Thermostatic radiator valves: what TRV settings mean

How a TRV actually works

Inside the head is a small capsule of wax, liquid or gas that expands as the room warms. As it expands it pushes a pin in the valve body, which gradually cuts the flow of hot water into the radiator. As the room cools, the capsule contracts, the pin springs back and more hot water flows in.

That means a TRV only controls its own radiator. It cannot turn the boiler on. The boiler is switched on and off by your programmer and the main room thermostat. If the room thermostat in the hall is satisfied, the boiler stops, and a bedroom TRV turned up to 5 will not bring it back on. Smart radiator heads that talk to a hub can work differently, and our guide to smart radiator valve compatibility covers those.

What the numbers on a TRV mean

Every manufacturer uses its own scale, so check the instructions for your valve if you have them. As a rough guide, many common valves use a scale like the one below. The snowflake or star setting is frost protection: the valve stays mostly closed and only opens if the room gets very cold.

Turning a TRV to 5 does not heat a room faster. The radiator gets the same hot water either way; the valve just shuts later. Set the temperature you want and leave it.

Setting TRVs room by room

Start by setting the main room thermostat to the temperature you want in the room it sits in, typically 18 to 21°C. Then set each TRV for its own room. Give the system a day to settle before changing anything, because a TRV reacts slowly and adjusting it several times in an evening makes the result hard to judge.

Keep the TRV head clear. Curtains drawn over it, furniture in front of it or a radiator cover all trap warm air around the sensor, so the valve thinks the room is warmer than it is and closes early. A TRV fitted in direct sunlight or next to a cooker has the same problem.

Do not fit or leave a TRV turned down in the same room as the main room thermostat. If the TRV shuts the radiator before the room thermostat is satisfied, the boiler keeps running with nowhere for heat to go. Most systems have at least one radiator, often in the hall or bathroom, left without a TRV or with its valve fully open for this reason.

Getting TRVs working again in autumn

The most common TRV problem shows up when the heating goes back on after summer. A valve that has sat closed for months can have a pin stuck in the closed position, leaving that radiator cold while the rest of the house warms up.

Before calling anyone, turn each TRV from its lowest setting to fully open and back a few times with the heating running, and then leave it on a normal setting. Often that frees the pin. If the radiator is still cold while its pipes are hot, the pin may be seized, and our guide on a TRV that is stuck or not working goes through the safe checks. Do not grip the pin with pliers or take the valve body apart; the valve holds system water under pressure.

It also helps to bleed any radiators that are cold at the top, and to check the boiler pressure on a sealed system before judging the TRVs. A radiator that is cold at the bottom has a different cause, usually sludge. At the end of the heating season, turning all TRVs fully open before the heating goes off for summer makes it less likely that pins will stick next autumn.

Common TRV problems

Most TRV complaints come down to a handful of faults. The table shows what each one usually means.

Fitting or replacing TRVs

Heads can often be replaced on their own, as long as the new head suits the valve body. Replacing the whole valve means the radiator has to be isolated or the system partly drained, then refilled, bled and brought back up to pressure. On a sealed system that is a job for someone comfortable working on central heating.

Since June 2022, Building Regulations guidance in England (Approved Document L) expects room-by-room temperature control, usually TRVs, when a boiler is replaced, where it is technically and economically feasible. If you are planning a new boiler, it is a good time to fit TRVs to radiators that still have manual valves. Our guide to radiator valve types compares the options.

Help with radiators and heating controls

Loyal Plumbers repairs radiators, valves and central heating systems across London and everywhere inside the M25, 24 hours a day, with 10 vans on the road. If a TRV is stuck, leaking or not controlling the room, or you want TRVs fitted across the house, call us or send an enquiry.

Tell us which radiators are affected, whether their pipes are hot, the make of valve if you can see it, and what the boiler pressure reads. That lets the engineer bring the right heads or valves.