Radiator Valves Explained: TRVs, Lockshield Valves and the Right Settings

Radiator valves control how much heat each radiator provides, but the two valves on a radiator do very different jobs. Understanding thermostatic radiator valves, or TRVs, and lockshield valves can help you improve comfort, avoid wasted energy and recognise when a heating fault needs professional attention.

What the two radiator valves do

TRVs control room temperature

A thermostatic radiator valve adjusts the flow of hot water into a radiator according to the temperature around it. Most TRVs have settings from 0 or frost protection to 5, although the exact temperatures vary between manufacturers. A setting of 3 commonly provides a comfortable living-room temperature, while bedrooms may feel better at 2 or 3.

A TRV does not make the boiler hotter, increase water pressure or heat a room instantly. It simply restricts or allows water through the radiator as the surrounding air reaches the selected temperature. The number is a target setting, not a measurement of how quickly the radiator will heat.

Keep the TRV head clear of curtains, furniture and radiator covers. If warm air becomes trapped around the valve, it may think the room is already hot and reduce the flow too soon.

• Use higher settings in rooms that need more warmth, rather than turning every radiator to maximum.

• Keep doors closed where appropriate so heated rooms do not lose warmth unnecessarily.

• Turn unused rooms down carefully, but avoid switching every radiator off during a heating complaint.

• Leave enough space around each TRV for air to circulate accurately.

Some radiators should not have a TRV

The radiator in the room containing the main wall thermostat is often left without a TRV, or kept fully open. This allows the thermostat to measure the room properly. If a TRV closes that radiator before the thermostat reaches its target, the heating controls may work against each other and other rooms can become uncomfortable.

There are exceptions depending on the heating system and control arrangement, so do not remove or alter a valve arrangement without advice. If you are unsure which room is the reference room, check where the thermostat is located and ask a qualified heating engineer.

Lockshield valves and radiator balancing

A lockshield valve is usually fitted at the opposite end of the radiator from the TRV. It controls the maximum flow through the radiator and is normally adjusted during system balancing, rather than for everyday temperature changes. Once set correctly, it is covered by a cap or requires a tool to turn.

It is usually best to leave a lockshield alone. Opening it fully or closing it by accident can change the flow around the whole system, leaving some radiators too hot while others stay cool. For more detail, read our guide to radiator balancing.

Common radiator valve problems

A stuck TRV pin

If a radiator remains cold when the heating is on, the TRV pin may be stuck after a long period of inactivity. With the heating off and the radiator cool, remove only the thermostatic head according to the manufacturer’s instructions. The small metal pin should move gently by a few millimetres and spring back.

You can carefully press it down and release it several times, but do not pull it out, force it with pliers or apply excessive pressure. If it will not move, the valve is leaking or the head is damaged, call an engineer.

Other safe checks

Check that the radiator valve is open, the radiator has been bled if it is cold at the top, and the boiler pressure is within the range shown in the boiler manual. Many sealed systems sit around 1 to 1.5 bar when cold, but the correct reading depends on the system.

Look for dampness, corrosion or staining around either valve. A seized valve, persistent leak or valve that will not respond should not be forced. Turn off the heating and arrange professional help, particularly if water is reaching electrical fittings or flooring.

Radiator troubleshooting guide

Radiator hot at the top only

A radiator that is hot at the top but cold lower down may have restricted flow or sludge inside it, especially if the temperature difference is substantial. Check that both valves are open and that the system has been balanced correctly. If several radiators have cold lower sections, ask about a system clean or power flushing.

Radiator will not turn off

First check the TRV setting and make sure the valve head is fitted correctly. A stuck pin, faulty TRV head or incorrectly adjusted lockshield can keep water flowing. Do not dismantle a pressurised or leaking valve, as replacing it may require draining part of the heating system.

TRV makes a clicking noise

An occasional click can occur as the valve responds to changing temperature, or as materials expand and contract. Repeated loud clicking, tapping or banging may indicate rapid water flow, a loose head or a balancing problem. Reducing the setting slightly may help temporarily, but recurring noise should be assessed by an engineer.

When to call a heating engineer

Get professional help for leaks, seized valves, repeated balancing problems or radiators that remain cold after basic checks. You should also arrange an inspection if the boiler pressure repeatedly drops, several radiators have cold spots, or the heating system needs frequent bleeding.

For guidance on a single cold radiator, see our radiator not heating advice. If the issue affects multiple radiators, a heating system assessment may identify sludge, poor circulation or balancing faults that simple valve adjustments cannot resolve.

For reliable advice with radiator valves, heating faults and power flushing, contact Pro Plumbers on 01732 444555. An experienced engineer can check the system safely, explain the settings and recommend the right repair.