What Is a Lockshield Valve? How It Works and How to Adjust It
The lockshield valve is the capped valve at one end of your radiator. What it does, how to tell it from the TRV, how to open, close and adjust it, and what to do when one leaks or won't turn.
A lockshield valve is the valve with a plain plastic cap at one end of a radiator, usually opposite the thermostatic or on/off valve. It controls how much hot water can flow through that radiator and is set once to balance the heating, not used day to day. Turn it anticlockwise to open and clockwise to close, using a lockshield key or small spanner after pulling off the cap. Close it together with the other valve to isolate a radiator.
Key Takeaways
- The lockshield is the capped valve with no numbers on it. The valve you turn day to day is the TRV or wheelhead at the other end.
- It sets how much flow a radiator gets, so it is used to balance the system and to isolate a radiator.
- Anticlockwise opens it, clockwise closes it. Count the turns before you close one so you can put it back.
- A radiator that stays cold while others heat up is often a lockshield that has been closed too far.
- A weeping lockshield is often a loose gland nut, but a corroded or seized valve needs replacing, which means draining down.
A lockshield valve is the valve with a plain plastic cap at one end of a radiator. It controls how much hot water the boiler can push through that radiator, and it is meant to be set once to balance the heating rather than turned every day. The valve you use to turn a radiator up or down is the one at the other end, usually a thermostatic radiator valve (TRV) with numbers on it. To adjust a lockshield, pull off the cap and turn the spindle with a lockshield key or small spanner: anticlockwise opens it, clockwise closes it.
What a lockshield valve does
Most radiators in UK homes have two valves, one at each bottom corner. One valve is for you: a TRV or a simple on/off wheelhead. The other is the lockshield, and it is there for whoever sets up the system.
Hot water takes the easiest route round a heating circuit. Radiators close to the boiler, or on short pipe runs, get more than their share, while radiators at the far end of the house get what is left. The lockshield is a flow restrictor. Closing it down a little on the radiators that heat up first pushes more water on to the ones further away, so the whole house warms at a similar rate. This is called balancing.
The lockshield also lets a radiator be isolated. With both valves closed, a radiator can be taken off the wall for decorating, or a leaking fitting can be repaired, without draining the whole system. The cap is there so the setting doesn't get knocked or fiddled with, which is where the "lock" in the name comes from.
How to tell it from the TRV
- Lockshield: a small, plain plastic cap, often white or chrome-effect, with no numbers. It spins freely or pulls off, and underneath is a square or flat-sided spindle.
- TRV: a larger head marked with numbers or a snowflake symbol, usually 0 or * to 5 or 6. It turns by hand and opens and closes automatically with room temperature.
- Wheelhead or manual valve: a ridged knob that turns by hand with no numbers. It works like a tap, simply open or closed.
On older systems both valves can look similar, and sometimes both ends have a lockshield. If neither end has a TRV or wheelhead, the radiator can only be controlled by the room thermostat or by adjusting the lockshield itself. Fitting a TRV on that radiator gives room-by-room control. Our TRV fitting and repair service covers swapping one in.
The lockshield is often on the return side of the radiator, but not always. It depends on how the radiator was piped, so don't assume flow direction from which valve is which.
How to open, close and adjust a lockshield valve
Take off the cap
Pull it straight up or unscrew it. Some caps are held by a small screw in the top. Don't lever it off with a screwdriver against the valve body, as the plastic snaps easily.
Use the right tool
A plastic lockshield key from a DIY shop fits most spindles and costs very little. An adjustable spanner or small grips also work. Pliers can round the spindle, so go gently.
Count the turns
Before closing a lockshield, turn it clockwise until it stops and count the turns, including part turns. Write the number on a piece of masking tape and stick it to the radiator. That lets you reopen it to exactly where it was.
Open it back up
Turn anticlockwise by the same number of turns. If you never counted, open it fully and then rebalance the system, because an unbalanced radiator will either overheat or starve others.
Using lockshields to balance radiators
If some radiators get hot quickly and others stay lukewarm, and bleeding hasn't helped, the system probably needs balancing. The basic method is:
- Turn the heating off and let the radiators cool.
- Open every TRV fully, and open every lockshield fully, counting the turns on each first.
- Turn the heating on and note the order in which the radiators get warm. The first ones to heat are nearest the boiler in pipe terms.
- Close the lockshield on the first radiator to heat almost fully, then open it about a quarter turn. Open the next radiator's lockshield a little more, and so on, leaving the last radiator fully open.
- Check again once the system is up to temperature, and fine tune.
Heating engineers use a pair of clip-on thermometers to set a steady temperature drop across each radiator, which is more accurate than touch. If you would rather have it done properly, see our radiator balancing service. Before balancing, bleed any radiators that are cold at the top. Our guide on how to bleed a radiator covers that. If one radiator stays cold no matter what the lockshield is set to, read why a radiator won't heat up.
Autumn is when unbalanced systems show up, because the heating is being asked to do real work for the first time in months. A cold bedroom at the end of the landing and a scorching radiator in the hall is the classic sign.
Leaking or stuck lockshield valves
Water weeping from under the cap
A drip from around the spindle is usually the gland nut, the small nut just below where the spindle comes out of the valve body. With the cap off, a gentle nip of about a quarter turn clockwise with a spanner often stops it. Don't overtighten or the spindle will seize. Our article on a leaking radiator explains how to tell a gland leak from a leak at the union nut or the valve body itself.
Water from the nut joining the valve to the radiator
This is the union or tail joint. It can sometimes be nipped up, but it often needs the radiator isolated and the joint remade with fresh sealant or a new tail. That is a job to book in if you aren't confident.
A valve that won't turn
Lockshields are often left untouched for years and can seize with limescale or corrosion. Don't force it with long-handled grips, because a snapped spindle means the valve has to be replaced. A little penetrating spray around the spindle and gentle back-and-forth pressure sometimes frees it. If you need to isolate the radiator urgently and the valve won't budge, turn the heating off and let it cool, and call a plumber.
When to call a plumber
Adjusting a lockshield and nipping up a gland nut are reasonable DIY jobs. Call a plumber if the valve body is corroded or cracked, the spindle has snapped, the leak is at a joint you can't reach, the system keeps losing pressure after a leak (see boiler pressure problems), or the radiators still heat unevenly after balancing, which can point to sludge that needs a power flush.
When you ring, say which valve is the problem, whether it is dripping or seized and whether the system is a sealed combi or an older open vented system with a tank in the loft. It helps us bring the right parts. Our labour is £95 an hour, the £60 call-out is waived when the job runs over 2 hours, and we confirm the price before starting.
Radiators heating unevenly or a valve leaking?
Tell us which valve and what it's doing. We'll explain the likely fix and confirm the price before any work starts.
📞 Call 020 7870 3200Or see our radiator repairs and pricing.
Frequently Asked Questions
Should a lockshield valve be fully open?
Not always. Radiators closest to the boiler usually need their lockshields partly closed so that hot water also reaches the radiators further away. Radiators at the end of the circuit are often left fully or nearly fully open. The right setting is whatever makes all the radiators heat up evenly.
Which way do you turn a lockshield valve to open it?
Anticlockwise opens it and clockwise closes it, looking down at the spindle once the cap is off. Count the turns as you close one so you can open it back to the same point afterwards.
Can I use the lockshield valve to turn a radiator off?
You can close it to isolate a radiator, usually together with the TRV or wheelhead at the other end, for example before removing the radiator or fixing a leak. For everyday control, use the TRV instead, so the balance of the system isn't lost.
Why is my radiator cold when the lockshield is open?
If the lockshield is open and the radiator is still cold, check the TRV pin is not stuck, bleed the radiator for air, and check whether other radiators are warm. A radiator that is cold at the bottom but warm at the top usually points to sludge rather than the valve.
Can you replace a lockshield valve without draining the system?
A lockshield valve sits on the pipe feeding the radiator, so replacing the valve body normally means draining at least part of the system. Some plumbers use a pipe freezing kit to avoid a full drain down. Replacing just the cap or tightening the gland nut does not need draining.