2.1: Thermostats – General

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Page ID: 18024

Aaron Lee
BC Campus

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7 Thermostats – General

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A BCcampus element has been excluded from this version of the text. You can view it online here: https://opentextbc.ca/basichvac/?p=39

Thermostats are what we refer to as “temperature-actuated switches,” meaning they will automatically open or close their electrical contacts upon a change in ambient temperature. This means that we can set the thermostat in the room to a comfortable level and allow the heating or cooling system regulate the temperature.

Thermostats that control heating loads will close their contacts when the temperature falls below its set point, while thermostats controlling cooling loads will energize upon a rising temperature. Some systems will allow a single thermostat to control both heating and cooling apparatus.

Normally closed contacts, normally open contacts, and normally closed and normally open contacts. — Schematic symbols for temperature-activated switches.

Inside the average residential home, thermostats are generally found in two varieties: line-voltage thermostats, which directly control the flow of current to the load, or low-voltage thermostats, which indirectly control current to the load.

Regardless of the type of thermostat used, it must be rated for the voltage and current that it is expected to operate at and control.

The location of thermostats is critical as well. As a rule, we place our heaters, either baseboard heaters or air vents, near the outside of rooms and near windows and other areas of high heat loss. For the most effective heating of a room, thermostats should be installed on the opposite wall, or as far away as possible from a heat source.

If the thermostat was installed directly above the heater, for example, it would sense a high temperature and switch the load off long before the room heated up to a desirable level. By installing it on the opposite wall, we ensure the heat has to travel through the whole room before the sensor opens the circuit, providing more balanced heating.

Some thermostats will have the temperature sensitive portion of their apparatus installed remotely from their electrical contacts. This type of thermostat is commonly used for in-floor heating and certain industrial processes, where temperature monitoring equipment is impractical to install.