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:

The wall wart is a classic design still common throughout North America. If you have one of these, you’ve almost certainly got a low voltage HVAC system.
Thermostat backplate with low gauge wires hooked into three terminals
Or, maybe you’ve got a programmable thermostat from the 90s. You should be able to pop the entire thermostat off its base and see what’s behind it. 24v HVAC systems are often wired with multi-colored thin wires that are flexible.
Photo of a 2010 Rheem furnace
Homes with central heat often have a furnace in the basement, garage, closet, or other “utility” area. This furnace heats the entire dwelling.

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 
Honeywell line voltage dial
This simple dial design is typical of line voltage heating systems.
Line voltage heating vent
Large vents that blow hot air are another typical sign of a line voltage heating system.

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:

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

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

A Mitsubishi Mr Slim unit – these heating and cooling systems are more affordable than central heating and cooling, and are found mounted close to the ceiling.

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

Common in North American apartments, this heater has a dial right on it – and it’s incompatible with smart thermostats.

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

Millivolt heaters

Millivolt heating is industrial revolution era heating technology, but that doesn’t mean it’s no good – it’s just not compatible with any of today’s smart thermostats.

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

12 replies on “Is your wiring compatible with a smart thermostat? Our guide to low voltage, line voltage, and more”

Thank you for writing this guide…it helps a lot. This article answered many questions the customers of Vine Smart Thermostat. I would like to have permissions to have a link to this article from our support webpage….Thanks again.

Hello Hanshan,

You have my permission to link to this page from your support webpage. Thanks for visiting!

I have a 3 low voltage Honeywell 5 + 2 day programmable thermostat with ONLY a red and white wire. what i need to know is if there is a wifi smart thermostat that can manage with 2 wires and not add the C wire – i don’t have a basement or attic and all the ceilings are finished. not about to make holes to run 3 new wires.

Hey there!

I just wanted to reach out to draw your attention to our new product, the Mysa Smart Thermostat! It was designed for line voltage heating systems like electric baseboard and fan-forced heaters.
If you have any questions please shoot me an email ( or have a look around our site:


Hi, I just got a Honeywell Lyric T-5 smart thermostat. The instructions said if you don’t have a C wire, but have an extra wire, to hook the extra wire to the C terminal at the air handler and the same extra wire at the thermostat to the C on the smart thermo. It didn’t work. Won’t lite up the thermo. There is another white wire on C at the air handler probably going out to the condenser along with a red wire. Any idea why this didn’t work?? Thanks

Hi thanks for all this info it’s helpful. I’m trying to figure out what smart thermostat we can use. We bought the Nest E only to find out it won’t work with our system as we have a Millivolt System. Is there anything we can buy that will work? Thanks for the help!

I currently have a lux tx500e 7-day programmable thermostat (has batteries). The manual says it only runs on 24 volt furnaces and no other (higher) voltages. There is no option to connect a C wire, nor is there a C wire in the wall. I only have 2 wires connected to the base plate (and have no other wires). I also have baseboard radiant heaters (have a hot pipe running through the middle of grates covered by a metal housing. Can I use a smart thermostat ? Which smart thermostat do you recommend. My furnace is in the basement. I am a renter and do not have access to the furnace. . . which is located in the basement. Also, I would not be able to run wires.

I have a WaterFurnace Premier 2 geo thermal system, and I’d like to upgrade the 8wire TA32E12 to a new WiFi enabled tstat. I’d like all the bells and whistles of new tech, such as remote smartphone control, weather forecasting, and data collection if possible. Any suggestions?


I have a Honeywell chronotherm IV plus programmable thermostat similar to the one in the picture with three batteries.. except without the batteries.

The thermostat just connected won’t power up and I just noticed the C wire only has 12 volts AC against the R wire.

Jumping the R-W does engage the furnace
And R-G does engage the summer fan.

But the thermostat itself needs 24volts ac.

It’s a vulcan gas ducted furnace installed new in 2000 yr.

Can I just step up the voltage… or have I done the wiring wrong.

4 Wire cable is about 15 feet long and correct 24 AWG.

Thank you very much for showing very much information on this webpage. I really learn very much on your teaching. It’s just a whole class for a day. I would treat you a beer if I see you some where, 🙂

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