above-described advancing of the binary counter 60 through four stages until the heat source 1 is shut down.

When the restarting after the first or second disabling caused by a partial or total obstruction reveals that the normal condition has been obtained, the heat source will thereafter operate without a disturbance for an indefinite time until another partial or total obstruction occurs.

Whenever the heat source is started, the detector circuit 1 6 is temporarily disabled for the start delay time by the one-shot multivibrator 66.

There will be a reverse flow when flue gas from another source is forced through the flue 10 into the interior 1 3 of the flow monitor 3 and the temperature sensed by the temperature sensor 14 will then be much lower than that sensed by the temperature sensor 30 and also lower than the intermediate temperature sensed by the temperature sensor 17. Signals representing the difference between the temperatures sensed by the sensors 14 and 1 7 are applied to the inputs 52 and 49 of the comparator 35. The desired difference between the temperatures sensed by the temperature sensors 14 and 1 7 can be adjusted at the potentiometer 51. If the temperature difference decreases below that desired value, the comparator will deliver an output signal to line 37, so that an output signal is delivered to lead 55.As a result, the heat source is disabled once or more often in the manner described in that the contact 64 is opened. The signals delivered to the output lines 36 and 37 might be sampled and counted to find out whether the associated heat source is disabled owing to a partial obstruction, a total obstruction or a reverse flow. The data thus collected could be used for an investigation and possibly for an improvement of the operating conditions of the associated heat source. The actuating signal delivered to the line 61 after each enforced inoperative interval will cause the binary counter to be reset after its fourth stage has been set so that the counter will have a zero count at the beginning of the next following sequence of operations.

In the embodiment shown in Figure 1, two temperature sensors 14 and 17 are associated with the flow monitor at respective outlets associated with the flue and the room. If this teaching is applied to the embodiment shown in Figure 2, only the comparator 35 will be used whereas the comparator 34 and the circuitry associated with it can be omitted as well as the OR gate 38 and only one desired temperature difference must be adjusted at the potentiometer 51 to permit a detection of a partial obstruction, total obstruction and reverse flow.

Claims

1. A fuel-heated heat source of the kind set forth, characterized in that at least two temperature sensors are provided, one of which is associated with the outlet communicating with the flue and the other with the outlet communicating with the room, and that the detector circuit is responsive to the difference between the temperatures sensed by the two temperature sensors and disables the heat source when said difference is less than a difference which can be adjusted at a set point adjuster.

2. A fuel-heated heat source as claimed in claim 1, wherein a third temperature sensor is additionally provided and is associated with the inlet at which the flow monitor communicates with the heat exchanger.

3. The fuel-heated heat source as claimed in claim 2, wherein the detector circuit generates a signal for disabling the heat source when the difference between the temperatures sensed by the temperature sensors and or between the temperature sensors and is less than the difference which has been adjusted at the respective set point adjuster.

4. The fuel-heated heat source as claimed in any one of claims 1 to 3, wherein the detector circuit includes a one-shot multivibrator, which disables the detector circuit for an adjustable time when the heat source has been started.

5. The fuel-heated heat source as claimed in any one of claims 1 to 4, wherein the detector circuit includes a multi-stage binary counter, the input of which is connected to comparators for processing signals representing the temperatures sensed by the temperature sensors, and that the several stages of the binary counter are connected to a final control element associated with the heat source by means of one-shot multivibrators which are adjustable to different delay times.

6. The fuel-heated heat source as claimed in claim 5, wherein the delay times for which the one-shot multivibrators are set differ by the factor 2, with a minimum time of five minutes.

7. The fuel-heated heat source as claimed in claim 5 or claim 6, wherein the last stage of the binary counter is connected to a bistable multivibrator, the response of which renders the final control element associated with the heat source inoperative so that it will not be automatically restarted.

8. The fuel-heated heat source as claim in any one of claims 1 to 7, wherein three temperature sensors in series with three fixed resistors are connected in parallel to each other to a source of operating voltage and the junctions between the temperature sensors and the associated fixed resistors are connected to inputs of comparators, one input being connected also to a reference voltage via an adjustable potentiometer.

9. The fuel-heated heat source as claimed in claim 8, wherein the outputs of the comparators are connected by an OR gate to one input of an AND gate having a second input, which is connected by a one-shot multivibrator directly to the output of a controller for generating a signal for starting the heat source.

10. The fuel-heated heat source as claimed in claim 8 or claim 9, wherein the one-shot multivibrators which are adjustable to different delay times, have outputs which constitute inputs of an OR gate, the output of which is connected to one input of an AND gate, the other input of which constitutes the output of the one-shot multivibrator.

11. The fuel-heated fuel source as claimed in claim 1, wherein a NOT circuit is connected between the one-shot multivibrator and the input of the AND gate.

1 2. The fuei-heated heat source as claimed in claim 11, wherein another input of the OR gate is constituted by the output of a bistable multivibrator.

1 3. A fuel-heated heat source constructed and arranged substantially as hereinbefore described and as shown in the figures of the accompanying drawings.