Controls Used in Air Conditioning Systems: Thermostats, Humidistat, Airstats

Controls Used in Air Conditioning Systems: Thermostats, Humidistat, Airstats
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The main purpose of the air-conditioning system is to create comfort conditions by controlling the temperature, humidity and flow of air inside the rooms. To provide the comfort conditions throughout the year, the air-conditioning systems are fitted with three controls: temperature controls, humidistats and airstats that control the temperature, relative humidity and flow of air inside the room respectively. Let us see all these in more details.

Temperature Control Devices – Thermostat

The temperature control devices maintain the proper temperature inside the room as per our comfort requirements. When the desired temperature is reached these devices switch off the compressor of the air-conditioning system and when temperature goes away to certain extent from the desired levels the compressor is restarted. The setting of the temperature is done by the user manually or automatically using the remote control devise of the air-conditioning system.

One of the most commonly used devise for controlling the room temperature is the room thermostat. It senses the temperature inside the room and causes the electrical contacts to the compressor open or close depending on the temperature inside the room. There are various types of thermostats, but the most commonly used is the bimetal room thermostat. It comprises of two metals which at particular temperature are of same length, but when their temperature is increased one metal increases in length more while the other increases less due to their different coefficients of expansions. When the bimetal is heated it tends to get bent which helps in opening or closing of the electrical supply to the compressor.

See the image below (source:

Thermostat (Source:


Humidistats are the devices that help controlling the humidification equipment fitted in the air conditioning system. When the air is heated its humidity is reduced and it may be desirable to add humidity to it. In some cases when the temperatures are very low, it may be necessary to heat the air and add humidity to it. In certain cases it may be necessary to remove the humidity from the air. Humidity is usually added in the air washers and it is usually removed by reducing the temperature of air below its dew point temperature or using some drier. The operation of these equipments is controlled by the humidistat.

The humidistats can be used either on line or low voltage. They consist of the nylon ribbon which is highly sensitive to the humidity inside the room. The ribbon wound on the bobbin acts as the control for the operation of the humidification or dehumidification equipment.

Nortec Humidistat


The airstats do not control the flow of the air through the air conditioning systems; rather they act as the fan safety cut-off devices. There are two types of fans that can be used in the air conditioning systems. The first type of fan absorbs the cool air from the cooling coil and throws it in the room to be cooled. The second type of fan absorbs the hot return air from the room and sends it to the cooling system. In the second case, sometimes the temperature of the return air becomes so high that there may be possibility of occurrence of fire. The airstat stops the operation of the fan in such cases.

The temperature control devise thermostat is used in all the air conditioning systems, while humidistats and airstats are fitted in equipments based on the type of the air conditioning system being used and its applications.