Low Voltage vs High (Line) Voltage: How to Identify Your Home’s Heating System

Before upgrading to a smart thermostat, it’s important to know what type of system your home uses — low voltage or high (line) voltage. The voltage type determines which thermostats are compatible with your heating or cooling system. Installing the wrong type can lead to poor performance or even damage.

What’s the Difference?

Low Voltage (12–24 V)

• Common in most North American homes.

• Controls central HVAC systems such as furnaces, boilers, or ducted heat pumps.

• Uses small control signals — the thermostat tells your system when to turn on or off.

• Wiring includes multiple thin, multi-colored wires (R, C, W, Y, G, O/B).

High Voltage (120–240 V)

• Also called line voltage.

• Used with electric heating systems such as baseboard, in-floor, or fan-forced heaters.

• These thermostats carry a higher electrical load — they directly power your heaters.

• Wiring uses thick black, red, or white wires — typically 2 or 4 conductors.

How to Tell Which Type You Have

These four checks will help you identify your thermostat type in just a few minutes.

1. Check how you heat or cool your home.

• Central systems (furnace, boiler, or ducted heat pump) → usually low voltage.

• Electric systems (baseboard, wall, ceiling, or in-floor) → usually high voltage.

• If your home has a thermostat in every room, it’s likely high voltage.

2. Look for voltage markings.

Remove the thermostat cover and check for a label:

• “120V” or “240V” → high voltage

• “24V” → low voltage

If unsure, a licensed electrician can confirm with a voltage meter.

3. Count the wires.

• Low voltage: 4–8 thin, color-coded wires.

• High voltage: 2–4 thick wires (often capped with marrets).

4. Check wire thickness.

• Thin wires → Low voltage (signal wires).

• Thick wires → High voltage (carry power directly).

Safety Tip: Always turn off power at the circuit breaker before removing your thermostat cover or inspecting wires.

Choosing the Right Smart Thermostat

Once you’ve confirmed your system type, you can choose a compatible smart thermostat:

Low Voltage (12–24 V) Systems

• Control central systems like furnaces, boilers, and heat pumps.

• Supported by smart thermostats such as Mysa for Central HVAC, Google Nest, and ecobee.

• For Mysa for Central HVAC: See Compatibility — Electrical Limits & Heater Types That Work.

High (Line) Voltage (120–240 V) Systems

Used with electric baseboard, in-floor, or fan-forced heaters.

Mysa offers smart thermostats purpose-built for these systems:

• Mysa for Baseboard Heaters

• Mysa for In-Floor Heating

• Mysa for Mini-Split Heat Pumps

Quick Reference Guide

Helpful Links

• Before You Buy

• Compatibility — Electrical Limits & Heater Types That Work

• Understanding Your C-Wire (and the Mysa C-Wire Adapter)