Which radiator valve is the lockshield, and which one do I adjust?

This is a small piece of knowledge that prevents a large amount of trouble. People with a cold radiator often open the lockshield to fix it, which does make that radiator warmer — by taking flow away from somewhere else in the house. Understanding what each valve does turns radiator problems from guesswork into a two-minute diagnosis.

Step by step

1. Look at both ends of the radiator.There is a valve at each bottom corner. One of them you can grip and turn by hand. The other has a cap that just sits there and will not turn usefully. That is the whole distinction.
2. Identify the control valve.Either a wheelhead — a plain knob with no numbers, giving simple on and off with nothing in between — or a thermostatic radiator valve, which is fatter, has a numbered scale with a frost symbol and settings up to five, and contains a wax capsule that pushes a pin into the valve body as the room warms.
3. Identify the lockshield.A plain cap, usually plastic and often colour-matched to the valve body, that pulls or unscrews off to reveal a small square or slotted spindle. It is turned with a spanner or a lockshield key, and it is deliberately awkward so that nobody adjusts it casually.
4. Understand what each one decides.The control valve decides how much water goes into the radiator. The lockshield decides how fast water is allowed out. Set together across the house, the lockshields are what make every radiator get its fair share — which is what balancing means.
5. Know which one to touch.The control valve, always. Turn a TRV up or down, close a wheelhead to shut a room off, adjust whatever you like — none of it affects any other radiator meaningfully. Touching the lockshield changes the balance of the whole system.
6. Handle unmarked old valves.On old installations neither valve may have any marking at all. The rule still holds: the one you can turn by hand is the control, the one requiring a tool is the lockshield. If both look the same, look for a decorative cap that lifts off to reveal a square, which is the lockshield.
7. Check which end is flow and which is return.With the heating on from cold, the pipe that gets hot first is the flow. On most installations the control valve goes on the flow, though modern TRVs are usually bidirectional and can go either way. If a TRV is on the wrong side for its design, it can chatter or fail to control.
8. Remember the TRV head comes off dry.The head unscrews from the valve body without releasing any water at all, because the body seals independently. That is why a stuck TRV pin can be freed and a dead head swapped with the system full — one of the few genuinely easy heating repairs.

Common mistakes

• Opening a lockshield to warm up a cold radiator — It works for that radiator and steals flow from another. You have moved the problem rather than solved it, and re-balancing the house afterwards is a longer job than diagnosing the original fault.
• Putting furniture or a long curtain over a TRV — The TRV senses the air immediately around itself. Trap warm air against it and it shuts the radiator down while the room is still cold. It is a very common cause of a room that never warms up.
• Turning every TRV to five to heat the house faster — The room does not heat faster; it just overshoots and heats past where you wanted before shutting off. TRVs are not throttles.
• Assuming a radiator with only one adjustable valve is faulty — Some radiators — often the one in the room with the room thermostat, and some towel rails — are deliberately fitted without a TRV so the system always has a path for the water.

Questions people ask