Glossary · Central heating, radiators and hot water
What is a lockshield valve?
A lockshield valve is the valve at the bottom of a radiator, opposite the thermostatic valve. It balances the system by controlling how much water leaves the radiator. It usually has a plastic cap and is set by the installer during installation, so it rarely needs adjusting and is not for everyday control.
Key points
- It sits at the bottom of the radiator on the opposite side to the thermostatic radiator valve.
- Glow-worm says it balances the system by controlling how much water leaves the radiator, and rarely needs adjusting.
- Vaillant says lockshield valves have a plastic cap that must be removed to adjust them with a spanner or key.
- Balancing sets each radiator's flow so rooms heat evenly. Vaillant says it is different from bleeding.
- For balancing, Vaillant says to close each lockshield then open it about a quarter turn, with far radiators a little more open.
How a lockshield valve works
A lockshield valve sits at the bottom of the radiator, on the opposite side to the thermostatic valve. Glow-worm says its purpose is to balance the system by controlling how much water leaves the radiator. It says the valve rarely needs adjusting because the installer sets it during installation.[1]Glow-worm, Why is my radiator cold at the bottom?
Water does not reach every radiator equally. Vaillant says the difficulty of flow to a radiator depends on its location, its connecting pipes and other factors. Balancing adjusts this so each radiator gets the flow it needs and the whole home heats evenly, rather than some radiators heating quickly while others stay cold.[2]Vaillant, How do I balance radiators
The valves at each end
| End | Valve | Job |
|---|---|---|
| One side, bottom | Thermostatic radiator valve or manual valve | Controls hot water flow into the radiator |
| Opposite side, bottom | Lockshield valve | Balances flow by controlling water leaving the radiator |
| Top | Bleed valve | Releases trapped air |
Balancing and bleeding are different jobs
Vaillant says to bleed a radiator when the system rattles or gurgles or when a radiator has cold spots near the top. It says to balance when room temperatures differ widely, certain radiators fail to heat up, or radiators take a long time to heat across the home.[2]Vaillant, How do I balance radiators An individual radiator can be bled, but a proper balance needs all radiators to be checked.[2]Vaillant, How do I balance radiators
Vaillant describes the steps as follows. You need a lockshield valve key or an adjustable spanner, and the valve has a plastic cap to remove first. With the heating off and cold, open all valves, turn the heating on and note the order in which the radiators heat up. Let the system cool again, then close each lockshield and open it about a quarter turn. Open the radiators furthest from the boiler a little more than the earlier ones. Repeat the cold start test to check the radiators are heating at close to the same rate.[2]Vaillant, How do I balance radiators
If you are not comfortable doing this, a heating engineer can balance the system.
When it matters for a cold radiator
Vaillant says that if the whole radiator is cold and there is no air, you should check the valve setting, and that a valve can be blocked by sludge or limescale. It recommends a professional heating engineer to unblock the radiator.[3]Vaillant, Why is my radiator not working? Glow-worm says that when a radiator is being flushed, the installer closes all the valves, including the lockshield, so the radiator does not refill when it is drained.[1]Glow-worm, Why is my radiator cold at the bottom?
Example
Aisha in Bradford has a radiator in the front room that heats slowly while the one beside the boiler is scalding. She turns on the heating from cold, notes which radiator heats first, then closes the lockshield on each and opens it a quarter turn. She opens the front room one slightly further. After a second cold start test the radiators warm at a similar pace.
What people get wrong
Many people use the lockshield valve like an on and off tap and close it to stop a radiator heating. It is a balancing valve, not a control, and changing one setting affects how the flow is shared across the system.
The other mistake is bleeding a radiator when the problem is balance. Vaillant separates the two, and bleeding will not even out room temperatures.[2]Vaillant, How do I balance radiators
The bottom line
A lockshield valve is the capped valve at the bottom of a radiator that sets how much water leaves it. The installer normally sets it and it rarely needs changing. If rooms heat unevenly, balancing the system is the job, and it follows a set order. If a radiator is cold, check the other valve and the air first.
Common questions
What does a lockshield valve do?
Glow-worm says it balances the system by controlling how much water leaves the radiator. Vaillant says balancing gives each radiator the flow it needs, so the whole home heats evenly.
Which is the lockshield valve and which is the TRV?
Glow-worm says the thermostatic valve is on one side of the radiator at the bottom and the lockshield valve is at the bottom on the opposite side. The bleed valve is at the top.
Should I turn the lockshield valve off to turn the radiator off?
No. Glow-worm says it rarely needs adjusting because the installer sets it. Use the thermostatic valve or the manual valve for everyday control. Changing a lockshield setting changes how the whole system balances.
What do I need to adjust one?
Vaillant says a lockshield valve key or an adjustable spanner. The valve usually has a plastic cap that has to be taken off first.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Glow-worm. Why is my radiator cold at the bottom?. Checked 11 November 2026. ↩
- Vaillant. How do I balance radiators. Checked 11 November 2026. ↩
- Vaillant. Why is my radiator not working?. Checked 11 November 2026. ↩