Residential heating and air conditioning systems use a variety of controls. Thermostats, aquastats, relays, timers or sequencers, are basically on and off switches.
Thermostats can be mechanical or electronic. Mechanical thermostats make use of the expansion rate of different metals. By welding two flat pieces of dissimilar metals together to make a bar, movement will occur favoring one side or the other as the bar is heated or cooled. Using the bar as a lever, with one end fixed in place, movement of the other end from temperature changes can be used to switch the heating or cooling on. Some manufactures attach glass bulbs with a bead of liquid mercury and electrical contacts inside to the lever for more accurate temperature sensing and switching. Correct mounting of a mercury bulb thermostat is critical to its accuracy. Dust can also affect performance. Mechanical thermostats are usually adjustable, but require special tools.
Most electronic thermostats are programmable. They use a thermistor or special sensor to determine room temperature and circuitry to switch the heating or cooling on and off. Some electronic thermostats are adjustable. The installation instructions will either explain the procedure or offer a phone number for technical help. Batteries can be used to power the thermostat or be used to retain programming in case of power outages.
Before removing or replacing a thermostat, determine the type of power supplied to it. Most homes with electric baseboard or wall cavity electric heat have thermostats that are fed with line voltage. Power must be turned off first to avoid injury. Deadly high voltage conditions could exist. Central heating and air conditioning systems use low voltage thermostats. Low voltage does not present a safety hazard, but damage can occur to the thermostat or other controls when disconnecting. Turn the power off.
Programmable thermostats offer new methods of conserving energy and increasing comfort. Installation can be done by the homeowner, but there are different thermostats for each type of system. Zoned systems with the selector switch (heat-off cool) at the thermostat require a thermostat with changeover terminals. Heat pumps need a two stage thermostat. Fuel burning thermostats might need a thermostat with extra terminals if equipped with air conditioning.
A programmable thermostat combines convenience with conservation. It may be programmed to conserve energy by lowering the demand for heat during inactive hours. Users may program the heat to remain at a lower temperature at night while they sleep and restore it to room temperature just before they get out of bed. Likewise, the thermostat can turn the heat down right after a person leaves for work and have it back up to room temperature when they return from work.