Is your wiring compatible with a smart thermostat? Our guide to low voltage, line voltage, and more

You’ve probably heard the term “low voltage” or “24v” while researching smart thermostats – but if you’re early in your research, you might not yet know the difference.

The difference is critical: today’s best-known smart thermostats (from manufacturers like ecobee, Nest, and Honeywell) are designed only for low voltage systems. You’ll need to be sure you have the right kind of heating and cooling system before you attempt to install a smart thermostat. However, heating and cooling systems in North America take many forms.

Here’s our guide to figuring out what kind of heating and cooling system you have in your home.

You probably have a low voltage (24v) system if…

• You have 2 to 9 (or more) wires
• Those wires are low gauge (rather thin compared to other electrical wiring)
• Your wires are multicolored (red, white, green, orange, blue, black)
• You have a furnace somewhere in your house – basement, garage, bathroom, etc
• You have central air conditioning, boiler, heat pump, or split system
• You have heating vents on the floors along the perimeters of your rooms
• Your thermostat settings affect multiple rooms (ie: you don’t have a thermostat in every room of your house)

Here are some classic elements of a low voltage system:

If you have a low voltage system, congratulations – your system meets the most basic requirement for a smart thermostat! You’ll still have to see if you have the right wiring, but at least you aren’t out of the running.

The rest of the heating and cooling systems detailed in this article are not compatible with mainstream smart thermostats.

You might have a line voltage (120v) system if…

• Your wires are thick and either 2 or 4 in quantity (probably just two or four, red and black or red and white in color)
• You operate your heater(s) on a room-by-room basis
• Each room in your house has its own personal thermostat dial
• Your thermostat(s) control just one heater apiece
• Your heating system is a radiant, convection, or resistance heater
• Your place is a cabin, ski lodge, cottage, small apartment, or city condo
• Heat in your home comes from a vent in the wall or a baseboard design 

Smart thermostats are still uncommon for line voltage systems, but one manufacturer is starting to make waves in this untapped market: Mysa. Try out their compatibility checker here: https://getmysa.com/

Other heating and cooling systems that aren’t compatible with today’s smart thermostats

Mini-split systems, such as the Mitsubishi Mr. Slim

These remote-controlled wall-mounted units are called mini-split systems. They can be used to heat or cool a space. They used to be rather uncommon in North America, but seem to be proliferating thanks to their affordability and the fact that they’re easier to install on an existing structure (compared to central heating/cooling).

One of our homes (built in 2011) has this kind of heating and cooling system – and it’s not compatible with any smart thermostat on the market that we know of.

Electric baseboard heaters

Alas, electric baseboard heaters have no way to tie into any of today’s smart thermostats.

Millivolt heaters

Millivolt heaters are a nifty old technology – they run without any external power – but here’s some good news for brave people with a DIY spirit: one clever guy shared his steps to hooking a Nest up to a millivolt heater. (We haven’t tried these steps and we caution against doing anything Nest says not to do, but… sometimes you gotta do what you gotta do to enjoy modern comforts.)

Further reading

• How can I tell if my current thermostat is low voltage (nest.com)
• Line voltage or low voltage (cadetheat.com)
• Why Cadet Heat no longer recommends installing a smart thermostat with their products (cadetheat.com)