How does a thermostat work?

When it's cold, we walk over and turn up the thermostat, and when it is hot, we walk over and turn on the air conditioner to cool us down. But, how does that little box on the wall actually control the temperature of our homes or offices? A mechanical thermostat is actually a very simple device. It is basically a thermometer attached to a switch that turns on your heater, whether your heat source is natural gas or electricity. If you remove the cover of your thermostat, you will see the inner workings and get a better idea of how a thermostat works. The top layer holds a mercury switch and a thermometer coil. The mercury switch is just a small vial filled with the liquid metal, mercury. Within this vial are three wires: one at the bottom of the vial; one at the left of the vial; and one at the right of the vial. As the temperature rises or cools, the vial of mercury gets tipped to the right or the left making the corresponding wire come into contact with the wire that runs along the ...  more

Thermostats control nearly all types of heating and cooling equipment, keeping room temperatures within a set range. By doing so, they ensure comfort, cut energy waste, and offer considerable convenience. A thermostat has a temperature sensor and an activating switch. Some are mechanical, others electronic. Not all thermostats work with all types of furnaces and heaters; a forced-air thermostat, for example, won't necessarily work with a heat pump. Most thermostats are either manual, electronic or "line-voltage" (used with electric baseboard and wall mounted heating). Programmable, electronic thermostats couple sensors with circuitry to do the job. Typical programmable thermostats can handle both heat-only systems-such as forced-air, steam, hot water, and, in some cases electric-and complete air-conditioning systems. Most are like minicomputers that can automatically align heating and cooling to changing daily needs, eliminating unnecessary energy usage and reducing energy costs.

A thermostat is a closed valve, held shut by spring tension. Once the coolant gets hot, a wax filled element in the upper part of the thermostat opens the valve. The heat melts the wax and expands it, that pushes the valve open. The thermostat is fully open at 20F above the rated temperature. As soon as the coolant temperature decreases, the wax cools off and contracts, allowing the valve to close. The thermostat ensures a constant operating temperature of the engine by open and shut the coolant circuit.  more

A thermostat's basic function is to regulate the operation of heating and cooling equipment in your home. You set the thermostat to the desired temperature and the thermostat turns the equipment on and off to maintain that temperature. A thermostat is essentially an automatic switch. The thermostat senses the temperature and turns the air conditioner or heater on and off as required to maintain the desired temperature. There are two basic types of thermostats: electromechanical and electronic. They both do the same thing but they do it in different ways. Additionally, of the two basic types, there are several varieties from which to choose. An electromechanical thermostat typically uses a bi-metal coil or strip which moves as it expands and contracts with temperature changes. Mounted on the bi-metal strip is a glass vial partially filled with mercury. As the vial is tilted on the back on the bi-metal strip, the mercury flows to the end of the vial where two electrical contacts are ...  more

Most central heating and cooling systems are connected to a thermostat on a low-voltage DC current. The furnace or air conditioner waits for a current on this DC line in order to operate. So when there's current on the thermostat line the furnace or AC runs, and heats or cools the house, and when there is no current, the furnace or AC stops adding heat to or extracting heat from the system.  more

Materials • Honeywell Thermostat (Model number T87F 2873) • Hair dryer with both heating and cooling options, or other heating/cooling method Procedure • The thermostat must already be mounted vertically on a board or backing that is firmly placed. The students should not handle the thermostat, but they should carefully examine it. • Locate the bimetallic coil and the mercury bulb. Sketch a diagram of the thermostat in your data table, labeling any important parts. • Plug in the hair dryer. Blow hot air on the bimetallic coil. Observe what happens and record your observations. • Switch the hair dryer so that it blows cold (or room temperature) air. Observe any changes in the thermostat structure and record these. • Alternate the temperature of air on the thermostat. Observe the time it takes for the change in temperature to affect the position of the glass bulb.  more

2003 Virginia SOLs • PS.1 • PS.7 Objectives Students will • understand the various parts of a thermostat, including the bimetallic thermometer and the mercury-filled bulb; • visualize the effect of hot and cold air on the thermostat; • understand how a thermostat is connected to a circuit that regulates the heat produced in a house.  more