Furnaces are great for keeping us warm in the fall and winter, but what good would they be without a thermostat?
The thermostat is the main control for your HVAC system. It’s a fairly simple controller that is basically a temperature sensitive switch for your home.
Simple as it may be, there are different types of thermostats. In this post, we review the three main types of thermostats in common usage in homes.
What are the main types of thermostats?
- Low-voltage thermostats
- Line-voltage thermostats
- Millivolt thermostats
It’s important to know what type of thermostat your furnace requires. All thermostats are not compatible with all furnaces and HVAC systems. If you are going to replace your thermostat, be sure to refer to your owners manual to ensure it is safely compatible with your furnace.
Related: Common furnace problems
Low voltage thermostat
Low-voltage thermostats are the most common and versatile type of thermostats you will find in homes. They are used to control forced air furnaces (gas and electric), gas, oil or electric boilers, radiant heat systems, air conditioners, single stage or multi-stage heat pumps, hydronic baseboard heating, and other temperature control systems.
The Low-voltage will commonly run on 24 volts and is powered by a step-down transformer wired into a standard 120-volt household circuit. The voltage may be higher or lower but 24 volts is the standard.
Low-voltage thermostats include digital/programmable, mercury bimetallic, and mechanical contact types.
Line voltage thermostat
Line-voltage thermostats are powered directly by a standard 120-volt or 240-volt circuit. They are used mostly in electric resistance heating systems, for example, electric baseboard heaters and in-wall heaters.
To check if you have a line-voltage thermostat, you should check the wires. The wire leads on a line-voltage thermostat will be thick 12-gauge or 14-gauge wires, similar to what you would see on a wall outlet. Low voltage thermostats will have very thin wiring.
Within this low voltage thermostat category, there are four major types.
- Mercury contact (traditional thermostat)
- Mechanical contact
- Electronic Programmable/IP Thermostat
Millivolt thermostats are the least common in homes and used mostly in direct vent or top vent wall furnaces and floor heaters that don’t use electricity. These heating systems will not work with standard low voltage thermostats. They are not connected to the household wiring system and do not require a step-down transformer.