Will a smart thermostat work with your boiler? OpenTherm, on/off and brand buses

Step 1: how does your boiler talk to the thermostat?

There are roughly three ways:

• OpenTherm. According to the OpenTherm Association, this is a manufacturer-independent way for modulating boilers and room thermostats to communicate. The thermostat then says not just "on" or "off" but also how hot the water should be. The connection is a two-wire low-voltage link where polarity doesn't matter.
• On/off (relay). The thermostat only switches the boiler on and off, like a switch. This works with nearly every boiler, but the boiler then doesn't modulate on the thermostat's instruction.
• The brand's own system. Some boilers talk over their own bus. Bosch names the EMS bus of Nefit, Bosch, Junkers and Buderus among others for its EasyControl. Vaillant uses eBUS and makes a separate module, the VR 33 OpenTherm, to connect an OpenTherm thermostat from another brand.

How do you know which you have? Google advises looking for the OpenTherm logo on the boiler or thermostat and, if it isn't there, asking the boiler maker with the model number. Google also says that some boilers in the Netherlands need an OpenTherm adapter.

Step 2: which thermostat fits which connection?

Per brand, the makers say this:

• tado: according to tado, the Smart Thermostat X supports almost all boilers with a relay (on/off) and modulating boilers through OpenTherm. tado does say the X supports fewer modulating boilers than the older V3+. The names of the wired Smart Thermostat X and the wireless Smart Thermostat X say "OpenTherm & Relay".
• Honeywell Home: according to Resideo, the T6R works with 24 to 230 volt on/off appliances and with OpenTherm appliances, boilers and heat pumps. The T6R is a wireless thermostat with a receiver at the boiler, such as the Lyric T6R. With evohome you choose between a modulating version and an on/off version; the name says which.
• Netatmo: according to Bticino, which like Netatmo belongs to Legrand, the modulating Smart Thermostat is for OpenTherm boilers; the thermostat talks wirelessly to a relay wired to the boiler.
• Remeha: according to Remeha, the wired eTwist connects to OpenTherm heating appliances through a gateway. Mind the name: there is an eTwist with gateway and an eTwist without gateway.
• Google Nest: Google names OpenTherm for the Nest Thermostat E and the third-generation Nest Learning Thermostat, each through its own Heat Link that goes at the boiler. In April 2025 Google said it will release no new thermostats in Europe: these two are sold while stock lasts and remain supported.
• Bosch EasyControl: according to Bosch it works with the EMS bus of Nefit and Bosch among others. OpenTherm and on/off boilers need an EasyControl Adapter; with on/off the boiler then doesn't modulate.

Wired or wireless?

A wired thermostat goes where your old thermostat was and uses the existing two wires to the boiler. A wireless thermostat needs a receiver or relay at the boiler, and you can then place it anywhere in the house, on a stand or on the wall. First check whether wires are already there and where you want to measure the temperature: the thermostat controls the heating by the temperature of the room it is in.

Bridge, Thread and internet

Some thermostats need a bridge as well as wifi. According to tado, the tado X devices use Thread and need a Bridge X, a Heat Pump Optimizer X or a supported Thread border router from another brand. Netatmo lets the thermostat talk to a relay that is on wifi. Check whether a starter kit includes the bridge.

If you want to use the thermostat with Google Home, Alexa or Apple Home, that is in the name or specifications too. Will it work with my hub? shows whether your hub can handle a thermostat's standard.

Heat pump or hybrid?

Here too, the connection decides. Honeywell names heat pumps for the T6R, Bticino names OpenTherm heat pumps as a heat source for the Netatmo thermostat, and tado has its own Heat Pump Optimizer X. With a hybrid heat pump, the heat pump's manual lists which thermostats fit; check there first.

Room by room: smart radiator valves

To control each room separately, you add smart radiator valves. tado says most radiator valves with an M30x1.5 thread need no adapter and that an RA adapter is in the box. Danfoss supplies adapters for RA, RAV and M30x1.5 among others with the Ally; the Ally works with Zigbee 3.0 through its own gateway or another Zigbee system. Radiator valves usually work within the system of the same brand, so choose them to match your thermostat: tado Radiator Thermostat X with a tado X, Netatmo radiator valve with Netatmo. All models are under smart thermostats.