Heating 11 September 2026

What the Numbers on Your Radiator Valves Actually Mean

The numbers on a radiator valve are not a heat setting. They are a room temperature target: turning the head from 3 to 5 does not make the radiator any hotter or heat the room any faster, it simply tells the valve to keep letting hot water through until the room is warmer before it shuts off.

That one misunderstanding sits behind a large share of the “my heating is not working properly” calls our engineers take across Kent every autumn. People wind every valve to 5 in October to warm the house up quickly, leave them there, and then wonder in December why the bedrooms are stifling and the gas bill has climbed. Here is how a thermostatic radiator valve actually works, and how we set them up on the homes we look after.

How does a radiator valve work?

A thermostatic radiator valve, or TRV, is a small self-contained thermostat sitting on the end of the radiator. Inside the numbered head is a capsule of wax or liquid that expands as the air around it warms. As it expands it pushes down on a pin in the valve body underneath, and that pin closes the waterway. When the room cools, the capsule contracts, the pin lifts and hot water flows again.

Nothing electrical is involved, and the head never talks to the boiler. It is purely mechanical, which is why an old TRV can seize up over the summer and why a new one costs nothing to run.

The important part is that the valve senses the air around its own head, not the water in the radiator. That is why a TRV tucked behind a thick curtain or boxed in by a sofa will shut off long before the rest of the room is warm - it is measuring its own little pocket of trapped heat.

What do the numbers on a radiator valve mean?

Each number corresponds to a rough room temperature at which the valve will close. Exact figures vary between manufacturers, and the scale is only ever approximate, but as a guide most heads work out something like this:

• * (frost symbol) - the valve stays shut unless the room drops to around 5-7°C, then opens just enough to stop pipes freezing
• 1 - roughly 12°C, for a room you want to keep from going properly cold
• 2 - roughly 16°C, a sensible bedroom setting for many people
• 3 - roughly 20°C, the usual living-room comfort point
• 4 - roughly 24°C
• 5 - the valve is effectively fully open and will barely ever close

So a valve on 5 is not “maximum heat”. It is a valve that has been told to keep going until the room reaches a temperature most homes never get to, which means it never switches off and the radiator runs whenever the boiler does.

Which way do you turn a radiator valve to turn it on?

On almost every TRV, turning the head anticlockwise opens it and moves you up towards the higher numbers. Turning it clockwise closes it down towards the frost setting. The number lined up with the small marker or arrow on the collar is the one that is active.

If yours turns but nothing changes at all, whichever way you go, the head has probably lost contact with the pin underneath and the valve needs looking at rather than adjusting.

What number should radiator valves be set to?

There is no single correct answer, but there is a sensible starting pattern that our engineers use when we set up or rebalance a system, and it is a long way from “everything on 5”.

• Living room and kitchen-diner: 3, occasionally 4 if the room is large or poorly insulated
• Bedrooms: 2 for most people, 3 if there is a young child or an older relative sleeping there
• Hallways, landings and utility rooms: 1 or 2, because you only pass through them
• Spare rooms and rooms used rarely: 1 or the frost setting, not fully off

Set it like that and then leave it alone for a few days. A TRV is a slow control - the wax takes time to respond and the room takes time to settle - so nudging it up and down every hour just keeps it hunting. If a room is genuinely not getting warm on 3, the problem is usually elsewhere: a radiator that needs bleeding, a system that has never been balanced, or a boiler flow temperature set too low for that particular room.

Should you turn radiator valves off in empty rooms?

Turn them down, not off. A room left completely unheated through a Kent winter gets cold walls, and cold walls attract condensation and mould, particularly in older solid-wall houses. A valve on 1 or the frost setting keeps the fabric of the room ticking over for very little gas.

There is a mechanical reason too. A TRV wound fully shut holds the pin at the bottom of its travel for months, which is exactly how valves seize. Our engineers see far more stuck valves in rooms that were “turned off for the winter” than in rooms that were simply set low.

Three controls people confuse

Most heating problems we are asked about on the phone come down to which of three controls is actually doing what. They are separate, and they need to agree with each other.

The room thermostat on the wall (or the Hive or Nest on your phone) is the master switch. When the room it lives in reaches its set temperature, it tells the boiler to stop firing for heating altogether. Every radiator in the house goes off, regardless of what its TRV says.

The TRVs are local trim. They cannot start the boiler; they can only stop hot water reaching their own radiator while the boiler happens to be running.

The boiler flow temperature is the dial on the boiler itself that sets how hot the water leaving it is. It affects how quickly every radiator warms and whether the boiler runs efficiently. We wrote a full guide to what your boiler flow temperature should be, and it is the one control of the three that most people have never touched.

Get these the wrong way round and you get the classic complaints: a thermostat set to 21°C but a living room that never feels warm because its TRV is on 2, or a bedroom on 5 that cooks while the hall thermostat, sitting in the coldest spot in the house, keeps the boiler firing all evening.

Why you should not fit a TRV in the same room as the thermostat

This is the rule our engineers explain most often. The radiator in the room where the wall thermostat lives should have a plain manual valve or a TRV set to its maximum, not a working TRV.

If that radiator has a TRV on 3, it shuts off at around 20°C. The wall thermostat, set to 21°C, is still waiting for the room to reach its target, so it keeps the boiler running. The boiler now cycles on and off indefinitely, heating every other room but never satisfying the one control that can actually switch it off. It is a surprisingly common cause of the “boiler never seems to stop” complaint, and it wastes a lot of gas.

When the valve, not the setting, is the problem

If a radiator does not respond to its TRV at all, the fault is in the valve, not your choice of number. Common signs are a radiator that is hot on the frost setting, one that stays cold on 5 while the pipe to it is warm, or a head that spins without resistance. These are jobs for a heating engineer, and they are worth fixing before the cold weather arrives rather than in the middle of it.

TRV work is on the water side of the system and does not involve the gas supply, but anything that involves adjusting or opening the boiler itself must be done by a Gas Safe registered engineer. All of our engineers are, and we are happy to check and set every valve in the house as part of an annual boiler service, or to sort a faulty one through our boiler and heating repair service.

If you would like a hand getting the controls in your home working together properly before the heating goes on for the winter, call our Tonbridge office on 01732 792 454 and we will get an engineer out to you.