What You Need to Know about Heaters and Thermostats: Relays and Transformers

By Mark Watola on 11/16/2020

So far, we’ve discussed heater and thermostat voltage as well as heater and thermostat wattage. In those we briefly mention the use of relays and transformers. Now we’ll go more in-depth about their importance and touch on thermostat wiring configuration.

What happens when you attach a low voltage (12-24V) thermostat to a line voltage (120-240V) heater?

The thermostat would not handle the electrical current and ultimately overload. Relays and transformers make it possible to connect a low voltage thermostat to a line voltage heater.

You might connect a low voltage thermostat to a line voltage heater because:

  • Low voltage thermostats are common in homes
  • You don’t want to replace your thermostat
  • You need a line voltage heater for its powerful heating capability

Some heaters with less wattage, like a smaller ceiling fan heater, may not require a transformer for a low voltage thermostat.


Relays use electromagnets to open and close circuits when signaled by a low power switch or device, in this case a low voltage thermostat. When activated, the thermostat will signal for the relay to power on or off the heater. Essentially, relays increase wattage capability and would help stop the thermostat from overloading.


Transformers use electromagnets to boost or reduce voltage. Primary windings, or coils, transfer electricity through electromagnets. The current then passes the secondary windings which increases or decreases the voltage. The more coils, the higher the voltage. The less coils, the lower the voltage.

A step-down transformer, with more primary coils and less secondary coils, could safely reduce 240V to 24V, suitable for use between line voltage heaters and low voltage thermostats.

Using Relays and Transformers Together

Some instances may require both a relay and transformer. Do not use more than four relays without a transformer for one thermostat.

Note: While relays and transformers can link thermostats and heaters, Cadet does not recommend installing their heaters with smart thermostats. On the other hand, Stelpro manufactures many of their heaters to operate with their Maestro line smart thermostats.

If you still need help deciding if you need transformers or relays with your thermostat and heater, contact us!

For more information check out our blog:

The Difference Between Single Pole And Double Pole Thermostats

How to Install a Single Pole or Double Pole Thermostat to a 240 Volt Baseboard Heater

Should You Switch to an Electric Heater?

Buyers Guide to Heaters: 120V vs 240V





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