A thermostat is a device for maintaining a constant temperature at a desired value. For this purpose it is equipped with a temperature sensing unit which detects any deviation of the actual temperature from the desired value and transmits information on this to a device which cancels the deviation.

The most commonly used principles for sensing changes in temperature are (1) unequal rate of expansion of two dissimilar metals bonded together (bimetals), (2) unequal expansion of two dissimilar metals (rod and tube), (3) liquid expansion, (4) saturation pressure of a liquid-vapor system (bellows), and (5) temperature-sensitive resistance element.

The simplest device of this kind is the direct-acting thermostat. It makes use of the fact that nearly all liquids expand on heating (Fig.1). The thermostat itself consists of a tube filled with a liquid which expands very considerably when it is heated (Fig.2). The connection to the control device which actuates the valve in the hot-water supply is established by a capillary tube which is also filled with liquid. If the air temperature in the room under thermostatic control rises above the desired level, the liquid in the sensing unit expands, overcomes the restraining force of a spring on the valve, and throttles or closes the latter.

Due to this, the flow of hot water or other heating medium is reduced, and less heat is supplied to the room. The temperature in the room will go down after a time, so that the liquid cools and contracts. The spring load on the valve once again exceeds the pressure exerted by the liquid and opens the valve. In this way the temperature in the room is kept constant within fairly narrow limits.

The desired value of the temperature is set on a graduated scale which has been calibrated by the makers. By rotation of the screw on the control device the valve spring is compressed to a greater or less extent by the liquid, so that the valve correspondingly opens more or less. The flow rate of the hot water thus increases or decreases, causing the temperature level in the room to rise or to fall (Fig 2).

A different type of device is the indirect-acting thermostat, which uses an auxiliary source of power e.g. electricity or compressed air for transmitting the impulses for effecting the change in temperature. In a device of this kind the sensing unit actuates a contact, i.e., an electrical contact is closed (Fig.3), for e.g., an electromagnetically controlled valve of the heating system is opened or closed. If the above liquid-filled sensing unit is employed, the expansion of the liquid due to a rise in temperature closes the contact.

The electric current which energizes the electromagnetic valve will close the latter. When the temperature falls, the reverse process takes place. When the pre-set minimum temperature is reached, and the liquid in the sensing unit has contracted a certain amount, the circuit is broken, so that electromagnet now ceases to keep the valve closed. Consequently the hot water flows through the system once again.

Another type of sensing unit is the bimetallic strip (Fig.4b).This consist of two thin strips of different metals bonded one against the other to form a composite strip. These metals undergo different amounts of thermal expansion. One end of the bimetallic strip is fixed. When the temperature rises, one metal expands more than the other, causing the strip to curve, so that its free end actuates a contact (Fig.4a).

A third type of sensing unit consist of a resilient bellows filled with a volatile liquid or gas (Fig.4c). When the temperature rises, the increase in volume of the vapour or gas in the bellows causes the latter to expand and actuate a contact. A fall in temperature has the opposite effect, i.e., the bellows contracts and may actuate another contact.