Lockshield valve: what it does and how to adjust it
A lockshield valve is the valve at one end of a radiator, usually under a plain plastic cap, that sets how much hot water flows through that radiator compared with the rest of the house. It is normally left partly open, not fully open and not closed, at a setting chosen when the system was balanced. You only need to touch it to balance radiators that heat unevenly, or to isolate a radiator completely for a repair. To adjust it, pull off the cap and turn the spindle underneath with a lockshield key or small spanner: clockwise closes it, anticlockwise opens it. Always count the turns so you can put it back.
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Discuss your plumbing workWhat a lockshield valve does
A central heating system pushes hot water round every radiator from one pump. Radiators close to the boiler get water first and most easily, and ones at the end of the run, often upstairs or in an extension, get less. The lockshield is there to even that out. Partly closing the lockshields on the nearer radiators pushes more of the flow on to the ones further away, so the whole house warms at a similar rate. That process is called balancing.
The other valve on the radiator, the thermostatic radiator valve (TRV) or the manual valve, is the everyday control for the room. The lockshield is set once and then left. That is where the name comes from: the cap shields the setting so it does not get knocked or fiddled with.
How to tell which valve is the lockshield
On most radiators it is obvious. The lockshield has a small plastic cap, often white or chrome, with no numbers, and it spins freely or does nothing when you turn it by hand. The TRV has a larger head numbered from 0 or * to 5 or 6. On radiators with two plain valves, the one you can turn by hand is the manual control and the one with a cap hiding a square spindle is the lockshield.
It does not matter which end of the radiator the lockshield is on. Installers fit it on either the flow or the return pipe depending on how the pipework runs, so you cannot tell which is which from the position alone.
Should a lockshield valve be open or closed?
Open, but usually only partly. A fully closed lockshield stops the radiator working completely, which is only right when you are isolating it. A fully open lockshield on every radiator means the system is not balanced, which is fine in a small flat but tends to leave the far rooms slow to warm in a larger house.
As a rough guide, radiators nearest the boiler often end up open a quarter to half a turn, and the furthest ones one to two turns or fully open. There is no single correct number, because it depends on pipe sizes, radiator sizes and the pump.
| What you see | Likely cause | What to do |
|---|---|---|
| Radiator stays cold, TRV is open | Lockshield closed or nearly closed | Open it half a turn at a time and wait for the radiator to warm |
| Radiator near the boiler roasting, far ones lukewarm | System not balanced | Close the nearest lockshields a little, working outward |
| Radiator whistles or hisses | Lockshield open too little, water squeezing through | Open it slightly; if it persists, check pump speed or valve |
| Water weeping from the spindle | Worn gland packing or O-ring | Tighten the small gland nut a quarter turn, or have it repacked |
| Spindle will not turn | Seized after years untouched | Stop before it snaps; the valve may need replacing |
How to adjust a lockshield valve
You need a lockshield key (a cheap plastic or metal cap that fits the square spindle) or a small adjustable spanner. Do this with the heating on so you can feel the effect, but take care with hot pipes.
- Pull or unscrew the plastic cap to expose the spindle.
- Before changing anything, close it clockwise, counting the turns until it stops, then open it back to the same point. Now you know the current setting and can always return to it.
- Turn anticlockwise to let more water through, clockwise to restrict it. Move in small steps, a quarter turn at a time.
- Wait at least 15 to 20 minutes after each change. Radiators are slow to respond, and adjusting too quickly is the most common reason balancing goes wrong.
- Put the cap back on and note the final setting for that radiator.
Balancing all the radiators
If some rooms heat well and others do not, and the cold radiators are warm at the top (so it is not air), balancing may solve it. Start with the heating off and the system cold. Open every TRV fully, and bleed any radiator with air in it first, because balancing a radiator with air in it gives false results.
Turn the heating on and note the order the radiators warm up. The first ones to get hot are nearest the boiler. Close their lockshields fully, then open them a quarter to half a turn. Work outwards, opening each lockshield a little more than the last, and leave the slowest radiator fully open.
For a more accurate result, engineers use clip-on thermometers on the flow and return pipes at each radiator and adjust the lockshield until the drop across each one is similar, commonly around 10 to 12°C on a traditional gas boiler system. Lower-temperature systems and heat pumps are balanced to different figures, so follow the installer's design. When you finish, set the TRVs back to their normal numbers.
If radiators are cold at the bottom, heat patchily or the system is full of black water, balancing will not fix it. That usually points to sludge, and the fix is cleaning the system, not adjusting valves.
Leaking, stuck or snapped lockshields
A small weep from under the cap usually comes from the gland around the spindle. Tightening the gland nut (the small nut the spindle passes through) by about a quarter turn often stops it. If it does not, the gland needs repacking or the valve replacing.
A leak from the nut where the valve joins the radiator, or from where the pipe enters the valve, is a union or compression joint rather than the spindle. See our guide to a radiator leaking from the valve.
If a lockshield will not turn, stop. A seized brass spindle can shear off, and then the radiator cannot be isolated or balanced until the valve is changed. Replacing a lockshield means isolating or draining that part of the system, so it is a job worth pairing with anything else that needs doing on the radiators.
When to call us
Call if a lockshield is leaking and tightening the gland has not helped, a spindle is seized or snapped, or you have balanced the radiators and some rooms are still cold. We have 10 vans covering London and everywhere inside the M25, 24 hours a day, and our engineers can replace radiator valves, balance the system or clear sludge while they are there. Our call-out charge is £60.
It helps to tell us how many radiators you have, which ones are cold, and whether the boiler is a combi or runs a hot water cylinder.
Common questions
- What does a lockshield valve do?
- It sets how much water flows through that radiator so every room heats at a similar rate. The TRV or manual valve at the other end is the everyday control.
- Should the lockshield be fully open?
- Not usually. It is normally part open, set when the system was balanced. Radiators far from the boiler may be fully open; ones close to it are often open under half a turn.
- Which way do I turn a lockshield valve to open it?
- Anticlockwise opens it and clockwise closes it. Count the turns from fully closed so you can return to the same setting.
- Can I turn the heating on and off with the lockshield?
- You can, but you should not. Use the TRV for everyday control. Changing the lockshield upsets the balance of the whole system, and old spindles wear or seize.
- Why is my radiator cold with the TRV on full?
- The lockshield may be closed, the TRV pin may be stuck, or there may be air or sludge in the radiator. Check the lockshield first, then the TRV pin, then bleed it.
Sources and further reading
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