Radiator Valve Types Explained

Heating controls are covered by Approved Document L, which is why modern installations require thermostatic control on most radiators. In practice, most radiators have two valves that look superficially similar and do completely different jobs. Confusing them is behind a lot of heating complaints, because adjusting the wrong one unbalances the whole system.

The Two Ends

The control valve

At one end, with something you are meant to turn — a numbered dial, a wheel head, or a smart head. This is the one you adjust day to day. It controls how hot that room gets.

The lockshield valve

At the other end, with a plain plastic cap covering a small square spindle. This is not a user control. It sets how much flow that radiator draws relative to every other radiator in the house.

It is set once during balancing and then left permanently. The cap exists specifically to stop people adjusting it.

Thermostatic Radiator Valves

The most common control valve in modern homes. A numbered head containing a wax or liquid capsule that expands as the room warms, pushing a pin down to close the valve.

It is a room thermostat, not a heat setting. The numbers correspond roughly to target temperatures rather than output — setting it to 5 does not make the radiator hotter, it just means the valve stays open until the room is warmer.

Getting the best from them

• Set 3 for living areas, 2 to 3 for bedrooms, and lower for hallways.
• Do not put a TRV in the same room as the main house thermostat — they fight each other. That radiator should have a manual valve instead.
• Keep the head unobstructed. A TRV behind a curtain or a sofa reads its own trapped heat and closes early, leaving the room cold.
• Open every TRV fully in summer. Left closed for months, the pin seizes and the valve is stuck shut come autumn.

A seized pin is the single most common TRV fault. Pull the head off and press the pin — it should spring back. If it is stuck, gentle taps or grips will usually free it. Our guide to thermostatic radiator valves covers this in detail.

Manual Valves

A simple wheel head that opens and closes flow, with no temperature sensing. Whatever you set stays set until you change it.

Still the right choice in the room containing the main thermostat, and sometimes on a towel rail where you want constant output.

Lockshield Valves

The plain-capped one, and the most misunderstood valve in the house.

Its job is balancing. Radiators nearest the boiler naturally get more flow than those furthest away, so the near ones are throttled slightly with the lockshield until every radiator warms at a similar rate.

If you take a radiator off, count the turns as you close the lockshield and reopen it by the same amount. Our guide to removing a radiator covers why.

Angled, Straight and Corner

This is about geometry rather than function, and it depends where your pipes come from.

• Angled — a 90-degree bend. Standard where pipes come up out of the floor.
• Straight — in line. Used where pipes come horizontally out of the wall or skirting.
• Corner — turns through 90 degrees in a different plane, keeping the head facing outward. Neater in tight spaces.
• H-block — a single fitting handling both flow and return, for radiators with a central bottom connection.

Measure and photograph your existing setup before buying. Getting angled instead of straight is the most common ordering mistake.

Smart Radiator Valves

The Energy Saving Trust has comparisons of smart heating controls if you are weighing them up. A smart TRV is a thermostatic valve with a motorised head, controlled from an app or a hub. They allow different schedules and temperatures per room, which is genuinely useful in a house where rooms are used at different times.

Worth knowing before buying:

• They need batteries, which need changing.
• They are physically bulkier, and can foul skirting or a windowsill in a tight spot.
• They control the radiator, not the boiler. Unless the system also has a compatible room thermostat, the boiler may keep firing while every valve is closed.
• They fit standard valve bodies, so usually only the head is replaced rather than the whole valve.

Flow and Return

Older thermostatic valves were directional and had to go on a specific side, marked with an arrow. Most modern ones are bi-directional and work either way, but check the arrow if there is one — fitted backwards, a directional valve chatters and closes unpredictably.

Frequently Asked Questions

Which end does the thermostatic valve go on?

Traditionally the flow side, though most modern valves are bi-directional. What matters more is that it is on the end where the head will not be blocked by furniture or curtains.

Can I put a TRV on every radiator?

Not on the one in the room with the main thermostat — the two controls fight and the boiler short-cycles. Building regulations require a means of keeping some circulation available.

Why is my lockshield capped?

Deliberately. It is a commissioning setting, not a user control, and adjusting it unbalances the system.

Do I need to drain the system to change a valve?

Usually yes, unless the pipework has isolation valves nearby. Changing just the thermostatic head needs no draining at all — the body stays in place.

If a Valve Needs Replacing

Changing a valve body means draining down, and a seized or leaking valve on old pipework can be awkward. We cover Milton Keynes and the surrounding area for heating repairs. Get in touch.