A smart radiator thermostat replaces the head on a radiator valve with a small battery-powered motor that opens and closes the valve on a schedule or on command. It is a simple way to give each room its own schedule, and some models are now very cheap: IKEA's Thread-based LILJEBAGGE already sells in Sweden for 199 kronor, and heise reports a German price of €14.99 that IKEA has not yet confirmed (our news item).

Cheap hardware does not help if it does not fit the valve, cannot talk to the rest of your system or regulates the room to the wrong temperature. Those are the three problems this guide covers, in the order you will meet them.

Step 1: check what is on your radiators

Do this before you buy anything. Take off one existing thermostatic head (it comes off by hand or with pliers; the valve body stays on the radiator and no water escapes) and look at what is underneath.

A threaded valve, about 30 mm across. This is the common case. The standard thread is M30×1.5. Eve Thermo screws straight onto it, and tado, Netatmo and Danfoss all support it, in some cases with an adapter from the box.

A threaded valve, about 28 mm across. Usually M28×1.5, found on valves from brands such as Herz, Comap and MMA. You need an adapter. Netatmo includes one for M28×1.5 in the box; Danfoss sells one for the Ally (014G0264).

A clip-on valve with notches or a clamp. These are mostly Danfoss valves: RA, RAV and RAVL, each with its own adapter. Eve Thermo ships with all three. Danfoss RTD valves need yet another adapter (014G0253 for the Ally). Netatmo also supplies adapters for M30×1 and Giacomini valves.

If you are not sure which thread you have, Netatmo's help centre describes a simple check. Measure the diameter of the valve body where the head attaches, then measure the thread pitch with a ruler: if three ridges span 3 mm, the pitch is 1.5 mm. Netatmo also runs an online compatibility tool where you can look up your valve model.

Setups that will not work

Some valves cannot be motorised by any of these devices:

  • Manual valves. If the head simply turns the flow on and off and there is no pin underneath, there is nothing for the motor to push. tado's help centre lists manual valves as incompatible.
  • Return temperature limiters (RTL). They limit the temperature of the water leaving the radiator or heating circuit rather than regulating the room. A Danfoss FJVR is one example. tado's compatibility guide excludes them.
  • One-pipe (monotube) systems. tado says its thermostats are not compatible with single-pipe radiator systems.

If your valve matches none of the supported types, Netatmo recommends asking a heating professional to replace it with a compatible one. That is a plumbing job, unlike fitting the head.

Step 2: choose a protocol, not just a device

The thermostat has to talk to something. There are three broad options, and the choice decides what else you need in the house.

Zigbee. The Sonoff TRVZB and the Danfoss Ally are examples; Danfoss describes the Ally as Zigbee Certified. You need a Zigbee coordinator, either a manufacturer's gateway or a USB stick running Zigbee2MQTT or a similar open-source stack. The upside is local control and a lot of exposed settings. Zigbee2MQTT's page for the TRVZB, for example, lists temperature calibration, open window detection, child lock, frost protection and a weekly schedule that the thermostat runs itself in auto mode.

Matter over Thread. Eve Thermo and IKEA's LILJEBAGGE work this way. You need a Matter controller and a Thread border router; many smart speakers and streaming boxes combine both, and Eve lists compatible hubs on its site. Eve says Thermo uses no cloud service and stores its schedules on the device, so heating keeps to the plan even when the phone or internet is gone. Keep one detail in mind: Eve describes Thermo as a Minimal Thread Device, a battery-powered endpoint that does not relay traffic for other devices. Thermostats will not strengthen your Thread network; mains-powered Thread devices do that. If you are new to the differences between Zigbee, Thread and Matter, our explainer covers them.

A manufacturer's own hub. TP-Link's Kasa KE100 is an example: according to heise, it costs around €30 and needs its own hub. Systems like this tie you to that company's hub and app, so check what it offers for local control and integrations before you buy.

Whatever the protocol, check what the thermostat does without a connection. According to heise, LILJEBAGGE works as a manual thermostat with buttons and a display if there is no hub, and it keeps its last setting when the internet goes down. That is a reasonable minimum to expect from any device.

Step 3: mount and calibrate

After mounting, the thermostat calibrates: the motor pushes the valve pin until it closes, so the device learns the stroke. If calibration fails, the thermostat cannot regulate properly, so check the following.

  • Make sure the pin moves. Valves that sat all summer under a fixed head can stick. Eve's error codes E1 and E2 both point to the same first check: take the thermostat off and make sure the valve pin can be pushed in freely.
  • Use the right adapter. Eve lists the wrong adapter as a cause of a calibration range error (E2).
  • Short-stroke valves. Eve says around 10% of valves have a slightly too short stroke. As a test it suggests putting a 2-cent coin into the thermostat's valve holder, and if that works it offers a small adapter that sits over the pin.
  • Do not pair before calibration succeeds. On the Sonoff TRVZB, Zigbee2MQTT's pairing instructions say the device must show the temperature after calibration; if it does not, mount it again and repeat the step.

Some newer models move the valve on their own from time to time. heise reports this for LILJEBAGGE, which does it to stop limescale from building up.

Step 4: fix the temperature reading

The thermostat measures temperature right next to the radiator, so the room can end up at a different temperature than the one you set. Eve puts it bluntly in its FAQ: the temperature Thermo shows is measured "way too close to the heater" and should be ignored.

There are two ways to correct it.

An offset. Eve's procedure is a good template for any brand:

  1. Set the offset to 0.
  2. Set a target temperature, for example 23 °C, and let the room heat for 2-3 hours.
  3. Measure the temperature in the middle of the room with a separate thermometer.
  4. If it reads 20.5 °C, set the offset to -2.5 °C.

tado documents the same idea for its Smart Radiator Thermostat X, with the offset set per device in the app. In Zigbee2MQTT, the TRVZB exposes it as local_temperature_calibration, from -12.7 to 12.7 in 0.2-degree steps.

An offset is a compromise: it corrects a fixed difference, and Eve itself warns that you may have to fine-tune it over the following days.

An external sensor. A room sensor placed away from the radiator gives the thermostat a better number. tado sells the Wireless Temperature Sensor X for this, which then becomes the room's measuring device and controls the radiator thermostats. tado recommends putting it about 1.5 m above the floor, away from radiators, direct sun and draughts.

In open systems, check how the external reading reaches the valve. The TRVZB has an external_temperature_input setting and a switch to choose between the built-in and the external sensor, but Zigbee2MQTT notes that something outside it has to keep that value updated, typically an automation in your smart home controller. Zigbee2MQTT also documents the failure case: if the external sensor goes offline, the TRVZB falls back to its built-in sensor and switches back when the external one returns.

Settings worth a look

Once the basics work, a few settings make a noticeable difference. The examples come from the Zigbee2MQTT page for the TRVZB, but most thermostats have equivalents:

  • Open window detection. On the TRVZB it turns the radiator off when the measured temperature drops by more than 1.5 °C within 4.5 minutes.
  • Frost protection. A minimum temperature (4-35 °C on the TRVZB) at which the radiator turns on even when the thermostat is off, so pipes do not freeze.
  • Control accuracy. With firmware 1.3.0 or newer, temperature_accuracy sets how far below the target the valve reopens. At the default of -1 °C and a target of 26 °C, the valve closes at 26 °C and opens again at 25 °C; at -0.4 °C it reopens at 25.6 °C.
  • PID control. smart_temperature_control turns on adaptive valve control based on a PID algorithm. When it is on, the opening percentage and accuracy settings are unavailable.
  • Schedules on the device. Up to six transitions per day on the TRVZB. Eve makes the same point for Thermo: a schedule stored on the valve keeps running when the phone or internet connection is gone.

Firmware matters here too. Several of these options appeared only in later versions, and the TRVZB supports over-the-air updates through Zigbee2MQTT, so update before deciding that a setting is missing.