How-to guide
Smart Radiator Valves (TRVs): Fitting, Adapters and Boilers
By Ismail Farooq · Updated 2026-10-30
In homes heated by radiators, which covers most of the UK, Ireland and continental Europe, the easiest smart upgrade is often on the radiator itself. A smart radiator thermostat (a smart TRV) replaces the numbered head on a thermostatic radiator valve with a battery-powered motor that you can schedule room by room. There’s no mains wiring, and in most homes no plumbing either.
What you’re replacing
A thermostatic radiator valve has two parts:
- The valve body, plumbed into the pipe at one end of the radiator. A small spring-loaded pin sticks out of it; pushing the pin in closes the valve.
- The head, the part with numbers (usually * and 1 to 5). It pushes on the pin as the room warms.
A smart head swaps in for the old head only. The valve body, the water and the pipes stay as they are, so you don’t need to drain the system. If a radiator has a plain manual valve (a wheel-head with no numbers) instead, it needs a new TRV body first, which is a plumbing job.
Will it fit? Valve threads and adapters
| Valve connection | Where you see it | What you need |
|---|---|---|
| M30 x 1.5 thread | The most common in the UK and Europe (for example Heimeier, Oventrop, Honeywell and many unbranded valves) | Usually fits directly |
| Danfoss RA (clip-on) | Very common in the UK, Scandinavia and on Danfoss-fitted systems | An RA adapter, included with most smart radiator thermostats |
| Danfoss RAV, RAVL, RTD | Older Danfoss valves | A specific adapter for that type; check the brand’s adapter list |
| Caleffi, Giacomini, Comap, Herz and others | Common in Italy, France and parts of Europe | A brand-specific adapter, sometimes sold separately |
The valve maker’s name is usually stamped on the body or printed on the old head. Take a close-up photo of the valve with the head removed and compare it with the smart thermostat maker’s adapter guide before you buy a full set.
Fitting a smart radiator thermostat
- Turn the old head to its highest setting (5 or max). That takes the pressure off the pin so the head comes off easily.
- Remove the head. Most unscrew with a collar; Danfoss RA heads come off by releasing a clip ring. The valve stays sealed; no water comes out, and the radiator will heat fully with no head on.
- Check the pin. Press it gently with your thumb or a flat object: it should spring back. A pin that doesn’t move (common after a summer off) can often be freed with a few gentle taps or a little penetrating oil. If it stays stuck, the smart head can’t control the radiator.
- Fit the adapter if you need one, then screw or clip on the smart head.
- Let it calibrate. Most smart heads run the motor in and out to find the valve’s closing point. Expect a short buzzing sound.
- Pair it with the hub or app and give the room a schedule.
Why they still need a boiler control
Radiator valves only decide how much hot water each radiator lets through. They can’t turn the boiler on. If the boiler is only told to fire by a room thermostat in the hallway, a smart TRV in a cold bedroom can open all it likes and still get no heat while the hallway is warm.
- Whole-home smart systems (for example tado, Hive, Drayton Wiser, Netatmo and Honeywell evohome) also put a controller at the boiler, so any room that needs heat can call for it. See UK smart thermostat wiring for how those receivers are wired.
- Standalone smart TRVs without boiler control still save energy by turning down rooms you don’t use, as long as the main room thermostat keeps calling for heat when the house needs it.
Bypass and the room with the wall thermostat
- Don’t put a TRV on every radiator if your boiler is controlled by a single wall thermostat. Leave the radiator in the same room as that thermostat without one (or set fully open). Otherwise the TRV can close before the room thermostat is satisfied, and the boiler keeps firing with nowhere for the heat to go. UK guidance for new heating systems says the same.
- Automatic bypass valve. If every radiator can shut at once, most boilers need an automatic bypass valve, or one radiator with no TRV, so water can still flow. If you hear the boiler cycling rapidly or pipes whistling after fitting smart heads, ask a heating engineer to check the bypass.
Common problems
- Radiator stays cold: a stuck pin, the wrong adapter so the head doesn’t reach the pin, or no boiler call (see above).
- Radiator always hot: the head didn’t calibrate or isn’t pushing the pin. Remove it, check the adapter is seated, and rerun calibration.
- Room reading is off: the head measures temperature next to a hot radiator, often behind a curtain. Many systems let you add an offset or a separate room sensor.
- Batteries drain fast: weak radio signal to the hub or a stiff valve pin making the motor work harder. Move the hub closer or add a repeater, and check the pin moves freely.
Not for electric radiators
Electric panel heaters, oil-filled radiators and storage heaters have no water valve. They’re controlled by their own built-in thermostat or by a mains switch, not by a smart radiator head.