Heating controls, TRVs and smart thermostats

The four layers

1. Time control – when the system runs. A programmer or timer.
2. Room temperature control – a room thermostat, switching the system off when the reference room reaches temperature.
3. Per-room control – thermostatic radiator valves, so each room holds its own temperature.
4. Boiler output control – compensation, varying the flow temperature so the boiler runs as cool as it can. This is the layer most houses do not have, and it is where the biggest saving is.

Most homes have the first three in some form. The fourth is frequently available in the equipment already installed and simply not enabled.

Thermostatic radiator valves

The cheapest genuinely effective control available. A TRV senses the air temperature around it and closes the valve as the room warms, so each room holds roughly its own setting rather than every room being heated to whatever the hall thermostat wants.

Why they seize

Inside a TRV body is a pin that the head presses on. Left in one position through a whole summer, that pin sticks – corroded or simply bound by deposits. In autumn the head turns but the pin does not move, so the valve is stuck wherever it was.

• Stuck open – that room overheats and you cannot turn it down.
• Stuck closed – that room never warms up. This is the most common cause of a single cold radiator.

The free fix: take the head off and look at the pin. It should move freely and spring back. If it is stuck down, work it gently with pliers or a small adjustable spanner – a drop of penetrating oil and patience, not force. Frequently it frees and the radiator works again.

The prevention: each autumn, wind every TRV fully open and fully closed a few times before the heating season. It takes five minutes for a whole house.

Positioning

A TRV reads the air around it, so where it sits matters. Behind a curtain, above a radiator cover, or in a draught from a door, it reads the wrong temperature and controls badly – typically shutting the radiator off while the room is still cold. Where the position cannot be changed, remote-sensor TRVs exist and solve it.

Compensation, and the one question to ask

A condensing boiler is most efficient when its return water is cool enough for the flue gases to condense. Compensation is the control that keeps it there.

Weather compensation

An outdoor sensor measures the external temperature and the boiler adjusts its flow temperature accordingly – cool on a mild day, hotter on a cold one. The house is heated gently and continuously rather than blasted and switched off, which is both more efficient and noticeably more comfortable.

Load compensation

The same principle using the gap between the current room temperature and the target, rather than an outdoor sensor. As the room approaches temperature, the boiler modulates down rather than running flat out then stopping.

Both require the control and the boiler to communicate, rather than the control simply switching the boiler on and off. That is usually over OpenTherm or a manufacturer’s own digital interface.

Choosing and setting up a smart thermostat

They are all broadly competent. The differences that matter are not the ones the marketing leads with.

Setup is where the benefit actually comes from, and it is the part most commonly skipped:

1. Boiler flow temperature set as low as the radiators allow.
2. Compensation enabled and configured, where the boiler supports it.
3. Schedule built around real occupancy, with realistic pre-heat.
4. TRVs set sensibly per room and explained to whoever lives there.
5. Hot water scheduled separately on a cylinder system.
6. The household shown how it works, in person.

Zoning

Splitting the system so different parts can be controlled independently.

• Upstairs and downstairs as separate zones is worthwhile in most two-storey houses. The two floors have genuinely different heating needs, different solar gain and different occupancy patterns, and bedrooms generally want to be cooler.
• Hot water as a separate zone on a cylinder system, so it is not being reheated whenever the heating runs.
• Beyond two or three zones, complexity and cost rise faster than the benefit for a typical house. Per-room control through smart TRVs achieves much the same thing more cheaply.
• Each zone needs its own thermostat and its own motorised valve, which is why retrofitting zoning is more than a controls job.

A note on new systems: there are requirements around zoning and control for new and replacement heating systems, which vary by property size. Any competent installer will build this in – it is worth asking what is being fitted and why.

Free improvements worth making today

1. Turn the boiler flow temperature down in stages and see whether the house still gets warm enough. Free, reversible, and frequently the biggest single saving available.
2. Exercise every TRV fully open and closed before the heating season.
3. Check the schedule matches when anybody is actually home. Many have not been changed since the day they were set.
4. Check the programmer clock, particularly after a power cut or a clock change.
5. Move anything blocking a TRV or a thermostat – curtains, furniture, radiator covers.
6. Turn down radiators in rooms nobody uses, but not fully off in winter – a cold unheated room causes condensation and pulls heat from adjacent rooms.
7. Set the hot water schedule rather than leaving it on continuously.