OpenTherm or on off thermostat: what the boiler must match

You are in the kitchen with a combi on the wall and a shortlist of thermostats that all mention an app. One box adds an OpenTherm logo and a line about using less gas. The choice is an OpenTherm or on off thermostat, and it is about the signal the room control sends. On/off remains a normal way to run a UK combi. OpenTherm changes how the boiler is asked to fire only when the boiler has the terminals and the installer leaves that mode switched on.

Peter F. updated this page in October 2026. We used Hive's shop and OpenTherm support pages, tado's UK boilers page, and the listing rows stored for our Alexa thermostat comparison on amazon.co.uk. The stored price for the top pick, the tado V3+ wireless starter kit, is £57.50. Hive's up to 12% wording is quoted as Hive's figure. We did not lab-test a boiler, and we did not copy a Worcester Bosch or Vaillant model list. A qualifying purchase can earn a commission at no extra cost to you.

In short

An OpenTherm or on-off choice is about the signal the room control sends. On/off, including a volt-free relay, is a valid way to call for heat on a UK combi. OpenTherm needs the boiler and the control to speak it, with the installer leaving that mode on. Hive limits OpenTherm to a single-channel combi and sends multi-zone back to relay. The up to 12% line is Hive's claim. tado says Thermostat X suits relay or OpenTherm only if your boiler has the terminals.

What on/off does to the boiler

On/off control is a switch on the call for heat. Below the set temperature the contact closes and the boiler may fire. At the set temperature the contact opens and the call ends. Installers call that a relay heating control, a volt-free thermostat, or a dry contact. Volt-free means the stat presents a switch, rather than feeding a mains live into the boiler terminals. It does not send flow temperatures. Put the sensor in a hallway, clear of a radiator, a lamp, and direct sun. On the boiler cupboard it feels the case, opens early, and leaves the rest of the house lagging.

A dry contact cannot modulate. Our digest records the KETOTEK KTF0155C as a dry-contact control with a maximum of 3 A. Three amps is the switch rating, not a gas saving. In the stored comparison that 3 A KETOTEK boiler thermostat is the best value row, at £44.99. The other KETOTEK at the same stored price is an electric underfloor thermostat, so it is the wrong box for a gas combi. A mains live on a pair that expects a volt-free contact is a job for a Gas Safe registered engineer.

What OpenTherm adds on a UK combi

OpenTherm is a protocol between a room control and a boiler that both understand. Left enabled, the control can ask the boiler to change output instead of only granting or refusing heat. The boiler can ease down as the room nears the set temperature. Makers call that modulating boiler control. People ask for it because a lower flow temperature, held for longer, is where a condensing combi takes more heat from the flue gases. The protocol cannot create modulation the boiler does not have, and a logo on the carton does not turn the link on.

Both ends have to speak it. A thermostat with the mark, wired as a relay, behaves as an on/off stat while relay is selected. A boiler joined only to a volt-free receiver is still being switched. Hive's support position, followed here, keeps OpenTherm for a single-channel combi and sends multi-zone work back to relay. tado's UK boilers page says Thermostat X suits relay or OpenTherm. Worcester Bosch and Vaillant ranges differ by model, so this page does not copy a forum list.

How you check without a model rumour

Read the controls section, then ask the person who opens the panel. Does the boiler name OpenTherm, or a modulating room-control connection? Is there a pair for that connection, separate from the volt-free on/off pair? Will the receiver stay on relay? Any no, or a promise to decide on the day, means you should buy for on/off. A second zone or a hot-water cylinder belongs in that conversation before you pay, because Hive's published limit puts that layout back on relay.

Load compensation, in the Boiler Plus wording for a new gas combi in an existing English home, means output eases as the room nears the set temperature. An OpenTherm link can be one way to do that when both ends support it. The logo is not the name of the measure, and a voice skill is a separate function. The combi boiler controls page separates a combi, which heats tap water when a tap opens, from a system boiler with a cylinder. Swapping only the room stat, with the boiler staying, sits outside that installation trigger.

Hive OpenTherm single zone, and where tado draws the line

Hive's OpenTherm support page sets the border used here: one channel, on a combi. Two heating zones go back to relay, and relay is on/off. The hub fee does not add the terminals. The stored row for a Hive thermostat with the Nano 3 hub is £164.99. The stored row without the hub is £104.96. Both are listing prices, not an installer's quote. tado's UK page allows Thermostat X on relay or on OpenTherm. The wireless Smart Thermostat X starter is stored at £129.99, and the wired Smart Thermostat X add-on at £79.99. A receiver left in relay is an on/off control with an app.

The comparison's top pick is a different tado kit. The V3+ wireless starter kit, B08LP1LS5T, is stored at £57.50. That top pick is a listing judgement on the Alexa thermostat comparison. It is not a finding that this kit answers the OpenTherm question, and this article adds no protocol claim for that model. The Hive and tado comparison places the products side by side for voice and heating. It is not a compatibility certificate for your boiler. Voice on a switch receiver does not start OpenTherm.

Why the percentage is not your bill

Hive publishes a figure of up to 12% alongside OpenTherm. The sentence is Hive's. It is not a reading from your meter, and it does not transfer to another brand or to a receiver left in relay. The bill also moves with the weather, radiator valves, how often the combi heats taps, and the tariff. A volt-free KETOTEK does not carry Hive's line, and a tado kit left in relay does not carry it either. Other stored rows are prices only: the Meross MTS215B at £59.99, a Beok two-wire combi thermostat at £42.99, and a Beok wireless boiler thermostat at £62.99. This article has no OpenTherm claim for those three. Hive's percentage is not load compensation, and an Alexa skill is not that measure.

What to ask before you buy

Open the boiler manual at the controls section before you order. You want the word OpenTherm, or a plain line that a modulating room control can connect. A book that only shows a volt-free on/off pair is telling you to buy for that pair. Leave forum lists of Worcester Bosch and Vaillant models alone.

1. Which terminals will you use?
2. Will the receiver be OpenTherm or relay?
3. If the quote says Hive, is this a single-channel combi, or does a second zone force relay under Hive's limit?
4. If the quote says tado Thermostat X, which of the two methods on tado's page will be commissioned?
5. Where will the sensor sit, and which hours will the schedule run?

A modulating boiler left hot all day can use more gas than an on/off boiler on a shorter schedule and a lower set temperature. The stored amazon.co.uk figures above, from £44.99 for the KETOTEK dry contact to £164.99 for Hive with the Nano 3 hub, are listing prices, not an installation quote. Gas work inside the boiler belongs with a Gas Safe registered business. Have the invoice say OpenTherm or relay. With no terminals, buy a clear on/off stat and spend your care on hours and set temperature. With the terminals, a single zone, and a combi, pay for the path the installer will enable.