Which thermostat suits your boiler?

On/off or modulating: the choice that matters

Every boiler and every thermostat can work the simple way: on/off. The thermostat lets the boiler burn until the room reaches the set temperature, then switches it off, and on again when it cools. Milieu Centraal says this causes temperature swings and uses more gas.

A modulating thermostat tells the boiler how hard to burn. When less heat is needed, the boiler turns its flame down instead of switching off. The room stays more even, radiators don’t keep getting very hot, and Milieu Centraal puts the saving at about 3% on heating costs.

Not every thermostat modulates, including some clock thermostats. The user manual will tell you.

What OpenTherm means

To modulate, the boiler and the thermostat must speak the same language. The common one is OpenTherm, a communication standard that works across brands. The OpenTherm Association describes it as a manufacturer-independent system for modulating boilers and room thermostats.

Two things to know:

• Both ends must support it. An OpenTherm thermostat can’t tell a boiler that doesn’t speak OpenTherm how hard to burn.
• Some makes use their own protocol. Their own thermostats can modulate, but a thermostat from another brand may not be able to.

The boiler manual usually says which protocols it supports. That’s the first thing I check.

Clock, smart or simple

A simple room thermostat holds one temperature. You turn it down yourself at night or when you go out.

A clock thermostat (klokthermostaat) runs a programme: warmer when you’re up, cooler at night or when you’re out. Milieu Centraal’s guide to clock thermostats says it saves gas because you can’t forget to turn the heating down.

A smart thermostat adds an app, and sometimes presence detection, self-learning warm-up or control per room. Milieu Centraal says it can save around 14% of heating gas in an average home where people don’t yet heat carefully. If you already heat carefully, or the home is well insulated, a well-set clock thermostat can save as much. What a smart thermostat does add is convenience.

Whichever you choose, make sure it modulates. Getting a new boiler anyway? Milieu Centraal’s tip is to ask for a modulating clock thermostat at the same time, see boiler installation.

Wired or wireless

Most thermostats connect to the boiler with a thin two-core cable. OpenTherm uses a low-voltage, two-wire connection, and it can also run wirelessly.

A wireless thermostat still needs a receiver connected at the boiler, and the wall unit usually runs on batteries. Its advantage is that you can move it. Milieu Centraal suggests taking a wireless room thermostat to the room where you want the heat, so the boiler responds to that room.

If a cable is already in place and the thermostat sits in a good spot, wired is simple and has nothing to charge.

Check these before you buy

• Your boiler’s make and model, from the label, and what its manual says about OpenTherm or on/off.
• What’s there now: the current thermostat and how it’s connected.
• Wi-Fi where the thermostat will hang, if you want app control.
• Heat pump plans. A thermostat made for a gas boiler often can’t be used with a heat pump. See my heat pump guide.
• Where it hangs: at eye height, not on a cold wall, in direct sun, near a window or door, or above a radiator.
• The radiators in that room: no thermostatic valves in the room with the thermostat, and the valves there fully open. Other rooms can have thermostatic valves.
• Renting? Ask your landlord first. My guide to a boiler in a rented home explains who is responsible for what.

What I do when I fit one

I don’t push a brand. I check whether the thermostat you like will work with your boiler, and tell you if it will only switch on and off.

On the visit, I remove the old unit, connect the new one at the boiler and on the wall, and check the boiler responds to it. Then we go through the settings together, so you know how to change the programme. If a radiator in that room has a thermostatic valve, I’ll point it out, and we can talk about the valves along with my radiator work.

My rate is €114.95 incl. BTW per hour, with no call-out charge. Materials, including the thermostat, are extra. See my prices.

Get a thermostat fitted

Send me on WhatsApp a photo of the boiler’s model label, your current thermostat and your postcode. Please don’t open the boiler casing for a photo. If the boiler itself has a fault, see boiler repair.

If you smell gas, leave the property and call 0800 9009 from outside. If a CO alarm sounds or you suspect carbon monoxide poisoning, get everyone outside and call 112.