OpenTherm vs relay thermostat
Updated: October 2026
OpenTherm vs relay thermostat is a question about the boiler's terminals, not about how modern the screen looks. You already know which boiler you have, and you want to know whether modulation exists at all. A relay or a dry contact is on/off. OpenTherm only changes the burner when the boiler supports that bus and the stat is wired to it. This page is the interface choice. It is not a combi shopping list and not an install procedure. The product rows live on the [Wi-Fi room thermostat comparison](/hub/thermostats/wifi-room-thermostat). Products Compared used the listings in this set plus public notes from Hive, Viessmann UK, tado, and Which?. We did not measure a saving. A qualifying purchase can earn a commission at no extra cost to you.
OpenTherm vs relay thermostat is a question about the boiler's terminals, not about how modern the screen looks. You already know which boiler you have, and you want to know whether modulation exists at all.
OpenTherm vs relay thermostat is a question about the boiler's terminals, not about how modern the screen looks. You already know which boiler you have, and you want to know whether modulation exists at all. A relay or a dry contact is on/off. OpenTherm only changes the burner when the boiler supports that bus and the stat is wired to it. This page is the interface choice. It is not a combi shopping list and not an install procedure. The product rows live on the Wi-Fi room thermostat comparison. Products Compared used the listings in this set plus public notes from Hive, Viessmann UK, tado, and Which?. We did not measure a saving. A qualifying purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Boiler interface | Kit type | Receiver | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  tado tado X wireless starter | OpenTherm or relay | Wireless starter | Wireless Receiver X listed | 8.8 | about £129.99 | View offer |
 Vibe Vibe Wi-Fi room stat | OpenTherm or volt-free for a combi | Wireless kit | Described; components line lists a bracket | 8.4 | about £49.95 | View offer | |
| Preis-Tipp |  KETOTEK KETOTEK boiler stat | Dry contact, no voltage. Title says 3 A and 220 V | Flush-mounted mains thermostat | No separate receiver listed | 8.2 | about £44.99 | View offer |
On this page
On/off is the default
Most UK heating controls are a switch. Viessmann UK describes that pattern directly: the stat calls for heat or it does not, and the boiler runs at its own flow setting until the call ends. A relay is that electronic switch. A dry contact is the same idea with no voltage on the pair. The Wi-Fi logo does not change the switch into a conversation with the burner. In this folder the KETOTEK is sold as dry contact for a boiler. The BEOK title is an on/off two-wire combi stat. The MincoHome is a mains wall stat and does not claim OpenTherm. Any of them can still be a sensible control when the boiler only offers a relay or a switched-live input. Which? makes the matching distinction: some smart stats stay on/off, and being internet-connected is not the same as modulating. If your manual never names a digital bus, on/off is the honest description of what you will get, including from a stat whose advert says smart.
OpenTherm only if both ends speak it
OpenTherm is load compensation. The control asks the boiler to reduce output instead of only cycling a fixed flow temperature on and off. Hive's support page describes relay as the on/off switch and OpenTherm as the protocol that can request a lower output. That only happens when both ends speak it. A newer boiler badge does not prove the terminals are there. Viessmann says older boilers usually lack the interface, and a recent model can still be on/off only. The proof is the boiler's external-controls page, or a terminal marked for OpenTherm, not the thermostat's marketing line. Hive's own up to 15 percent line is Hive's claim about its OpenTherm setup. It is not a measured saving for the products in this folder, and we are not repeating it as your bill. If the manual lists only a volt-free or switched-live pair, buying a stat that mentions OpenTherm does not create the bus. You would still be running relay, which remains a valid way to call for heat. The wider on/off case is in OpenTherm or on/off heating.
What these listings actually claim
Naming OpenTherm on an amazon.co.uk title is a claim about the product, not proof that your combi has the terminals. In this set, OpenTherm is named on the wired tado X add-on B0CWPG6G5W, the wireless tado X starter B0D9QBPV51, the tado X starter B0H2MF6S5R, and the Vibe, which states OpenTherm and volt-free switching for combination boilers. The three cheaper wall stats stay in the on/off or dry-contact description. tado's UK compatibility page says tado X works with boilers that have a relay or an OpenTherm connection, and that X is not V3+. A separate tado help article about other product lines says X supports OpenTherm modulating control only, and that a non-OpenTherm modulating V3+ boiler may not carry over. Those two pages do not say the same thing. Do not collapse them into tado X is OpenTherm-only or tado X is relay-only. The compatibility page still offers relay. The help page narrows modulation and tells you to contact tado when the old bus was something other than OpenTherm. Hot-water schedules are a different topic, on the Wi-Fi room thermostat on a combi page only when you do not have a cylinder.
Zones, extra cable, and V3+
OpenTherm in the wild is often a single heating channel. Hive's worked example, and Hive is not a product in this folder, limits OpenTherm to a single-channel combi receiver. For multi-zone they tell you to stay in relay mode. If you have two heating zones, assume you are on on/off until the boiler maker and the control maker both say the bus can do otherwise. A review on B0H2MF6S5R said the OpenTherm benefit needed a low-voltage cable from the boiler to the stat, and that this had not been flagged at purchase. Budget for that cable as a possibility, not as a terminal recipe. tado's relay note says the hardware terminal is normally open, the app can switch relay behaviour, and normally closed is the wrong choice if the stat is on the boiler directly, while some underfloor systems need it. One install only fired correctly after normally open and normally closed were swapped. Follow the app's question rather than trying both at random. tado X will not join a V3+ home. If your V3+ boiler used a digital bus other than OpenTherm, tado's help page says it may not be an X job and that you should contact them. When the bus is absent, stay on relay.
Common questions
What is the difference between OpenTherm and a relay thermostat?
A relay thermostat is an on/off switch. It calls for heat, and the boiler uses its own flow setting. OpenTherm is a protocol that can ask a compatible boiler to reduce output, which is load compensation. Without OpenTherm terminals at the boiler, the clever stat still behaves as a switch.
Does a Wi-Fi thermostat automatically modulate the boiler?
No. Wi-Fi gets the schedule and the phone app onto your network. Modulation needs a shared language such as OpenTherm, wired to the matching terminals. Which? separates internet-connected controls from modulating ones. A colour screen does not add that language.
How do I know if my combi supports OpenTherm?
Read the boiler manual's external-controls section, or the installer menu, for an OpenTherm connection. A new badge on the case is not enough, which is Viessmann's warning. If the page only shows volt-free or switched-live terminals, the combi is an on/off appliance from the stat's point of view.
Is dry contact the same as OpenTherm?
No. Dry contact is a volt-free switch. The KETOTEK listing is that on/off case. OpenTherm is a low-voltage digital bus with its own terminals. Putting mains thinking, or a dry-contact pair, onto an OpenTherm input is a mismatch, not a shortcut.
Can I use OpenTherm on two heating zones?
Do not assume so. Hive limits its OpenTherm example to a single-channel combi receiver and tells multi-zone jobs to stay on relay. Your boiler and your control both have to document a multi-zone bus before you plan around it. Otherwise on/off per zone is the control you actually have.
Will tado X run a boiler that is not OpenTherm?
tado's compatibility page says tado X works with a relay or an OpenTherm connection. The help page narrows X modulation to OpenTherm and says other digital buses from a V3+ boiler may not carry over. Use relay when that is the terminal you have, and ask tado if the old system used a different bus.
Why did OpenTherm need an extra cable?
A review of B0H2MF6S5R said the modulating connection needed a low-voltage cable from the boiler to the stat, which the purchase page had not made obvious. Relay wiring you already have is not automatically that bus. If the cable run does not exist, you are choosing on/off or a new cable, not a setting in the app.
Is on/off control still valid?
Yes. On/off is the normal UK pattern, and it is the correct description when the boiler has no OpenTherm terminals. A relay or dry-contact Wi-Fi stat can still give you a phone schedule and a room setpoint. It will not trim the boiler's output the way a shared modulating protocol can.
Related pages
Sources
Public UK sources for Part L in England, Boiler Plus, and Gas Safe. Listings are the amazon.co.uk captures for these seven ASINs, not a live reload on 17 October 2026. Prices on amazon.co.uk move.
- Approved Document L (retrieved 17 October 2026)
- BEAMA heating-controls note (retrieved 17 October 2026)
- MHCLG circular, 3 April 2018 (retrieved 17 October 2026)
- HSE gas work (retrieved 17 October 2026)
- Gas Safe Register, home improvements (retrieved 17 October 2026)
- Which? smart thermostat buying guide (retrieved 17 October 2026)
- tado UK compatibility (retrieved 17 October 2026)
- Flameport combi thermostat terminals (retrieved 17 October 2026)