GB2236877A

GB2236877A - Fan-controlled PTC heating apparatus

Info

Publication number: GB2236877A
Application number: GB9016918A
Authority: GB
Inventors: William Ian Macfarlane
Original assignee: I O INNOVATION DESIGN CONSULTA
Current assignee: I O INNOVATION DESIGN CONSULTA
Priority date: 1989-08-03
Filing date: 1990-08-01
Publication date: 1991-04-17
Also published as: GB9016918D0; GB8917745D0

Abstract

Fan controlled PTC (positive thermal coefficient) heating apparatus for use in domestic applications maintains a desired room temperature by controlling the speed of the air through the PTC element by electronic control of the speed of a cooling fan. As the room temperature approaches the desired temperature, the fan slows down to a speed sufficient to maintain the room temperature. At the desired temperature, the fan moves very slowly and any further rise in temperature causes the heating element to be switched off to ensure that no heat is generated thus saving power and limiting the rise of temperature in the room. The fan speed is controlled by varying the firing angle of a triac switch, relative to the zero crossing points of fan current. The fan or fan and heater may be controlled by a clock to switch on and off, or vary the room temperature during a day. Alternatively they may be controlled by a proximity detector. The heater may be used in conjunction with an ioniser or a dehumidifier.

Description

FAN-CONTROLLED PTC HEATING APPARATUS The present invention relates to fan controlled PTC heating apparatus, particularly but not exclusively, for use in domestic applications.

Heaters which use electricity at a normal daytime tariff such as fan heaters have generally been considered at a disadvantage to storage heaters using white meter and similar tariffs. However, in many cases, this has been shown to be incorrect. The storage heater, even when fan operated, both leaks heat and has a high thermal lag. As a result heat is supplied at periods when it is not required and is often not available when it is required, leading to the use of 'top up' heating directly from the mains. It has been estimated that usage of electricity in this way can be as high as 60% greater than the consumption in directly supplied heating, substantially offsetting the gains (if any) from the smaller tariff.

This inefficient use of power can be more costly than a system in which heat is used only when required and is controlled to a level just sufficient to meet the current need.

Thermostatically controlled fan heaters are a part way stage to this type of system. However, unless they are used in combination with separate time switches they are normally switched on, on entrance to a room which has then to heat up. The thermostats are also of the on-off type causing the fan to run at full speed (its noisiest condition) or to be completely off. A large hysteresis of up to about 5% is normally provided to prevent very short cycles of operation. Electronic power switching of these devices to give continuously variable control is expensive and may create interference in the mains supply, which is not allowable under new IEC recommendations.

Heaters using elements constructed of positive temperature coefficient (PTC) material have a number of advantages over conventional hot wire elements. In one form the PTC heating elements consist of blocks of a ceramic material which are cooled by an arrangement of aluminium fins. The material has an electrical resistance characteristic with temperature which gives a constant (or slightly reducing) resistance from - 30 up to about 1700/l800C, above which temperature the resistance value increases very rapidly.

As the resistance rises the power used by the element reduces in proportion and the element temperature automatically limits to around 200 C. This makes the device inherently safer than the higher, and potentially unlimited, temperature of a conventional linear resistor heater. Forcing an air flow through the element radiator fins, cools the material, and increases the power drawn by the element.

However, with some existing PTC heaters the temperature output relies on the self limiting temperature of a PTC element and the heater operates inefficiently and does not regulate effectively the ambient room temperature.

An object of the present invention is to provide a fan controlled PTC heating apparatus which obviates or mitigates at least one of the aforementioned disadvantages.

This is achieved by varying the speed of the air through the PTC element by electronic control of the cooling fan speed to get a straightforward and convenient control of the heat output to maintain a substantially constant temperature in the room. In particular, as the room temperature rises to within a degree of preset value the fan slows down to maintain the temperature. At preset temperature the fan is moving very slowly, and any further rise in temperature causes the element to be switched off to ensure that no heat at all is generated thus saving power and limiting temperature rise in the room.

In accordance with one aspect of the present invention there is provided a PTC heater with a variable speed fan drive which is electronically controlled to maintain one or more set temperature levels, said PTC heater being in combination with electronic control means for switching the PTC heating element off completely when the temperature is above a preset value.

Conveniently a PTC heater includes an electronic timer for switching the device on and off at set times.

Alternatively the unit can be switched off or on at different temperature levels. A programmable digital clock with an LCD display with a battery back-up can be used to enable the clock to keep time during periods where said clock is disconnected from its voltage supply.

The PTC heater is conveniently combined with a negative ion generator disposed at the air output for creating a supply of negative ions in the room. The combination of the fan controlled PTC heating apparatus with the negative ion generator acts as a climate controller and dust precipitator. Advantageously the fan controlled PTC heating apparatus includes a mechanical cleanable air filter (eg. activated carbon) for removing larger dust particles from the atmosphere.

Such a fan controlled PTC heating apparatus controls the heat input into a room in an efficient manner and automatically switches off completely when the temperature in the room is higher than the set temperature. The unit, once set, can be left to work indefinitely. A number of such devices could be located in the rooms of a house to provide an effective heating system with each room controlled for its own optimum temperature and time requirements. Each fan controlled PTC heating apparatus may be coupled to an infra-red or ultra-sonic 'presence' or 'occupancy' detector which actuates the heater to switch on when someone enters the room and to switch off when the room is empty.

According to another aspect of the present invention there is provided a control circuit for use with a fan controlled PTC heater having a PTC heating element, said control circuit comprising means for monitoring the zero crossing of the current through the fan, means for measuring the difference between the ambient temperature in a room and a desired temperature; comparison means for comparing a signal representative of the temperature difference with a signal representative of the zero crossing of the current and for generating a control signal; solid state switch means connected in series with the fan for receiving the control signal for switching on or off the voltage applied across the fan in accordance with the temperature difference.

Conveniently the voltage across the PTC element is monitored and is used to switch the PTC element on or off depending on the temperature difference.

In accordance with another aspect of the present invention, there is provided a fan controlled PTC heating apparatus comprising a fan, a PTC.heating element circuit electrically connected in parallel with the fan motor circuit, a solid state switch means connected in series with the fan for controlling the supply of power to said fan, sensing means for sensing the fan current zero crossing and for generating a phase reference signal respresentative of the fan current zero crossing, temperature sensing means for sensing the difference between the ambient temperature and a preset temperature and for generating a signal proportional to said temperature difference, voltage control timing means coupled to said sensing means and to said temperature sensing means for receiving said phase reference signal and said temperature difference signal, said voltage control timing means providing a pulsed timing control signal to said solid state switch means for controlling the switching thereof, whereby as the ambient temperature rises toward the preset temperature the pulsed signal alters and the altered signal is supplied to the solid state switch to control the switch to reduce the voltage applied to the fan so that the fan speed is reduced.

Preferably the solid state switch is a triac.

Alternatively the solid state switch is provided by inverse parallel thyristors, or power MOSFETS.

Conveniently the temperature sensing means is a thermistor.

Preferably the preset temperature value is adjustable so that the desired temperature of the room can be varied.

Preferably said solid state switch means is controllable to switch off power to said PTC element when said ambient temperature exceeds said preset temperature.

A separate sensing circuit is provided using the voltage across a PTC heating element as a reference and combining the voltage with.a signal representative of the temperature difference to provide a bi-polar pulse waveform to control the switching of said solid state switch.

According to another aspect of the present invention there is provided a method of controlling the speed of a fan in a fan controlled PTC heater comprising the steps of: sensing the fan current zero crossing and using the fan current zero crossing to generate a phase reference signal; monitoring the ambient temperature and comparing the ambient temperature with a preset temperature and providing an output signal dependent on the temperature difference;; combining the phase reference signal with the temperature difference signal to provide a pulsed fan control signal, applying said pulse fan control signal to switch means coupled to said fan to control the voltage applied to the fan whereby as the temperature difference increases the voltage applied to the fan is reduced so that the fan output drops and the power in the PTC element drops due to its increasing resistance whereby the heat output from the fan controlled PTC heater is also reduced.

Preferably also the method includes the switching off the heating element when the ambient temperature rises above the preset temperature.

The heating element is controlled by using the voltage across heating element to provide a reference signal and combining the reference signal with a temperature difference signal to create a bi-polar waveform, and applying said waveform to a solid state switch coupled in series with said PCT heating element so that the voltage applied to the PTC heater element is switched off if the temperature exceeds the preset value.

These and other aspects of the present invention will become apparent from the following description, when taken in combination with the accompanying drawings in which: Fig. 1 is a circuit diagram of a fan controlled PTC heating apparatus in accordance with an embodiment of the present invention, and Figs. 2a to 2i depict various waveforms which are created in the circuit for controlling operation of the fan.

The fan controlled heating apparatus which is generally indicated by reference numeral 10 comprises a positive temperature coefficient (PTC) heater element 12 and a fan drive motor 14. The PTC heating element 12 and fan motor 14 are connected parallel between the live and neutral supply rails which are connectable to the main voltage supply. As will be explained the fan control heating apparatus operates so as to control the supply voltages applied to both the PTC heater element 12 and the fan motor 14 with the result that the heat output of the apparatus is varied by varying the voltage applied to the fan motor 14 which in turn varies the speed of the fan.

The heating apparatus is basically designed to operate to maintain a temperature in a room or other environment at a substantially constant level and when the temperature in the room reaches a preset temperature level the fan 14 is slowed down. The heat output from the PTC heating element is generally sufficient to maintain the temperature in the room at the desired level, but if the temperature in the room rises above this level both the heating element 12 and the fan 14 are switched off as will be later described in detail.

A triac Q2 is connected in series with fan 14 as shown and the speed of the fan is controlled in relation to the ambient temperature and set temperature so that, as the ambient temperature approaches the preset temperature, the conduction angle of the triac Q2 is reduced lowering the a.c. voltage applied to the fan. Because the fan motor is highly inductive the fan motor current lags the applied voltage by nearly 90 as best seen in Figs. 2a, 2b, and 2c. The voltage across the triac Q2 is measured by the exclusive norgate 16 which is connected as a non inverting buffer. The ratio of resistance R17 and R18 is chosen so that the gate 16 switches just after each current zero crossing in Q2.Resistor R19 and capacitor C5 provide positive feedback and the output of exclusive NOR gate 16 is a squarewave as shown in Fig. 2d with a small phase lag 3 relative to the current through the triac Q2. The squarewave signal is fed to one input of exclusive NOR gate 18.

The second phase delay is caused by passing the squarewave shown in Fig. 2d through a buffer amplifier 20 and the delayed signal is fed to the other input of exclusive NOR Gate 18. The output of the exclusive NOR gate is a sharp negative going pulse as shown in Fig. 2e, with a duration equivalent to the delay time of buffer amplifier 20. The negative going pulse discharges capacitor C2 through resistor R20 and diode D2 when the output of the exclusive NOR gate 18 is low and C2 charges again through R20 when gate is high. The resultant signal shown in Fig. 2f is a "sawtooth waveform" which is synchronous with a line frequency but which is in phase with the fan motor current.

It is now convenient to turn to another part of the circuit dealing with the sensing of air temperature. The temperature of the air drawn into the fan controlled PTC heater measured by a voltage divider formed by resistor R2 and thermistor Th1 and this voltage is compared with the reference voltage set by voltage divider R1 and R4. The voltage giving the temperature setting on potentiometer P1 sums with the voltage on thermistor Thl via resistor R3 so that the two inputs to differential amplifier 22 are equal for thermistor temperatures in the range 2 e C to 28 C (approximately) depending on the setting of potentiometer P1.Resistor R5 sets the proportional gain of the amplifier 22 giving an effective full output range for approximately 1" C change in the temperature of thermistor TH1.

The output of amplifier 22 is limited by resistors R6, 7, 8 to within maximum output and minimum values of the ramp voltage on capacitor C2 and is compared with this voltage by the comparator 24. Resistor R12 and capacitor C3 provide positive feedback to ensure a cleans switching of the comparator and rectangular waveform output of the comparator 24 is capacatively coupled via C4 to exclusive Norgate 26 which is connected to the non-inverting buffer.

As best seen in Fig. 2f when the ramp voltage on C2 passes through the voltage at the junction of R6, 7, 8 the comparator IC 24 switches low resulting in the output of the comparator having a shape as shown in Fig. 2g.

Capacitor C4 with resistance R14 differentiates the negative going pulse to create the waveform as shown in Fig. 2h.

The negative going pulse is applied to the input of exclusive NOR gate 26 and this buffers the pulse to create a negative going pulse as shown in Fig. 2i.

Resistor R 15 provides positive feedback to ensure clean switching. This pulse occurs at a delay after the zero current point which is variable depending on the output of the proportional amplifier 22 and therefore on the difference in temperature as measured by thermistor TH1 and the preset temperature.

This negative pulse triggers the triac Q2 with a negative gate pulse at an angle depending on temperature and therefore varying the voltage applied to the fan motor and therefore its speed. As the fan load falls at a rate much higher than the drop in speed a relative simple voltage control is sufficient to provide stable conditions right down to zero speed.

As the fan speed drops the power in the PTC element also drops due to its increasing resistance and the heat output is accordingly reduced. At zero fan speed the heat output is typically about 8% to 10% of full output providing a very wide range of proportional control.

The PTC heater element 12 is connected to the line voltage through a thermistor TH2 and triac Q1. The gate pulses for triac Q1 are provided by an exclusive NOR gate 27 which generates a bipolar pulse waveform as will be described. The bipolar pulse waveform has a positive going pulse at the start of the positive half cycle of the applied mains voltage signal and negative going pulse on the negative half cycle, creating minimum radio interference. When the room temperature is below or at the desired temperature, triac Q1 is fully on. When the room temperature rises above the desired temperature triac Q1 switches off as will be described ensuring that no heat is produced from the PTC element thus saving maximum energy.

Exclusive NOR gate 27 operates in a similar manner to gate 16, however, as the PTC heater element is purely resistive, gate 27 uses the line voltage as a reference and produces a square wave having a positive going pulse synchronised with the positive half cycle of the input voltage signal, and a negative going pulse synchronised with the negative half cycle of the input voltage signal.

A positive feedback resistor R22 is provided to ensure clean and fast switching of gate 27. However, this resistor creates a small phase lag and this phase lag is compensated for by the provision of capacitor C7 which provides a small corresponding voltage phase lead at the input of gate 27. This ensures that the edges of the positive and negative going pulses of the square wave output of gate 27 are exactly at the zero-crossings of the voltage signal applied to the PTC element 12 and triac Q1. The square wave output of gate 27 is fed via capacitor C6 and resistor R23 to the gate of triac Q1 and is perfectly in phase with the line voltage ensuring that triac Q1 is fully on and no line interference is produced..

When the desired temperature in the room is reached i.e. when the voltages at the two inputs of differential amplifier 22 are equal, the output of amplifier 22 goes towards its maximum potential and if, however, the temperature of the room rises further above the desired level, the voltage applied to the inverting terminal (from the junction of R2, R3 and Thl) is less than the voltage applied to the non-inverting terminal (from the junction of R1 and R4) and the output of amplifier 22 goes still higher. Comparator 28 senses when the output of amplifier 22 has gone high and consequently the output signal of comparator 28 goes low.The low output of comparator 28 causes the output of gate 27 to remain low and therefore the bipolar pulse waveform hereinbefore described is not applied to the gate of triac Q1 and the triac Q1 is effectively switched off and the PTC heating element is therefore disconnected from the electrical supply.

An ON/OFF switch 32 is connected between resistor R4 and the -5V rail. When this switch 32 is open, the series connected resistors R1 and R4 are disconnected from the 5V rail and a reference voltage is not provided to the non-inverting input of amplifier 22 the output of which goes high causing triacs Q1 and Q2 to switch off as hereinbefore described.

Thermistor TH2 is a negative temperature coefficient thermistor to reduce the inrush current on start up when the element is cold. Initially it is a high resistance and as the thermistor heats up the resistance decresaes while the resistance of the PTC element increases providing a smooth transition as power is applied to the PTC heating element.

Various modifications may be made to the embodiment hereinbefore described without departing from the scope of the present invention. In the diagram, the PTC fan controlled heater circuit is shown connected to the circuit of an ioniser generally indicated by reference numeral 34. The ioniser 34 is the subject of applicant's co-pending application. However, any suitable form of ioniser may be disposed at the air outlet of the PTC heater so that negative ions are generated at the air outlet of the heater and these ions are emitted into the air of the room increasing the level of negative ions in the air.

A programmable time clock may be connected to the PTC heater control system allowing the system to be switched on or off as required at different times of the day or night. A preferred form of clock is an electronic digital clock with a liquid crystal display. The clock should have a battery back up to ensure that the timing of the clock is not lost due to interruptions in the power supply for example, when the PTC heater control system is unplugged from the mains supply or during cuts in supply.

The clock could be programmed to provide output signals which switch either the fan or the fan and PTC heater element on. The fan and PTC element could be switched off for specified periods, for example during the night or during periods of the day when a room is not being used. In this case the clock would operate in the circuit of ON/OFF switch 32. Alternatively, the clock could be used to switch the value of the desired temperature in the room depending on the time of day, for example to maintain a low heat level in a room to prevent pipes freezing. The clock could also be used to actuate the fan alone for certain periods. This is particularly useful for systems with ionisers whereby the fan passes a stream of air over the ion emitter to distribute ions into the atmosphere.Such a system would be particularly useful in green houses where a PTC heater having a power rating of 1/3 KW would be used and where the ion stream may help plant growth. It may also be desirable to use the fan alone or with the ioniser for comfort whent the ambient temperature is high. In this case the control circuit may be switched so that the fan speed is controlled by the setting of the temperature setting potentiometer so that a desired level of ventilation is obtained.

A fan-controlled PTC heater has various applications, for example a PTC heater of about 1 - 1.8 KW could be used to maintain the temperature in rooms such as lavatories, bathrooms and kitchens. With rooms having hand washing facilities the system may be provided with a wall mounted PTC heater having a suitable proximity device, for example an infra red detector, to detect the proximity of a pair of hands held adjacent to the air outlet of the PTC heater, and to turn the fan fully on to provide an air flow to allow hand drying. Removal of the hands from the proximity of the air outlet causes the fan to return to its previous speed of rotation, or to a speed of rotation which maintains the temperature of the room at a desired level. With the addition of an air ioniser air quality is improved.It will be obvious that continuously variable control of the air outlet temperature is possible by a similar means to that described above and the heater could therefore be used for hair drying and similar applications also.

An application for a PTC heater with a power rating of 1 KW is to maintain a desired temperature in the interior of a car during severe frost conditions outside the car, for example temperatures down to - 30 C overnight. Such a PTC heater control system should shut down safely if the car is covered by snow for example, or if the heater is inadvertently covered by a rug or other object within the car.

The PTC heater could be used in combination with a germicidal lamp which may be disposed at the air inlet of the heater producing an air flow over the lamp.

Furthermore, an electronic de-humidifier (Peltier junction type) may be connected to the apparatus. Such a device extracts moisture from the cold air which passes into the PTC heater. In a portable humidifier, the collected water would be retained in a container having as small an evaporation surface as possible and this container would need to be emptied at regular intervals. In permanently installed dehumidifiers, the condensate could be piped to drains or to the exterior of the house.

The various devices which could be combined with the PTC heater control could be modular to allow these devices to be readily interchangable and replaceable for maintenance.

Advantages associated with the present invention are that the speed of the air through the element is varied by electronic control of the cooling fan speed to provide a straightforward convenient control of the heat output and to maintain a constant temperature in the room. As the room temperature rises the fan slows down and switches off when the set temperature is reached. In addition a further rise in temperature causes the element to be switched off ensuring that no heat at all is generated thus saving power and limiting temperature rise. The system is inherently safe because of the limited temperature characteristics and heaters can be made which will be safe even if accidentally covered. Such heating apparatus in accordance with the present invention will not set fire to readily combustible materials such as tissue paper, lint and the like. The fan controlled PTC heater can be left to operate continuously providing a regulated temperature in a room with minimal consumption of electricity and the distributed heating system may be provided by using a plurality of such units in the home or work place requiring minimal interference from the user because it is self regulating.

Claims

1. A PTC heater with a variable speed fan drive which is electronically controlled to maintain one or more set temperature levels, said PTC heater being in combination with electronic control means for switching the PTC heating element off completely when the temperature is above a preset value.

2. A PTC heater as claimed in Claim 1 wherein said heater includes an electronic timer for switching the device on and off at set times.

3. A PTC heater as claimed in Claim 1 wherein said heater is switched off or on at different temperature levels.

4. A PTC heater as claimed in Claim 2 wherein a programmable digital clock with an LCD display and a battery back up is used to enable the clock to keep time during periods where said clock is disconnected from its voltage supply.

5. A PTC heater as claimed in any of Claims 1 to 4 wherein a negative ion generator is disposed at the air output for creating a supply of negative ions in the room.

6. A PTC heater as claimed in any preceding claim wherein said heater includes a mechanical cleanable air filter.

7. A PTC heater as claimed in any preceding claim wherein said heater is coupled to an infra-red or ultra-sonic 'presence' or 'occupancy' detector which actuates the heater to switch on when someone enters the room and to switch off when the room is empty.

8. A PTC heater as claimed in any preceding claim wherein said heater further comprises a control circuit including, means for monitoring the zero-crossing of the current through the fan; means for measuring the difference between the ambient temperature in a room and a desired temperature; comparison means for comparing a signal representative of the temperature difference with a signal representative of the zero crossing of the current and for generating a control signal, and solid state switch means connected in series with the fan for receiving the control signal and for switching on or off the fan in accordance with the temperature difference.

9. A PTC heater as claimed in claim 8 wherein the voltage across the PTC element is monitored and is used to switch the PTC element on or off depending on the temperature difference.

10. A PTC heater as claimed in any of claims 1 to 7 wherein said heater includes a PTC heating element circuit electrically connected in parallel with the fan motor circuit, a solid state switch means connected in series with the fan for controlling the supply of power to said fan, sensing means for sensing the fan current zero currentand for generating a phase reference signal representative of the fan current zero crossing, temperature sensing means for sensing the difference between the ambient temperature and a preset temperature and for generating a signal proportional to said temperature difference, voltage control timing means coupled to said sensing means and to said temperature sensing means for receiving said phase reference signal and said temperature difference signal, said voltage control timing means providing a pulsed timing control signal to said solid state switch means for controlling the switching thereof, whereby as the ambient temperature rises towards the preset temperature the pulsed signal alters and the altered signal is supplied to the solid state switch to control the switch to reduce the voltage applied to the fan so that the fan speed is reduced.

11. A control circuit for use with a fan controlled PTC heater having a PTC heating element, said control circuit comprising means for monitoring the zero crossing of the current through the fan, means for measuring the difference between the ambient temperature in a room and a desired temperature; comparison means for comparing a signal representative of the temperature difference with a signal representative of the zero crossing of the current and for generating a control signal; solid state switch means connected in series with the fan for receiving the control signal and for switching on or off the voltage applied across the fan in accordance with the temperature difference.

12. A control circuit as claimed in Claim 11 wherein the voltage across the PTC element is monitored and is used to switch the PTC element on or off depending on the temperature difference.

13. A fan controlled PTC heating apparatus comprising a fan, a PTC heating element circuit electrically connected in parallel with the fan motor circuit, a solid state switch means connected in series with the fan for controlling the supply of power to said fan, sensing means for sensing the fan current zero crossing and for generating a phase reference signal representative of the fan current zero crossing, temperature sensing means for sensing the difference between the ambient temperature and a preset temperature and for generating a signal proportional to said temperature difference, voltage control timing means coupled to said sensing means and to said temperature sensing means for receiving said phase reference signal and said temperature difference signal, said voltage control timing means providing a pulsed timing control signal to said solid state switch means for controlling the switching thereof, whereby as the ambient temperature rises toward the preset temperature, the pulsed signal alters and the altered signal is supplied to the solid state switch to control the switch to reduce the voltage applied to the fan so that the fan speed is reduced.

14. Fan controlled heating apparaturs as claimed in Claim 13 wherein the solid state switch is a triac.

15. Fan controlled heating apparatus as claimed in Claim 13 wherein the solid state switch is provided by inverse parallel thyristors, or power MOSFETS.

16. Fan controlled heating apparatus as claimed in any of claims 13 to 15 wherein the temperature sensing means is a thermistor.

17. Fan controlled heating apparatus as claimed in any of Claims 13 to 15 wherein the preset temperature value is adjustable so that the desired temeperature of the room can be varied.

18. Fan controlled heating apparatus as claimed in any of Claims 13 to 17 wherein said solid state switch means is controllable to switch off power to said PTC element when said ambient temperature exceeds said preset temperature.

19. Fan controlled heating apparatus as claimed in any of Claims 14 to 18 wherein a separate sensing circuit is provided using the voltage across a PTC heating element as a reference and combining the voltage with a signal representative of the temperature difference to provide a bi-polar pulse waveform to control the switching of said solid state switch.

20. A method of controlling the speed of a fan in a fan controlled PTC heater comprising the steps of: sensing the fan current zero crossing and using the fan current zero crossing to generate a phase reference signal; monitoring the ambient temperature and comparing the ambient temperature with a preset temperature and providing an output signal dependent on the temperature difference;; combining the phase reference signal with the temperature difference signal to provide a pulsed fan control signal, applying said pulsed fan control signal to switch means coupled to said fan to control the voltage applied to the fan whereby as the temperature difference increases the voltage applied to the fan is reduced so that the fan output drops and the power in the PTC element drops due to its increasing resistance whereby the heat output from the fan controlled PTC heater is also reduced.

21. A method as claimed in Claim 20 further comprising the step of switching off the heating element when the ambient temperature rises above the preset temperature.

22. A method as claimed in Claim 20 or Claim 21 further comprising the steps of controlling the heating element by using the voltage across the heating element to provide a reference signal and combining the reference signal with a temperature difference signal to create a bi-polar waveform, and applying said waveform to a solid state switch coupled in series with said PCT heating element so that the voltage applied to the PTC heater element is switched off if the temperature exceeds the preset value.

23. A PTC heater substantially as hereinbefore described with reference to the accompanying drawings.

24. A control circuit substantially as hereinbefore described with reference to the accompanying drawings.

25. Fan controlled heating apparatus substantially as hereinbefore described with reference to the accompanying drawings.

26. A method of controlling the speed of a fan in a fan controlled PTC heater substantially as hereinbefore described with reference to accompanying drawings.