Boiler not responding to the thermostat: on/off, OpenTherm or a broken wire

Short answer: if your boiler does not respond to the thermostat, it is usually the connection and not the boiler. Check four things: fresh batteries and a set temperature above room temperature; the two thermostat wires (with on/off: bridge the wires at the boiler; if it fires, the boiler is fine); whether the thermostat speaks your boiler's protocol (on/off, OpenTherm or a brand-specific bus such as EMS or eBus); and, if wireless, whether the receiver is still paired. A modern modulating boiler with a simple on/off thermostat does work, but uses noticeably more gas.

Three types of control

A on/off thermostat simply closes a contact: the boiler goes fully on or completely off. An OpenTherm thermostat talks to the boiler and tells it how much heat is needed, so the appliance modulates and can run at a lower temperature. In addition, manufacturers have their own protocols, such as EMS for Bosch and Nefit or eBus for Vaillant and Remeha; these only work with thermostats of the same brand or with an interface module.

This distinction matters when you replace a thermostat. A universal OpenTherm thermostat on a boiler with its own protocol will not work without a module, and conversely, with an on/off thermostat you lose the modulation of a modern boiler.

The boiler does not respond: what to check

Start with the thermostat itself: fresh batteries, display on, set temperature higher than room temperature. It sounds obvious, but it is the most common cause of a cold home.

Then move on to the wire. Two thin cores run from the thermostat to the boiler, often undisturbed for years behind skirting boards. A kinked or worn-through core looks exactly like a faulty boiler. With an on/off system you can rule this out by briefly connecting the two wires at the boiler: if the appliance then fires, the boiler is fine and the problem is in the wire or thermostat.

If the connection is digital, the boiler often reports it itself. Vaillant shows F.12 when there is no signal from the user interface, Bosch reports 1010 O for an interrupted EMS bus, AWB reports 1.1 for a communication fault. More about this in our guide on PCBs and communication faults.

Wireless: its own pitfalls

With a wireless thermostat there is a receiver at the boiler. If it loses the connection, the boiler often falls back to an emergency mode: a fixed temperature continuously, or nothing at all. Check whether the light on the receiver shows the correct status and whether the thermostat is still registered. With some brands, the devices have to be paired again after changing the batteries.

Do not place the thermostat above a radiator, in the sun or next to an outside door. It then measures its immediate surroundings rather than the room, and it seems as if the boiler is not doing what is asked of it.

What does a thermostat or module cost?

Part	Manufacturer number	Price
Vaillant VR33 OpenTherm module (enables OpenTherm on eBus appliances)	8000020151	€ 71.95
Itho Daalderop Spider smart thermostat, OpenTherm	03-00476	€ 132.95
Nefit room thermostat	87183102980	€ 184.95

Individual thermostats and controls can be found under thermostats and controls; complete smart thermostats are in the category thermostats.

Why OpenTherm saves gas

A modulating boiler can reduce its output to sometimes a fifth of the maximum. With OpenTherm it makes use of this: with a small heat demand it burns low and long, with a low flow temperature. It is precisely that low temperature that makes the flue gases condense, and that is where the high efficiency comes from. With an on/off thermostat the boiler keeps switching fully on and off again; that is less efficient and causes more wear because of the number of starts.

If you plan to install a heat pump later, a low flow temperature is the direction to go anyway. Independent advice on energy-efficient heating can be found at Milieu Centraal.

Frequently asked questions

Why does my boiler not respond to the thermostat?

Usually because of flat batteries, a broken or loose thermostat wire, a thermostat that speaks a different protocol from the boiler, or a wireless receiver that has lost the connection. With an on/off thermostat you can test this by briefly connecting the two wires at the boiler: if the boiler then fires, the problem is in the wire or thermostat. Digital connections often report it with a code, such as Vaillant F.12 or Bosch 1010 O.

Does every smart thermostat suit my boiler?

No. First determine which protocol your boiler speaks: on/off, OpenTherm or a brand-specific bus such as EMS or eBus. With a brand-specific bus you need a thermostat of that brand or an interface module. If it is not in the manual, look at the rating plate and ask us.

My boiler is running continuously at maximum. How can that be?

Often because the boiler is no longer receiving a valid signal and falls back to a safe emergency mode, or because an on/off thermostat has been connected incorrectly. Check the wiring and the maximum output setting.

Can I replace the thermostat myself?

Yes, this is low voltage and is not part of the gas section of the boiler. Disconnect the boiler from the power, note which wire was on which terminal and work carefully. If you are unsure about the protocol, ask beforehand.

Why is my house not getting warm while the thermostat is asking for 21 degrees?

Then the control is probably working but the heat output is not. Check whether the radiators get warm; if they stay cold, read our guide on radiators that stay cold.