GB2060215A - Automatic control of temperature - Google Patents

Abstract

A temperature control arrangement for an air-conditioning system includes at least one device for heating air and one for cooling air. Operation of the heating and cooling modes occurs simultaneously over a coincident band of temperature values. Additionally, the humidity of the temperature controlled space may be controlled.

Description

SPECIFICATION Variable-adjusting controller The present invention relates to variable-adjusting controller, and is especially concerned with an electronic controller serving to control the operation of an air-conditioning unit.

Hitherto, such control has generally been effected by means of a temperature sensor positioned to give a signal indicative of the ambient air temperature, and switching means connected to receive that signal and to control the operation of heaters and coolers in the air-conditioning unit in dependence upon whether the sensed air temperature lies in a cooling band of temperatures or a heating band of temperatures. In previous air-conditioning units, those two bands are separate and lie on opposite sides of a predetermined desired temperature, or set point. The cooling band lies above the set point, and the heating band lies below it. As a result, there may be energy-wasting hunting between cooling and heating as the air temperature oscillates above and below the set point. Such hunting is also undesirable because of the frequency with which the cooler is switched on and off.Most coolers include refrigeration compressors, and the operational life of a compressor is seriously reduced by too frequent turning on and off. Furthermore, the accuracy with which air temperature can be kept close to the set point is reduced by virtue of the two separate bands, giving rise to a relatively large overall range of temperatures allowed by the control.

It is an aim of the present invention to reduce these deficiencies. Accordingly, in one of its aspects, the invention provides a controller for controlling operation of a control unit having at least two variableadjusting devices, one for heating air or otherwise increasing a variable, and another for cooling air or otherwise reducing that variable, comprising an input for connection to a sensor to receive signals indicative of the value of that variable, and switching means, connected to the said input, for changing operation of the unit from a heating or variable-increasing mode to a cooling or variable-reducing mode when the said variable rises above a first pre-set value, as indicated by signals received by the said input, and from the latter mode to the former when the said variable falls below a second pre-set value, below the first pre-set value, so that the range between the first and second pre-set values is a coincident band for both variable-increasing and reducing modes of operation of the unit.

According to another of its aspects, the invention provides a method of controlling operation of a control unit having at least two variable-adjusting devices, one for heating air or otherwise increasing a variable, and another for cooling air or otherwise reducing that variable, comprising maintaining a variable-increasing mode of operation of the unit unless and until that variable rises above a first pre-set value whereupon the operation of the unit is changed from the variable-increasing mode to a variablereducing mode, operation on the latter mode being maintained unless and until the variable falls below a second pre-set value, which is below the first pre-set value, whereupon the operation of the unit is changed back to the variable-increasing mode, so that the range between the first and second pre-set values is a coincident band for both variable-increasing and variable-reducing modes of operation of the unit.

In one simple form of the invention, the variable-increasing mode of operation comprises switching on a heater when the temperature of the ambient air falls below the set point, and switching the heater off when the temperature rises above the set point, perhaps with a small hysteresis.

Similarly, the variable-reducing mode of operation comprises switching on a refrigeration compressor when the air temperature rises above a preset value and switching off the refrigeration compressor when the temperature falls below a lower preset value. In the case of the cooling operation, the two preset temperature values at which the refrigeration compressor switches on and off are desirably spaced apart and above and below the set point to provide a greater degree of hysteresis in order to limit the frequency of switching of the compressor. Nevertheless, if the cooling capacity of the compressor is considerably in excess of the heat generation in the space being cooled, excessively frequent switching of the compressor may still occur. To avoid this, some form of modulation of the cooling effect is desirable.One way of achieving this is by allowing simultaneous operation of the heater and the cooler when the sensed temperature falls below the set point. Frequent switching of the heater may then occur, modulating the resultant cooling effect, whilst the compressor is permitted to run for longer periods. A less energy-wasteful way of modulating the cooling effect is to bypass some of the hot refrigerant gas delivered by the compressor directly into the evaporator, instead of switching on the heater.

Preferably, the devices of the unit which are used during the variable-increasing mode of operation, and those used during the variable-reducing mode, are used in mutually exclusive manner.

Thus, when the upper limit of the coincident band is exceeded, the device- or devices for heating, for example, are inhibited and those for cooling are allowed, and vice versa when the temperature falls below the lower limit.

An example of a controller in accordance with the present invention is illustrated in the accompanying drawings, in which: Figure 1 is perspective elevational view of an air-conditioning unit fitted with a controller embodying the invention, the unit having two facing panels removed to reveal the various components of the unit; Figure 2 is an explanatory diagram showing one very simple form of control for the airconditioning unit; Figure 3 is a further explanatory diagram showing how the unit of Figure 1 is actually controlied; Figures 4 to 7 show circuit diagrams of the controller of the unit; Figure 8 is an explanatory diagram showing a further simple form of air-conditioning unit control; and Figure 9 shows a circuit diagram for a controller to effect the form of control shown in Figure 8.

The air-conditioning unit shown in Figure 1 has a rigid welded steel frame 2 and a number of components each with their own integral support members for attachment to the steel frame 2. The components comprise a direct-driven, forward curved centrifugal fan 4, a disposable or cleanable air filter 5 having an efficiency of 95% on BS 2831 No. 2 test dust, an hermetic refrigeration compressor 6 of approximately 5 h.p. capacity operating on R22 refrigerant connected to supply the refrigerant through a generously-sized copper tube/aluminium fin cooling coil with associated refrigeration controls (not shown, but placed behind the filter 5), a two-stage electric air heater 8, each stage being of 6 kw and with fully sheathed eiements and safety overheat thermostats (not shown), and a Vapac (Registered Trade Mark) electrode boiler steam humidifier 10 with an outlet connected to a humidifier steam discharge pipe 1 The compressor 6 is resiliently mounted with flexible pipe connections 12 and 1 3.

The compressor 6 and heater 8 are controlled by a coincident-band, energy-conserving, solidstate controller 14 using plug-in printed circuit boards. The controller 14 is connected to operate independence on an ambient temperature sensor (not shown) and an ambient humidity sensor (not shown). The unit is also provided with an electrical panel 1 6 with fuses, motor starters, contactors and safety interlocks. An illuminated operating display 1 8 of the unit has an on/off switch and five lights which respectively indicate whether or not the following components are in operation: (1 ) the first stage of the heater 8 (2) the second stage of the heater 8 (3) the compressor 6 (4) a hot gas by-pass or injector (not shown) of the compressor 6 (5) a de-humidifier (not shown) associated with the compressor 6.

The humidifier 10 is operated independently and is switched on when a humidity sensor (not shown) indicates that the air is too dry.

The frame 1 and components of the air-conditioning unit are enclosed by panelling 20. This includes two identical quick-release lift-off panels fitted to both front and rear of one unit (the front ones being removed in Figure 1 to reveal the components of the unit). Each panel is provided with a compressible sealing gasket (not shown) to prevent air leakage, and the interior is lined with a noninflammable acoustic material 22. Access to all components for maintenance can be gained by removal of the front panels only. Discharge grilles (not shown) are provided in the panelling according to the nature of the air-flow desired. Upflow units have horizontal grilles on the top. The grilles having inclined vanes to provide good supply air diffusion.

The scheme shown in Figure 2 illustrates theeway in which one particular circuit operates the airconditioning unit. In the event that the total heu.lvutput of people and equipment in the room or rooms being air-conditioned is not sufficient to raise the ambient temperature to the desired set point B, the unit will be set by the controller in the heating mode of operation. In this mode, the heater 8 (Figure 1 ) is switched on when the ambient temperature is below the set point B, and is switched off when the temperature rises above B. There is a slight hysteresis in the switching, to make allowance for small and rapid fluctuations in air temperatures at the sensor.If and when an electrical appliance or other heat generating device in the room or rooms is switched on, the ambient temperature will continue to rise beyond the set point B even after the heater has been switched off. As the ambient temperature rises through point C, the controller switches the unit to its cooling mode of operation. The compressor 6 (Figure 1) is turned on, and a heater inhibit is latched. The cooling which ensures counters the effect of the heat generating device that was switched on.

When operation of the heat generating device ceases, the ambient temperature will fall, eventually, below point C, and as it does so the operation of the unit reverts back to the heating mode.

Thus the heating and cooling modes of operation occur over a coincident band between points A and C.

If the cooling effect of the compressor over-compensates for the operation of the heat generating device, the heater inhibit could be omitted so that the heater will be switched on to work simultaneously with the compressor when the ambient temperature falls below point B. This, however, would be energy wasting, and a more desirable method therefore would be to retain the heater inhibit and add a latch for switching on a compressor modulator, such as the hot gas by-pass (not shown) of the unit.

The performance of the controller 1 4 actually used in the air-conditioning unit of Figure 1 is set out diagrammatically in Figure 3. In the heating mode, the first stage of the heater 8 (Figure 1 ) is switched on when the ambient temperature fails below T4, and is switched off when the temperature rises above T4. The second stage of the heater 8 is similarly switched at temperature T3. In the cooling mode, the compressor is switched on, and the hot gas by-pass is operated when the temperature falls below T4. A rise through Ts changes the unit's operation from the heating mode to the cooling mode, and a fall through T2 effects the reverse change. Thus, as with the Figure 2 scheme, heating and cooling occur over a coincident band.In addition, provision is made for a demand for de-humidification to override inhibiting of the compressor by the temperature control over the temperature range T, to T2.. In the event that the humidity sensor indicates that the ambient air contains too much water vapour, the compressor is switched on to operate on a smaller portion of incoming air than usual, so that water vapour is condensed out of the air stream. This operation will be allowed until either the humidity sensor indicates that the air has been dried out sufficiently, or the ambient air temperature has fallen below T,.

In either event, operation reverts back to temperature-dependent control alone. It will be appreciated that, if the reversion occurs owing to a temperature drop below T1, a switching-back to humidity control directly the temperature rose back above T, would bring the temperature down again, and the ensuing rise and fall in temperature in fairly quick succession would cause undesirable short cycling of the compressor. To avoil this, there is a slight hysteresis in the switching at T1, and a two-minute time delay follows switching on a rise of temperature through T, before the compressor is switched on for further de-humidification. When this occurs, the compressor will be operating simultaneousl.y with both heater stages.

A typical setting would be as follows: T, = 19.40C, T2= 20"C, T3= 21 OC, T4=220Cand T5= 250C, making the set point 21 .50C.

The circuitry of controller 14 (Figure 1) by which the foregoing switching operations are performed is shown in Figures 4 to 7. The controller is designed to be "transient proof".

Figure 4 shows the basic connections. Two connection points 1' and 2' are connected to a live terminal L' via the compressor of the unit, points 3' and 4' via the hot gas by-pass, and pairs of points 13', 14' and 1 5', 1 6' via the two heater stages of the unit. Contacts from a "make on rise" contact of a humidistat are connected between points 6' and 8', and temperature sensor type YSI 44106, having a resistance of 1 Ok ohms at 250C, between points 7' and 8'. Points 9' and 10' are connected to a neutral terminal, and a 20 vac isolated bVA across points 11' and 12'.

The circuitry in Figure 5 shows a differential amplifier 24 having its two inputs connected respectively to the connection point 7' (via a resistance and capacitance circuit) and a set point adjuster 26. The set point can be varied, by adjusting a potentiometer 28, from 1 50C to 250C. The output from the differential amplifier 24 is connected to a band width adjuster 30. The latter has a variable resistance 32 by which the proportional band width from T2 toT5 in Figure 3, can be varied from 20 to 60C. The adjuster has five outputs A.B, C, D and E, and its circuitry is such as to cause the signal at these outputs to change respectively at temperatures T5, T4, T3,T2,and T1 of Figure 3, with T2 to T6 being spaced apart equally with one third of the band width between adjacent settings, and T, spaced below.T2 by one fifth of the band width. The set point lies mid-way between T3 and T4.The logic values at the outputs A to are dependent upon the temperature indicated by the Y.S.I. sensor, and are given by the following table:

Temperature t Output t < T1 Tl < t < T2 T2 < t < T3 T3 < t < T4 T4 < t < Ts t > Ts A O O O O O 1 B O O O O 1 1 C O O O 1 1 1 D O O 1 1 1 1 E O 1 1 1 1 1 The outputs of the adjuster 30 are connected to the next part of the circuitry as shown in Figure 6.

Connection point 6', connected to the humidistat (not shown), makes a sixth input to this part of the circuitry. The six inputs are respectively connected, via resistance and capacitance circuits, to inverters 34 to 39. Outputs of inverters 34 and 35 pass to inputs of a latch 40. One output L of this latch 40 leads to a time delay device 42 and an AND gate 44, whilse the other, L, goes to an OR gate 46 and another AND gate 48. The output of inverter 38 feeds a time delay device 50, an OR gate 52 and, via an inverter 54, and AND gate 56. The output of the AND gate 56 is connected to a second input of the OR gate 46, the OR gate 52 has a second input connected to a second output of the time delay device 50, and an output connected to turn on a light emitting diode 57. The output from the inverter 39 feeds another input to the AND gate 56 and, via an inverter 58, the AND gate 48.The AND gates 44 and 56 also have inputs connected to outputs T, and""2 of the time delay devices 42 and 50 respectively. A time delay device 60 has an output connected to a third input of the AND gate 44. Four outputs H,, H2 COMP and HGB of the circuitry of Figure 6 are provided respectively by the outputs of four AND gates 62 to 65 each with two inputs. The inputs of the AND gate 62 are connected respectively to the output of the AND gate 44 and to that of the inverter 36, those of the AND gate 63 to the output of the AND gate 44 and that of the inverter 37, those of the AND gate 64 to the output of the time delay device 60 and that of the OR gate 52, and those of the AND gate 65 to the output I of time delay device 60 and that of the AND gate 48.

Finally, Figure 7 shows how the four outputs of the circuitry shown in Figure 6 ar connected to four triac outputs 66 to 69 connected to pairs of connection points 13', 14'; 1 5', 16'; 1 2'; 3', 4'; and 9', 10 of Figure 4 respectively.

Operation of the circuitry will now be described in detail.

The latch 40 behaves as a bi-stable multivibrator, and determines in which mode, cooling or heating, the unit operates. A logic value of "1 " at the output Lof the latch 40 occurs afterchanges from "1" to "0", or when the ambient temperature falls below T2 as shown by the foregoing table. The logic value of the output L of the latch 40 will therefore then be "0". The result is that the cooling mode is inhibited and the heating mode is allowed. This occurs because the AND gate 44 is supplied with a "1 '' signal from L, but the AND gate 48 receives a "0" signal from L.The AND gate 44 will allow the heater outputs H, and H2, via the AND gates 62 and 63 respectively, provided the time delay devices 60 and 42 give "1" signal outputs (which they will after the controller has been switched on for a sufficient time to allow the circuitry to settle, and a sufficient time has passed since switching of the latch 40.) Thus, H, will be at the "1" level when B is "0" (i.e. when the ambient temperature is below T4 in Figure 3) and H2 will also be at "1" when C is "O" (i.e. when the ambient temperature is below T2 in Figure 3).

This gives the desired switching for the heating mode.

In the event that A changes from "0" to "1", i.e. the ambient temperature rises through T5 in Figure 3, the latch 40 is switched so that L goes to the "0" level and L to the "1" level. This inhibits the heaters and allows operation of the compressor and the hot gas by-pass for fhe cooling mode. The presence of a "1" signal at L results in a "1" input signal to the AND gate 64 via the OR gate 46, and since the other input to the AND gate 64 is already at the "1" level by virtue of the time delay device 60, a sufficient time for this having elapsed since the circuitry was switched on, the output from the AND gate 64 is at the "1" level and the compressor is switched on by the triac output 68 (see Figure 7).

In the event that the ambient temperature falls below T4, B becomes "0" and the AND gate 48 is fed with a "1" signal by the inverter 36. Since the dehumidification switch between connection points 6' and 8' is off at this time, the inverter 39 is fed with a "1" signal and the output from the inverter 58, and the associated input to the AND gate 48, is also therefore at the "1 " level. The third input to the AND gate 48, from the output L of latch 40, is also at the "1 " as already explained by virtue of the switching of operation modes, and consequently the AND gate 65, having one input at the "1" level by virtue of the AND gate 48 and its other input also at the "1" level by virtue of the time delay device 60, switches the triac output 69 to operate the hot:gas by-pass. This gives the desired switching for the cooling 'mode.

If the ambient temperature falls through T2 (shown in Figure 3) D changes from "1" to "0" the latch 40 switches so that L becomes "0" and "L" goes to the "1" level, and the operation of the unit reverts to the heating mode.

The inputs E and 6' come in to play if and when the ambient air humidity rises above a predetermined level set by the humidistat connected between connection points 6' and 8'. This causes the voltage level at the output F of inverter 39 to change to the "1" level. The AND gate 56, already having two inputs at the "1" level by virtue of the input E being at that level, now has its third input at "1", thus feeding OR gate 46 with a "1" signal to ensure that the compressor is operated irrespective of whether the latch 40 is ordering a cooling or heating operation mode. Furthermore, the output F of the inverter 58 is switched to the "0" level thus inhibiting the hot gas by-pass by virtue of a "0" signal thus received by the AND gate 48.Moisture is thereby condensed out of the incoming air, unless and until the temperature falls below T1 (see Figure 3), in which case E changes to the "0" level causing the inverter 54 to feed a "0" signal to the AND gate 56 thereby inhibiting the compressor. Since the temperature is below T2, the latch 40 will have been switched for the heating mode, and the heater stages wiil raise the temperature back into the proportional band. The inhibiting of the compressor by input E is not removed directly the temperature rises above T1, partly due to hysteresis of the switching, and partly because of the time delay device 50. This avoids rapid switching on and off of the compressor during dehumidification.

Figure 8 shows a method of operating a simple air-conditioning unit having only one heater and one cooler, for example a refrigeration compressor. This method is a slightly modified version of the scheme shown in Figure 2. The modified version is similar to the Figure 2 scheme in that the heater is switched on and off, with small hysteresis, as the air temperature falls or rises through the set point, temperature Y during the heating mode of operation. Likewise, the compressor is turned on as the temperature rises through Z, and is turned off as the temperature falls through X, during the cooling mode with X below Z and X and Z equidistant from or symmetrically displaced around Y regardless of the width set for the coincident band X to Z.Furthermore, if the temperature is carried through and above Z following a heating mode of operation, the heater is inhibited and the compressor is switched on to change operation of the unit to the cooling mode. Where the modified method differs from the Figure 2 scheme is in the point at which changeover from the cooling mode to the heating mode occurs.

This point is at W, below point X, where the compressor is switched off, by an amount equal to half the coincident band, regardless of the width of that band or the value of the set point Y. Thus, when the compressor is switched off, the heater will not switch on unless and until the temperature falls by an amount equal to half the coincident band width. It has been found that this is more effective than using a time delay to delay switching of the heater.

Controller circuitry for effecting the method of Figure 8 is shown in Figure 9. It comprises a voltage stabilizer 80, a set point adjuster 82, a band width adjuster 84, switching logic circuitry 86, and transistor-switched compressor and heater reiays 88 and 90. The set point adjuster 82 and band width adjustor 84 are very similar to those shown in Figure 5, and will now be described in detail. It will suffice to mention that four outputs 92, 94, 96 and 98 from the band width adjuster 84 supply "1" level signals to the logic circuitry 86 as the temperature rises above the points W, X, Y and Z of Figure 2 respectively.

Operation of the switching logic circuitry 86 will now be described in detail. When the temperature rises above point Z, outputs 94 and 98 are at the "1" level. An AND gate 100 whose inputs are connected to these two outputs thus delivers a "1" signal at its output to trigger the switching transistor of the compressor relay 88. Because of the feedback line 102 to maintain the input connected to the output 98 at the "1" level, the compressor will not be switched off unless and until the temperature falls below point X to switch the output 94 to the zero level.Should the temperature continue to fall below point W, so that all the bands with adjuster outputs are at zero, and in particular the output 92, an inverter 104 connected to the output 92 provides a "1" signal to one of two inputs of a further AND 1 06. The other input to that AND gate is connected to receive a signal from a further inverter 1 08 which in turn is connected to the output from the AND gate 100. Thus the further AND gate 106 only emits a "1" signal when the compressor has been switched off and the temperature has fallen below point W.This "1" signal is fed to an input of another AND gate 110 to allow the latter to pass a "1" signal to the switching transistor of the heater relay 90 in dependence upon the voltage level of the bandwidth adjuster output 96 connected to the other input of the AND gate 110 via an inverter 112. Thus if the temperature is below the set point Y, the output 96 is at the zero level, the inverter 110 output is at the "1 " level, and the heater is switched on. Conversely, if the temperature rises above point Y, the output 96 is at the "1" level, the inverter 110 output is at the zero level, and the heater is switched off. This occurs provided the AND gate 110 is supplied with an allow signal from the AND gate 106. A feedback line 114 ensures this until such time as the inverter 108 output falls to the zero level following a rise in temperature above the point Z. A zero level signal from the AND gate 106 is then received by the AND gate 110 to inhibit the heater.

Although the foregoing description describes how the controller circuitry maintains the ambient air temperature within a pre-set proportional band in which the cooling and heating bands are coincident, it will be appreciated that the controller could equally well be used to control some other variable, for example the humidity of the surrounding air, or the temperature of a certain object rather than of the surrounding air, or the content of a given solute in a flowing solution, or any variable where one mode of operation is required to increase the variable, and a different mode, other than mere reversal of the first mode, is required to decrease the variable.

Claims (25)

1. A controller for controlling operation of a control unit having at least two variable-adjusting devices, one for heating air or otherwise increasing a variable, and another for cooling air or otherwise reducing that variable, comprising an input for connection to a sensor to receive signals indicative of the value of that variable, and switching means, connected to the said input, for changing operation of the unit from a heating or variable-increasing mode to a cooling or variable-reducing mode when the said variable rises above a first pre-set value, as indicated by signals received by the said input, and from the latter mode to the former when the said variable falls below a second pre-set value, below the first preset value, so that the range between the first and second pre-set values is a coincident band for both variable-increasing and reducing modes of operation of the unit.

2. A controller according to claim 1, comprising means for switching on the variable-reducing device when the said variable rises above a first given value and means for switching it off when the said variable falls below a second given value, lower than the first given value, to effect the variablereducing mode of operation of the unit.

3. A controller according to claim 2, in which the said first and second given values coincide with the said first and second pre-set values.

4. A controller according to any preceding claim comprising means for switching on the variableincreasing device when the said variable falls below a predetermined value, and switching it off when the said variable rises above a predetermined valve, to effect the variable-increasing mode of operation of the unit, with or without some hysteresis.

5. A controller according to claim 4, when appended to claim 2 or claim 3, in which the switching means are so constructed or set that the first and second given values are symmetrically positioned above and below the said predetermined value.

6. A controller according to any one of claims 1 to 3, for controlling a control unit having two devices for increasing the said variable, comprising means for switching on one of them when the said variable falls below a first predetermined value and switching it off when the variable rises above that predetermined value, and means for switching on the other of them when the said variable falls below a second predetermined value, which is below the first, and switching it off when the variable rises above the second predetermined value.

7. A controller according to claim 6 when appended to claim 2 or claim 3, in which the said first and second pre-set values are symmetrically positioned above and below the said first and second predetermined values.

8. A controller according to claim 7, in which the spacing between the said first and second predetermined values is the same as that between the first pre-set value and the first predetermined value and that between the second pre-set value and the second pre-determined value.

9. A controller according to any preceding claim, in which switching means are arranged to switch on the or one of the variable-increasing device to modulate the reduction of that variable during a variable-reducing mode of operation of the unit when the variable falls below a pre-defined value.

1 0. A controller according to any one of claims 1 to 8, in which switching means are arranged to switch the variable-reducing device to a less effective operating mode when the variable falls below a pre-defined value.

11. A controller according to claim 9 or claim 10 when appended to claim 6, in which the said predefined value coincides with the said first predetermined value.

12. A controller according to any preceding claim, comprising a latch connected to inhibit the variable-increasing mode of operation of the unit during the variable-reducing mode, and vice versa.

1 3. A controller according to claim 12, in which the latch comprises or behaves as a bi-stable multivibrator.

14. A controller according to claim 2 or any one of claims 4 to 1 3 when appended to claim 2, in which the said first pre-set value coincides with the said first given value.

1 5. A controller according to claim 2 or any one of claims 4 to 14 when appended to claim 2, in which the said second pre-set value is spaced below the said second given value by an amount substantially equal to half the spacing between the first and second given values.

1 6. A controller according to any preceding claim, connected to operate such a control unit.

1 7. A controller according to claim 16, in which the variable-increasing device or devices comprise a heater or heaters, and the variable-reducing device comprises a cooler, the variable being the air temperature.

1 8. A controller according to claim 1 7 when appended to claim 10, in which the cooler is a refrigeration compressor and the less effective operating mode involves switching on hot-gas by-pass means of the compressor, or opening ducting by which hot air generated by the compressor is fed into its intake.

1 9. A controller according to any preceding claim comprising a further input for connection to a further sensor to receive signals indicative of the value of a further variable, and further switching means, connected to the said futher input, for bringing about a change in the operation of the control unit to bring the actual value of that further variable closer to a desired value when it differs from that desired value by more than a predetermined amount.

20. A controller according to claim 19, in which the change in operation of the unit brought about by the further switching means overides the variable-increasing or reducing modes of operation mentioned in claim 1.

21. A controller according to claim 19 or claim 20, when appended to claim 17 or claim 18, in which the further variable is the humidity of the ambient air.

22. A controller according to claim 21 when appended to claim 18, in which the compressor is used to reduce the air humidity.

23. A controller connected to control operation of an air condition control unit, substantially as described herein with reference to Figure 2, Figures 8 and 9, or Figures 2 and 3 to 7 of the accompanying drawings.

24. A method of controlling operation of a control unit having at least two variable-adjusting devices, one for heating air or otherwise increasing a variable, and another for cooling air or otherwise reducing that variable, comprising maintaining a variable-increasing mode of operation of the unit unless and until that variable rises above a first pre-set value whereupon the operation of the unit is changed from the variable-increasing mode to a variable-reducing mode, operation on the latter mode being maintained unless and until the variable falls below a second pre-set value, which is below the first pre-set value, whereupon the operation of the unit is changed back to the variable-increasing mode, so that the range between the first and second pre-set-value is a coincident band for both variableincreasing and variable-decreasing modes of operation of the unit.

25. A method according to claim 24, using a controller according to any one of claims 1 to 23.