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

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:

Recommended Reading

Electric heaters offer many advantages over gas heaters. Easy installation, long life, higher safety, and a higher efficiency make these heaters a great option for your home.

A single pole thermostat features two electrical wires and no off feature. A double pole thermostat features an off function and four electrical wires.

When choosing a heater, you need to choose the right voltage: either 120V and 240V. Read our guide to learn the differences between 120V and 240V and how to figure out which one you need.

A pole refers to the number of separate circuits that a switch operates off of. Each thermostat operates differently and offers their own separate benefits!

Mark Watola
Mark Watola

Mark graduated with a B.S in Communications from Kennesaw State University in 2020. Enlisted in the United States Marine Corps from 2012 to 2016, Mark operated as a Photojournalist and Correspondent at the Marine Corps Base Camp Lejeune Public Affairs Office. Learning from his time in the military, Mark prides himself on having an adaptable and mission-based mindset with a willingness to work cooperatively to craft quality content.

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