Good control means the right temperature in the right room at the right time, and a boiler running as cool and as steadily as it can. Zoning, TRVs, load or weather compensation and a sensible schedule all contribute. The smart part is the least important of them.
What controls are actually for
Three separate jobs, often confused with each other:
- When the heating runs – the programmer or timer.
- How warm the house gets – the room thermostat, and the TRVs on individual radiators.
- How hard the boiler works to achieve it – compensation, which is the one almost nobody has and the one that saves the most.
Most houses have the first two, in some form, and almost none have the third. That is where the opportunity is.
Compensation, the control nobody talks about
Weather compensation
An outdoor sensor measures the external temperature, and the boiler adjusts its flow temperature accordingly. On a mild day it runs cool; on a cold day it runs hotter. Because a condensing boiler is most efficient at low return temperatures, a system that only runs hot when it genuinely needs to is meaningfully more efficient than one that runs at a fixed high flow temperature all winter.
It also produces better comfort, because the house is heated gently and continuously rather than blasted and then switched off. The temperature swing in a compensated house is noticeably smaller.
Load compensation
The same idea using the difference between the room temperature and the target instead of an outdoor sensor. The boiler modulates down as the room approaches temperature, rather than running flat out then stopping. Most modern boilers support it and most installations do not use it.
Both require the boiler and the control to actually talk to each other – typically over a manufacturer’s digital interface or OpenTherm – rather than the control simply switching the boiler on and off. That is the detail that determines whether a smart thermostat is doing anything clever or merely being a timer with a nice screen.
Does it modulate the boiler, or does it just switch it on and off? A control that only switches, however attractive its app, cannot deliver compensation. Whether it can depends on the boiler as much as the thermostat, and it is worth establishing before buying either.
TRVs and zoning
Thermostatic radiator valves are the cheapest genuinely effective control there is. They let each room hold its own temperature, so bedrooms are not heated to living room temperature and a sunny room does not overheat. They are inexpensive, and a house without them is heating every room to the same setting regardless of use.
- One radiator must not have a TRV – the one in the room with the main thermostat – so there is always a flow path when everything else closes.
- TRVs seize, especially if left in one position all summer. Exercise them each autumn by winding fully open and closed a few times. A stuck-open TRV overheats a room; a stuck-closed one leaves it cold.
- Position matters. A TRV behind a curtain or above a radiator cover reads the wrong air temperature and shuts off too early. Where that is unavoidable, a remote sensor version solves it.
- Smart TRVs give per-room scheduling and are genuinely useful in a house with very different room usage. They also need batteries changing, and they are considerably more expensive than standard ones.
Zoning at the system level – upstairs and downstairs on separate circuits and separate thermostats – is worthwhile in most two-storey houses, because the two floors have genuinely different heating needs and different solar and occupancy gains. Beyond two or three zones, the complexity and cost rise faster than the benefit.
Choosing a smart thermostat
They are all broadly competent and the differences that matter are not the ones the marketing emphasises.
| Consideration | Why it matters |
|---|---|
| Does it modulate the boiler? | The single biggest determinant of whether it saves you anything |
| Does it support your boiler’s protocol? | OpenTherm or the manufacturer’s own interface. Check compatibility before buying |
| Wired or wireless? | Wireless is easier to fit and needs batteries. Wired is more reliable and needs a cable |
| Does it work without the internet? | It should. A thermostat that stops heating your house when the broadband drops is not acceptable |
| Can somebody else in the house use it? | The best control is one the household will actually operate rather than working around |
| Per-room control? | Only worth the cost if rooms are genuinely used very differently |
| Hot water control? | Needed on a cylinder system. Combi systems do not have a hot water channel |
The honest position: a smart thermostat properly set up on a compensating boiler with functioning TRVs is a real improvement. A smart thermostat fitted as a straight replacement for an old timer, switching the boiler on and off at a fixed high flow temperature, gives you an app and very little else.
Setting it up, which is the part that gets skipped
Fitting a control takes an hour. Setting the system up around it takes longer and is what actually makes the difference:
- Boiler flow temperature set as low as the radiators allow, rather than left at the factory maximum.
- Compensation enabled and configured, where the boiler supports it.
- Schedule built around when the house is genuinely occupied, with realistic pre-heat times rather than optimistic ones.
- TRVs set sensibly per room and explained to whoever lives there.
- Hot water scheduled separately from heating on a cylinder system.
- The household actually shown how it works, in person.
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 take advantage. Fitted as a like-for-like timer replacement on a boiler running at a fixed high flow temperature, the saving is small. The setup matters more than the device.
Is it better to leave the heating on low all the time?
For a high thermal mass system – underfloor, or a very well insulated house – steady low-level running with compensation is genuinely more efficient and more comfortable. For a draughty house with radiators, heating when occupied is better. It depends on the building, which is why the blanket answers you see online are unhelpful.
Why does one room always overheat?
Usually a TRV that has seized open, or a TRV positioned where it reads the wrong temperature – behind a curtain, above a radiator cover, or in a draught. Both are cheap to resolve.
Can I fit a smart thermostat myself?
Wireless ones are often designed for self-installation, and many people manage. What is usually missed is the boiler side: enabling modulation, setting the flow temperature, and configuring compensation. That is where the benefit is.
What is OpenTherm?
A communication standard that lets a compatible thermostat and boiler exchange information, so the thermostat can tell the boiler to run at a particular output rather than simply switching it on. It is what makes load compensation possible. Not all boilers support it.
Do I need a thermostat for hot water as well?
On a cylinder system, yes – a cylinder thermostat, with the hot water on its own schedule. A combi has no store, so there is nothing to schedule.
What happens when you call
Call and describe it
You speak to an engineer, not a call centre. Most jobs can be narrowed down on the phone.
We look at it properly
A real diagnosis before a price, and a straight answer if it is not worth doing.
A written quote
Itemised, so you can see what you are paying for.
The job, and the paperwork
Certificates where the work needs them, and what will need attention next.
Where we do this work
Across Norwich and the whole of Norfolk. If you are on the edge of the county, call and ask — we will tell you honestly whether we are the right people.
- Norwich
- Wymondham
- Dereham
- Attleborough
- Aylsham
- North Walsham
- Cromer
- Diss
- Loddon
- Long Stratton
- Watton
- King's Lynn
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.