controlling radiant heat with arduino


I am new to the community! Excited to build my first project!!
I need help or guidance with the HW side of things.

I have radiant in-floor heat installed throughout my house. There is a thermostat in every room, that is connected to an actuator which sits on top of a manifold and opens it and closes it when thermostat calls for heat. When the actuator opens it also starts a pump for that manifold. The thermostats run on 24VAC. The challenge I have is that these thermostats are pretty dump, you can only set a temperature and that’s it. I thought of replacing them with smart/programmable thermostats, but that is going to cost a lot, as I have 20+ throughout the house. So instead I am looking to build an arduino project.

My goal:

  • Leave the thermostats where they are and functional, but connect arduino to each section so that i could call for heat either from the thermostat or from arduino, i.e. Thermostat would be set to 68F, but I would call for heat from arduino (i don’t need to read temperature as I am accustomed to my infloor heat and I know i need to call it for about 30mins in certain sections to get to desired temperature), and then stop calling for the night from arduino and if the temperature dips at night the thermostat would call for heat on its own.

The thermostats are wired with 2 wires, and are powered thru these wires with 24VAC. Can i just simply plug arduino relay board into these two wires, and control the relay to fire on/off? If not how else would I need to wire this in.

Details of current design:
The thermostats I am using are:
Uponor a3030101 (
they say the thermostat uses two modes " differential mode and pulse width modulation (PWM) mode" not sure what it mean or if it matters

See attached diagram I found online, which is almost identical with my wiring, the only difference is that my thermostats are wired to each actuator and here they have two thermostat controlling all of the actuators so either one can call for heat. So same concept. See the red diagram, would this work? would I need anything else in the schematics or simply connecting the wires here would work?

The question is, if the relay option is Ok, can I simply connect to the same wire nuts where the 2 inputs are from thermostats are wired from?

The thermostat used to be a switch with bimetallic with quicksilver. The bimetallic would change the tilting and the quicksilver makes contact or not. You can use a relay parallel with it, to call for heat.

These day, it is much more complicated. The heating of a floor is with certain (low) temperatures and certain pressure. The thermostat is often a modular type in Europe. For those it is not possible to use a relay.
When many zones are created, the thermostats could still be like the old on/off types (Honeywell has simple thermostats with a super-cap so they don't need a battery), and the on/off controls a valve for that zone. And every thermostat calls for heat.
A single pump is often regulated by the central heating unit, and a over-pressure valve is used, in case all valves to the zones are closed.

That thermostat in the link has already two different modes. PWM or differential. I think the differential mode is like in the old days.

I don't understand your drawing. I was expecting that a single thermostat is connected to a single actuator.

Thank you for quick response.

sorry for the confusion with the diagram. See attached new diagram, this is exactly my current setup with many more Tstats connected to A (thermal actuators).

Based on your answer I am not sure if it is OK to add the relay where I am proposing or not. If yes, for this setup, is my understanding correct that I would connect it as “Normally Open”?


If you are certain that the thermostats are ON / OFF only and don't supply a varying voltage or current or contain a current limiting resistor, then a relay should work, red to NO and brn to COM.

That drawing I can understand. I agree with outsider.
Suppose that everything is as we expect it to be, but a bug in the sketch activates the relay every second. That could damage the central heating unit. It can be dangerous. Perhaps you should change the relay not faster than every 10 minutes.