The saving comes from the boiler modulating rather than switching, from a flow temperature set as low as the radiators allow, and from a schedule that matches occupancy. The app is the least important part.
The one question that matters
A control that only switches, however good its app, cannot deliver compensation – and compensation is where the saving is. Whether it can depends on the boiler as much as the thermostat, so check compatibility before buying either. This is the question that separates a genuine improvement from an expensive timer.
A condensing boiler is most efficient when its return water is cool enough for the flue gases to condense. A boiler left at its factory flow temperature, switching on and off against a room thermostat, spends much of its life too hot to condense. It is a condensing boiler that is not condensing.
A modulating control tells the boiler what output to run at, not merely whether to run. That lets it hold a low flow temperature on mild days, which is most of the heating season.
What they can and cannot do
| Feature | Real benefit | Notes |
|---|---|---|
| Modulation / compensation | Substantial | The main event. Needs boiler support, usually OpenTherm or a manufacturer protocol |
| Schedule that matches occupancy | Real | Achievable with any programmer. The smart part makes it easier to change |
| Remote control from an app | Convenience | Genuinely useful for irregular schedules and second homes, less so for a saving |
| Geofencing / presence detection | Modest | Works well for some households and poorly for others |
| Learning / optimum start | Modest to real | Varies enormously between products |
| Weather data from the internet | Modest | Not the same as a genuine outdoor sensor wired to the boiler |
| Per-room smart TRVs | Real, in the right house | Useful where rooms are used very differently. More batteries, more cost |
| Usage reporting | Indirect | Useful because it changes behaviour, not because the device saves anything |
The honest summary: a smart thermostat properly set up on a compensating boiler with working TRVs is a real improvement. The same device fitted as a straight timer replacement, with the boiler left at factory settings, gives you an app.
Choosing one
- Check boiler compatibility first, specifically for modulation. Not all boilers support it and not all controls do it.
- Does it work without the internet? It must. A thermostat that stops heating the house when the broadband drops is not acceptable.
- Wired or wireless? Wireless is easier to fit and needs batteries; wired is more reliable and needs a cable.
- Hot water channel? Needed on a cylinder system. A combi has no store to schedule.
- Can everybody in the house use it? The best control is the one the household will operate rather than work around. This matters more than any feature.
- Is it tied to a subscription or to a service that could be withdrawn? Worth knowing before committing.
- Do you actually want per-room control? Only worth the cost where rooms are genuinely used very differently.
The setup is where the benefit comes from
Fitting the device takes an hour. Setting the system up around it takes longer and is what actually changes the bill – and it is what most installations skip.
- Enable modulationWhere the boiler supports it. Wiring the control in without enabling the protocol leaves it switching, which defeats the purpose.
- Configure compensationWeather or load, with a sensible curve. This is a separate step from enabling it and it is frequently left on defaults.
- Set the flow temperature as low as the radiators allowIn stages, checking the house still gets warm on a cold day. Free, and often the single biggest saving available.
- Build the schedule around real occupancyNot around the factory example, and with realistic pre-heat rather than optimistic.
- Set the TRVs sensibly per roomAnd leave one radiator without a TRV, so the system always has a flow path.
- Schedule hot water separatelyOn a cylinder system.
- Show the household how it worksIn person. A control nobody understands gets overridden.
More on the underlying principles in heating controls, TRVs and smart thermostats, and on the regulatory requirement for new boilers in Boiler Plus.
Where a smart control is genuinely worth it
- Where the boiler supports modulation and it will actually be enabled.
- Irregular occupancy – shift work, working from home some days, a second home.
- A household that will engage with it. The usage reporting changes behaviour, and that is a real effect.
- Rooms used very differently, where per-room smart TRVs earn their cost.
- Replacing a control that has no room thermostat at all, which is a large step up regardless of how smart the replacement is.
And where it is not: a household that wants the heating on a fixed schedule and never thinks about it again, on a boiler that does not support modulation. A good conventional programmer and thermostat, with the flow temperature set properly and the system balanced, will serve them better for less.
We will tell you which you are. It is not in our interest to sell somebody a control that will not do anything for them.
Have a smart thermostat that has not saved you anything? It is usually because the boiler was never set up to modulate with it.
Questions we get asked about this
Will a smart thermostat save me money?
It can, if it modulates the boiler and the system is set up to exploit that. Fitted as a like-for-like timer replacement with the boiler at factory settings, the saving is small.
What is OpenTherm?
A communication standard letting a compatible thermostat and boiler exchange information, so the thermostat can request a particular output rather than just switching the boiler on. It is what makes compensation possible, and not all boilers support it.
Which brand is best?
The differences that matter are whether it modulates your boiler, whether it works without the internet, and whether your household will actually use it. Those are more informative than feature comparisons.
Can I fit one myself?
Wireless ones are often designed for self-installation and many people manage. What usually gets missed is the boiler side – enabling modulation, setting the flow temperature, configuring compensation – which is where the benefit is.
Do I need per-room smart TRVs?
Only where rooms are genuinely used very differently. They are considerably more expensive than standard TRVs and they need batteries changing.
What if the internet goes down?
A properly chosen control keeps heating the house from its local schedule. One that does not is not acceptable, and it is worth checking before buying.
Regulations and standards referenced on this page
- Approved Document L: conservation of fuel and power (the Boiler Plus measures apply in England) — Ministry of Housing, Communities and Local Government
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