Subject:- Refrigeration and Air Conditioning

Topic:- Duct Design, Controls & Applications

Difficulty:- Medium

A thermostat is a key component of any HVAC system. It maintains the space temperature by cycling the heating or cooling equipment on and off to maintain a desired temperature which is the thermostat ‘set point’.

There are basic three types of thermostats in the HVAC industry:

a) Mechanical Thermostats

b) Digital/Electronic Thermostats

c) Programmable Thermostats

Digital thermostat

 They have a digital LCD temperature and set point display and can be designed for single or multiple stages of heating or cooling.

 They use a device called thermistor to measure temperature. This is a resistor which changes its electrical resistance with temperature. More the temperature, greater is the resistance.

 The thermostat runs current through the thermistor and measures the resistance. It uses this resistance to calculate the temperature of air in the room with help of a microcontroller.

 This temperature is then compared to preset temperature for the heater or air conditioner.

 If the temperature is too high, the thermostat sends signal to the air conditioner to turn it on. Once temperature is back below threshold level, another signal is sent to turn it off. Similar functioning takes place for a heater.

 The programmable settings for thermostat allow economic benefits since the heating can be turned down when it is not needed at some time of the day.