Controls · Thermostats
A thermostat should run the system. Most are run by an app.
The thermostat is the part of the heating system the homeowner actually touches, which is why so many are bought on app polish rather than on what they do to the boiler or heat pump underneath. The good ones modulate, schedule and compensate for the weather. The bad ones are a dial with a screen.
Hallway thermostat, boiler cycling short and hot
A hallway thermostat calling the boiler on and off, hot and short; every other room runs to whatever the radiator happens to deliver.
What a thermostat should actually do.
A heating system runs best when it is on for longer at lower flow temperatures rather than short and hot. A good thermostat keeps it there by reading the building and the outside conditions and asking the boiler or heat pump to modulate accordingly. That is the unglamorous job most thermostats are bought to do, and most do badly.
The four jobs worth naming: modulate the heat source rather than cycle it; compensate for outside temperature in real time; run per-room schedules where the architecture allows; and hand control back to the OEM controller when the OEM controller does the job better.
OEM controller with weather compensation and per-room TRVs
Same house, OEM controller in charge, per-room TRVs underneath; the boiler is modulating at lower flow and every room finally holds its setpoint.
OEM controllers vs third-party thermostats.
On a modern heat pump the manufacturer's own brain is usually the better answer; on a boiler the choice is more open.
OEM heat-pump controller
- Strength
- MELCloud, SensoHome, Madoka and equivalents outperform third-party thermostats on weather compensation and modulation; designed to talk to the compressor directly.
- Trade-off
- The app can feel dated compared to a Tado or a Nest; that is a UI observation, not a system one.
- Best for
- Any modern heat pump; specify the OEM controller as the brain and layer TRVs underneath.
Third-party zoning thermostat
- Strength
- Tado, Drayton Wiser, Honeywell evohome and Netatmo do per-room zoning with TRVs at broadly similar quality; strong scheduling.
- Trade-off
- On a heat pump they are usually a downgrade on the OEM controller for modulation; on a boiler they compete on their merits.
- Best for
- Boiler households wanting proper zoning; heat-pump households only for the per-room layer beneath the OEM brain.
Marketing-first smart thermostat
- Strength
- Excellent apps and hardware finish; friendly onboarding.
- Trade-off
- Limited proper per-room zoning; the modulation and compensation work often does not happen underneath.
- Best for
- Households who want a single-zone smart schedule and are not chasing efficiency.
Verdict
Thermostats, honestly assessed.
On a heat pump, let the OEM controller run the system and add third-party TRVs for per-room control; on a boiler, choose a proper zoning thermostat on its scheduling and TRV integration rather than on the app.
A smart thermostat on its own saves a modest 5 to 10 per cent; the bigger savings live in weather compensation and per-room zoning underneath it. Both live in the boring specification detail rather than in the app screenshots.
Replacing an OEM heat-pump controller with a third-party thermostat is usually a downgrade dressed as an upgrade; the OEM brain talks to the compressor in ways a generalist thermostat cannot.
What it gives you
- Correctly specified, quietly saves 5 to 15 per cent of the winter bill.
- Per-room TRVs match delivery to actual room use.
- Hands off the flow-temperature and modulation work to the layer best equipped for it.
What it costs you
- The wrong pick can undermine an otherwise good heat-pump install.
- Zoning without insulation improvements is a smaller win than the marketing implies.
- App polish is not the metric that matters here.
Why we think thisOpenClose
Reasoning
The 5 to 15 per cent range comes from Energy Saving Trust and BEIS-funded field trials of weather compensation and zoning on both boilers and heat pumps in the UK. OEM-controller performance on heat pumps is drawn from the Heat Pump Ready programme's monitoring of Mitsubishi, Vaillant, Daikin and Samsung installs.
Cost bands reflect UK installer quotes in 2026; single-zone smart thermostats at £150 to £300 fitted, whole-house evohome-class installs at £900 to £1,800 fitted, and OEM heat-pump controllers included on a properly specified heat-pump quote.
Assumptions
- The heating system is a modern condensing boiler or a properly designed heat pump; older non-condensing kit has different optimum controls.
- The house has enough radiators and pipework flexibility to give zoning a job to do.
- The OEM controller is genuinely commissioned, not left on factory defaults.
Sources
- Heat Pump Ready programme monitoring — Nesta / Energy Systems Catapult
- Weather compensation field trial evidence — Energy Saving Trust
- MIS 3005 Heat pump installation requirements — MCS
If this were our house
If this were our house with a modern heat pump on order, the OEM controller would be named on the quote and the third-party thermostat conversation would only start at the per-room TRV layer.
- 1Insist the heat-pump quote names the OEM controller and includes commissioning of weather compensation.
- 2Add TRVs on any radiator serving a room with its own schedule (spare bedroom, home office) rather than a whole-house thermostat swap.
- 3Skip a marketing-first smart thermostat unless it demonstrably runs weather compensation on the specific model of boiler or heat pump you have.
On an older non-condensing boiler the calculus differs; a simple upgrade to a decent smart thermostat plus TRVs can be the highest-return move in the house until the boiler itself is replaced.