Can Mysa Smart Thermostat control high-wattage electric in-floor heating with a relay?

Last updated: July 6, 2026

Yes, in some cases. The Mysa Smart Thermostat (Low Voltage) can control an electric in-floor heating system when it's paired with a properly rated relay (or contactor), an external 24V transformer, and a Mysa C-Wire Adapter. Together, these separate Mysa's low-voltage control circuit from the high-voltage heating load.

This is a custom setup, not a standard swap. Mysa doesn't directly replace a line-voltage in-floor thermostat here. Because your heating load stays at line voltage, a qualified electrician or HVAC professional should design and install the relay and transformer.

When this setup works

This configuration works when all of the following are true:

  • Your floor-heating load is switched by a relay or contactor rated for your system's voltage and wattage.

  • Your Mysa Smart Thermostat is powered by a suitable 24V transformer.

  • A Mysa C-Wire Adapter provides isolation between the thermostat's power circuit and the relay/control circuit.

  • Separate transformer sources stay isolated (for example, using separate Rh and Rc connections where applicable).

Connecting a floor sensor

If your installation uses a compatible underfloor temperature sensor, connect it to the thermostat's S1 and S2 sensor terminals. This works the same way as the floor-sensor input on Mysa for In-Floor Heating. Even so, have your installer confirm that the sensor and the rest of the system are compatible before relying on it.

Sizing the relay correctly

The relay must be rated for the heating load it controls. For example, a 240V electric in-floor system drawing around 3,800W needs a relay or contactor rated for that voltage, wattage, and continuous heating load. An undersized relay can overheat or fail, so this step matters.

What to have ready

To confirm whether this setup will work for your system, gather:

  1. Your heating system's voltage and total wattage.

  2. Photos or model numbers for your relay/contactor, transformer, and existing thermostat wiring.

  3. Whether your floor sensors are already installed, and their rating if you know it.

  4. Confirmation that a qualified installer is handling the high-voltage wiring.

Prefer a simpler option?

If you'd rather install a direct line-voltage in-floor thermostat without building a custom relay setup, we recommend Mysa for In-Floor Heating instead — as long as your system falls within that product's supported load and wiring requirements.