Millivolt thermostats are used to regulate millivolt heating systems, which are different from the popular low voltage systems and the electric powered line voltage systems. A millivolt heating system is usually a floor heater or a wall heater that doesn’t use electricity; for example, a gas fireplace or a gas-powered wall heater. Millivolt systems are regulated using thermostats specifically designed for the system, commonly known as ‘millivolt thermostats’.

In homes across the USA, there are currently 3 types of heating systems in use.

  1. Low voltage systems (like furnace, boiler, heat pump)
  2. Line voltage systems (like baseboard heaters)
  3. Millivolt systems

A thermostat is always designed to cater to one type of system, and it can not be used interchangeably. In this article, we write about millivolt thermostats and review them in detail. Before delving into thermostat models, let’s first have a quick overview of millivolt systems.

Millivolt type heaters are rare these days and no one installs a millivolt system in a new construction. About 90% of the heating systems used in America are low voltage types like a furnace heating system or a boiler type or a heat pump system. However, millivolt systems are still found in older constructions and people continue using those millivolt heaters.

Popular Millivolt Thermostat Models

Now that we’ve seen a bit about millivolt heating systems, let’s see the different millivolt thermostat models available to choose from. As I write above, millivolt systems are found only in older constructions. So you don’t have too many choices here, and you won’t find WiFi-enabled smart thermostats compatible with millivolt systems.

Note: If you’re a DIY enthusiast, you may find this article on wiring Nest thermostat for millivolt heater very interesting. Nest thermostats or any other smart/WiFi thermostats are not suitable to be used with millivolt heating systems. This guy Chris Vale tweaked things to use Nest on his gas fireplace.

Honeywell CT53K1006

Honeywell TH1100DV

This model from Honeywell (CT53K1006) is a non-programmable thermostat, designed specifically for millivolt systems like floor fireplaces or wall furnaces. This model has two wires Red and White and is compatible with standard 750 millivolt systems (and lower, say 550mV or other voltage levels ).

CT53K1006 is mercury-free and is safe to use. There are 5 temperature set points, starting from 50 degree F, range up to 90 degree F (incrementing 10 degree for each set point in between). So you can set 50, 60, 70, 80 or 90 degrees on this thermostat model.

Highlight: As I noted above, CT53K1006 is a specifically designed millivolt thermostat. Such thermostats are guaranteed to work with most of the millivolt systems. There are thermostat models designed for both 24 volts and millivolt systems alike, which may not be compatible with all types of millivolt systems out there.


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