Types of Thermostats

Electromechanical or Manual thermostats: These are old-fashioned mercury units which consist of internal coils to expand or contract in response to temperature change. These types of thermostats are getting obsolete because of following two reasons:

1. Digital thermostats are able to more accurately register and respond to temperature changes
2. Purchase of products containing mercury has been restricted or banned in many states

In spite of above mentioned reasons manual thermostats remain popular owing to their low cost, the familiarity of their controls' design and ease of their use.

Manual digital thermostats: They make use of an electronic temperature sensor to register changes in the room temperature. These changes are then compared with the settings selected by the user. If there is a difference between the room temperature and the set temperature, a command is sent to the heating or cooling system necessitating action. For manual digital thermostats we need to physically adjust the setting of our preferences whenever we would like to adjust the room temperature. New digital thermostats have no moving parts to measure temperature and instead rely on thermistors or other semiconductor devices like a resistance thermometer or resistance temperature detector. They use either a relay or a triac to act as switch to control the HVAC unit. Units with relays will operate millivolt systems, but often make an audible "click" noise when switching on or off.

Programmable digital thermostats: These are up gradation of the digital manual thermostats and also referred to as setback thermostats or clock thermostats. They are more convenient to use and can help save energy costs. Once we program the thermostat to fit our lifestyle and schedule, the thermostat works automatically year-round. We simply need to program the temperatures into the memory of the unit, along with the times of day that we would like the changes to occur. Based on the model, a thermostat can be programmed in following three ways:

1. 5 - 2 programming: Maintains a set schedule for five days (weekdays), and then switches to an alternate schedule for the other two days (weekends).
2. 5 - 1 - 1 programming: Maintains a five day schedule (weekdays), then switches to first individual day (Saturday), then second individual day (Sunday).
3. 7 day programming: Allows the maximum flexibility for those with frequently-changing schedules by allowing us to program each day individually.

Other Advanced Thermostats: Various highly developed models have touch screens and the aptitude to work with home automation or building automation systems. More expensive models have a built-in PID controller, so that the thermostat knows in advance how the system will react to its commands.