Replacing a Lockshield Valve on a Radiator

Its job on the radiator

Hot water from the boiler enters the radiator through the flow side, where the thermostatic radiator valve (TRV) with its head usually sits. Cooled water leaves through the return side, and that is where the lockshield valve is fitted. In the trade it is also called a return valve or a balancing valve. It does two things:

• it closes off the radiator, so with the flow valve also shut, the radiator can be removed without draining the whole heating system,
• it lets the installer limit the flow of water through that radiator, so radiators close to the boiler do not "steal" hot water from those further away.

This limiting, called pre-setting or balancing, is done with an inner spindle under the plastic cap, usually turned with a small spanner or an Allen key. Homeowners do not touch it during the year. A heating engineer sets it when balancing the system, and after that the valve quietly does its work for years.

Why the valve gets replaced

• It drips at the spindle, where the adjusting screw comes out of the valve body. The gland packing or O-ring loses its elasticity over time.
• It weeps at the union nut between the valve and the radiator, usually after many years or after being overtightened.
• It will not close, or it lets water through when closed, because the seat is coated with deposits.
• The body is cracked, for example after freezing in an unheated house or after overtightening.
• The radiator is being replaced and the old valve no longer fits in size or connection.
• When the system is balanced, it turns out the old valve has no pre-setting at all.

In practice it often happens that nobody has touched the valve for years. On the first attempt to close it the spindle turns, but water keeps flowing, or it starts dripping right at the spindle. That is a good reason to close and reopen these valves now and then, even when nothing needs repairing.

The price list has three related items that are easy to mix up. The difference is which end of the radiator is being dealt with, and how much of it.

• Replacing the lockshield valve (this page): one valve on the return side is replaced, with no head. The thermostatic valve on the flow side stays. This suits you when the problem is only on this side and the flow valve is fine.
• Replacing the thermostatic valve: deals with the flow side, the TRV body and usually the head as well. A different part, a different function, a different setting.
• Installing radiator valves: fitting valves to a new radiator or new pipework, usually both ends at once, often including connecting to the pipes from scratch.

When is the return side alone enough? When the TRV is fairly new, seals well and controls the temperature properly. If the flow valve is the same age and in the same condition as the lockshield, it is worth considering replacing both at once. The most time-consuming parts are isolating and draining, then refilling and bleeding, and those happen once whether one valve is changed or two. Do not mix new and very old parts without thinking, though: a new TRV from a different manufacturer can have a different body length, and then the pipework needs adjusting on both sides.

A special case is a radiator with a bottom central connection, where flow and return run through one twin valve block, often called an H-block. On these, a separate lockshield usually cannot be replaced; the whole block is changed. If you have this type of radiator, say so in advance so the engineer brings the right part.

In a house

In a house with its own boiler you control draining and refilling yourself, so arranging a date is simpler. It helps if you have ready:

• clear access to the radiator, furniture moved and curtains tied back,
• any radiator cover or window board removed if it blocks access,
• information on whether the system holds plain water, antifreeze (glycol) or an inhibitor,
• somewhere to empty the drained water and access to the drain-off cock,
• a photo of the valve and any markings on it, if you want to send it beforehand.

Antifreeze does not go down the drain. It is collected in containers and usually reused, or topped up with fresh fluid according to the manufacturer's data. Allow for this when planning, as collecting it takes time and containers.

In a flat with communal heating

In a block of flats, the heating pipework is usually a shared part of the building, and draining it is decided by the building management or the owners' association. If the radiator cannot be reliably closed off on both sides, the whole riser has to be drained, which affects the neighbours above and below you as well. So:

• agree a date with the building management in advance; outside the heating season this is much easier,
• find out who drains and refills the riser and who bleeds the neighbours' radiators,
• ask whether the building requires a specific valve type because of system balancing or heat cost allocators.

Replacing a valve like for like is ordinary maintenance. If more is changing, for example moving the radiator or altering the pipework, check with the building management and, where relevant, your local building authority whether anything needs to be arranged. Local building regulations differ, and the authority decides on the specific case, not general advice.

An alternative to draining the riser is pipe freezing. The engineer uses a special freezing kit to form an ice plug inside the pipe and changes the valve without draining. This works on copper and steel of common sizes, usually not on plastic, and not every engineer owns the equipment.

Shape: straight or angled

An angled valve is used when the pipe comes out of the wall or floor at right angles to the radiator connection. A straight valve is used when the pipe runs in line with the connection, usually from the side. The replacement should have the same shape, otherwise the pipework has to be reworked. On radiators with bottom side connections from the floor, angled valves are more common; on old column radiators with pipes coming out of the wall, straight ones are typical.

Connection to the pipe

• Threaded steel pipe: a valve with a female thread, usually half an inch. Sealed with hemp and jointing paste or with thread-sealing cord.
• Copper pipe: a compression connection with an olive sized to the pipe, commonly 15 mm, or a male thread with a matching compression adaptor.
• Plastic and multilayer pipe: most often a Eurocone connection with an adaptor for the exact size and type of pipe. No hemp or tape here; the cone and O-ring do the sealing.

The radiator tail

The valve connects to the radiator through a tail, which screws into the radiator, and the valve is pulled onto it with a union nut. The tail usually comes with a new valve and is normally replaced as well. The old one may have a worn sealing face, and the new valve might not seat against it properly.

Extra features

• Pre-setting: standard today, not a given on older valves. Essential if the system is being balanced.
• Drain-off: some valves have a small drain point that lets you empty the radiator straight into a hose. This helps a lot if the radiator has to come off later.
• Shut-off without losing the setting: better valves separate the shut-off from the pre-setting, so after closing and reopening there is no need to set it again.

The detail that deserves the most attention is the body length, meaning the distance from the radiator to the pipe connection. If the new valve is longer or shorter than the old one, the pipe will not line up. With copper and plastic this can be taken up by bending or trimming the pipe; with steel it often means reworking a section of pipe. So it pays to measure the old valve and choose a replacement with similar dimensions.

1. Checking the situation: the engineer checks the valve type, the pipework and whether the TRV on the flow side can be closed. If the flow side will not close either, a larger part of the system has to be drained.
2. Recording the setting: before removal, the engineer counts how many turns the old valve was open, so the new one can be set similarly. If the system was balanced from a calculation, it helps to have those values to hand.
3. Switching off and isolating: the boiler is switched off, left to cool and the flow valve closed. Then, depending on the situation, the radiator, a branch or the whole system is drained, or the pipe is frozen.
4. Removal: the union nut at the radiator and the connection at the pipe are loosened. On old valves it is important to hold the pipe and the tail with a second spanner, so the pipe in the wall does not twist and no joint elsewhere is disturbed.
5. Removing the old tail: done with a radiator spanner or tail key. If it is seized, careful heating or a penetrating spray helps, never brute force on the radiator.
6. Cleaning: the threads in the radiator and on the pipe are cleaned of old sealant and rust, and the pipe thread is checked for damage.
7. Fitting the new tail and valve: the tail is sealed according to its type, either with an O-ring or with thread sealant. The valve is connected to the pipe and the union nut tightened with feel.
8. Refilling and bleeding: the flow valve and the new valve are opened, the system refilled, the radiator bled and others too if needed. The pressure is topped up to the value given by the boiler manufacturer.
9. Leak check and setting: the joints are checked cold, then with the heating running. The pre-setting is adjusted and the cap fitted.

A straightforward case

• The TRV on the flow side closes tightly, so only the radiator is drained.
• The pipework is copper or plastic and can be eased into position.
• The new valve has the same body length as the old one.
• The radiator is freely accessible, with no covers or built-in furniture.
• The system holds clean water without antifreeze.

A more difficult case

• Old steel pipework with corroded threads. The pipe thread may be damaged during removal, and then a piece of pipe has to be cut off and rethreaded, or an adaptor fitted.
• A tail seized in the radiator, not unusual on old steel radiators.
• The flow valve will not close, so the whole system or the riser has to be drained.
• A flat in a block, where everything waits on the building management and neighbouring flats also need bleeding.
• Sludge and deposits in the system that come loose once it is opened and clog the new valve. In that case it is worth flushing the system and checking the filter at the boiler.
• Pipes buried in the wall right next to the radiator, with no room to move them.

How long it takes

Swapping one valve with matching dimensions usually takes roughly one to two hours. Draining, refilling and bleeding take longer, especially when the whole house is drained. With steel pipe alterations or a seized tail, the job can stretch to half a day. Nothing needs to dry or cure, and the heating can run as soon as the system is refilled and bled. Air keeps coming out of the water for several days, though, so it is sensible to bleed the radiators again after a few days and check the pressure.

• Pre-setting not adjusted: the new valve stays fully open and the radiator draws more water than it should. The result is that distant rooms do not warm up properly, or the valve hisses and whistles.
• Overtightened union nut: the brass cracks, sometimes only weeks later. A hairline crack first just looks damp and later drips.
• Wrong sealing: hemp or PTFE tape wrapped onto a Eurocone or the cone seat of the tail. The joint then leaks, because the material stops the sealing faces from meeting.
• Flow and return swapped: the TRV belongs on the flow. If both valves are replaced and the sides get swapped, the thermostat controls badly and the valve may knock. You can tell the flow side because that pipe is warmer when the heating is on.
• Poor bleeding: the radiator is warm only at the bottom, gurgles, or the boiler cuts out on low pressure.
• Pressure not checked: after refilling the pressure is not topped up, or it is set too high. When the system then gets hot, the pressure relief valve opens and water drips below the boiler.
• Twisted pipe: loosening without holding the pipe turns it and disturbs a joint inside the wall or floor. This is only discovered when the plaster becomes damp.

If you live in a house with its own boiler, copper or plastic pipework and a replacement valve of the same dimensions, you can do this yourself. Do not attempt it in a block of flats without agreement from the building management, on old steel pipework with corroded threads, or with antifreeze in the system if you have nowhere to collect it. Operate the boiler only as its manual describes; anything involving the boiler's electrics or gas side belongs to a qualified professional, and gas work only to a registered gas engineer.

1. Switch off the boiler and let the system cool down.
2. Find and note the old valve's setting by counting the turns to fully closed.
3. Close the TRV on the flow side by turning the head to zero or fitting the manual shut-off cap.
4. Put old towels under the radiator and have a shallow tray ready.
5. Drain the radiator or the relevant part of the system; opening the bleed valve lets the water run out faster.
6. Loosen the union nut and the pipe connection, holding against them with a second spanner.
7. Unscrew the old tail and clean the thread in the radiator.
8. Screw in the new tail with the sealing the manufacturer specifies.
9. Connect the valve to the pipe and tighten the connection and the union nut with feel, without extending the spanner.
10. Set the pre-setting to the noted value or as the manufacturer recommends.
11. Refill the system, bleed the radiator and top up the pressure as the boiler manual describes.
12. Switch on the boiler and check the joints when hot, after cooling and again the next day.

If the pipe thread gets damaged during removal or a joint still weeps after tightening, call a heating engineer. Further tightening usually makes things worse.

A finished valve does not look like much, so it is worth spending a few minutes checking it at handover and again after a few days of heating. Check:

• Dry joints at the tail, the union nut and the pipe connection
• A dry spindle area, including after closing and reopening the valve
• That the valve closes and opens again without force
• The cap is fitted and you have been told the pre-setting value
• An evenly warm radiator once the heating is on, with no gurgling
• System pressure in line with the boiler's data, straight after refilling and after a few days
• Undamaged pipes and plaster where the pipes pass through the wall
• A dry floor under the radiator and under the boiler
• In a block of flats, confirmation from the building management that the riser is refilled and bled