How-to guide
Line Voltage vs Low Voltage Thermostats: Check Before You Buy
By Ismail Farooq · Updated 2026-10-29
Nest, ecobee, Honeywell T-series and Amazon thermostats are low voltage (24 V). Connecting one to a line-voltage circuit destroys it instantly and is a shock and fire hazard.
Signs you have line voltage
- Electric baseboard or wall heaters with a thermostat on the wall.
- Thick wires (like lamp or outlet wire) joined with wire nuts instead of thin wires on small screws.
- Labels such as 120 V, 240 V, L1, L2, LINE, LOAD on the old thermostat.
- Only two to four heavy wires, usually black and red (and sometimes white).
Smart options for line voltage
Look for thermostats built for it, such as Mysa or Sinopé baseboard models, and match their amperage/wattage rating to your heaters.
Millivolt systems
Gas wall furnaces, floor furnaces and fireplaces that work during a power outage are often millivolt systems powered by the pilot light. They need a millivolt-rated thermostat; 24 V smart thermostats won’t work.
Choosing a line-voltage smart thermostat
- Match the voltage. Baseboard heaters are usually 240 V in North America, sometimes 120 V. The thermostat must be rated for the same voltage.
- Check the load. Add up the wattage of every heater on that thermostat (it’s on each heater’s label) and divide by the voltage to get amps. For example, two 1,000 W heaters at 240 V draw about 8.3 A. The thermostat’s current or wattage rating must be higher than that.
- Double-pole vs. single-pole. A double-pole thermostat can disconnect both hot wires on a 240 V circuit when turned fully off; a single-pole one breaks only one. Replace like with like unless an electrician advises otherwise.
- Fan heaters and radiant ceilings. Some models aren’t approved for fan-forced heaters or radiant ceiling cable. Check the product’s compatibility list.
How to recognize a millivolt system
Millivolt thermostats are usually connected with just two thin wires to a gas valve with terminals marked TH, TP or TH-TP, and the appliance keeps a pilot light burning. There is no transformer and no 24 V at the wires. If you see this, look for a thermostat sold specifically as millivolt-compatible.
When to call an electrician
If the thermostat box has aluminum wiring, scorched or brittle insulation, more wires than the old thermostat’s diagram shows, or you aren’t sure the breaker you turned off is the right one, stop and get an electrician. Line-voltage work is mains wiring, not low-voltage thermostat wiring.