Radiator valves

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Nearly all radiators are fitted with valves and they are nearly always fitted at the bottom of the radiator, at opposite ends. There are manual valves and thermostatic valves. There are also electrically controlled radiator valves but they are much less common.
You need to be careful when dealing with radiator valves as central heating water is rarely clean and is often really black and filthy. We use tightly woven sheets when working with radiator valves. When we buy cotton decorators' sheets, we put them through a "boil" wash which shrinks them and tightens up the fabric. The tight weave then traps any black sediment.

Thermostatic Radiator Valves

Honeywell VT200 thermostatic radiator valve showing numbered temperature scale.

Honeywell VT200 Classic thermostatic radiator valve

Thermostatic Radiator Valves (TRVs) open and close automatically as the room temperature changes. They sense the air temperature in the room. TRVs don't cause the boiler to fire up and send water round the heating system. The heating has to be on anyway. When a heating demand is on, the radiator will heat up if the TRV is open.
If the room gets warm enough, the TRV will start to close down, restricting the water flow through the radiator. If the room air gets too warm, the TRV will stop the flow through the radiator completely, until the room gets cooler. The radiator will start to cool down and may go cold if the room stays too warm. That's normal.

TRVs are very good a keeping the room at a steady temperature.  Since they sense the air temperature they can also close off a radiator  if another source of heat such as a fire or sunlight makes the room too warm.
TRVs have a numbered scale on them, lower numbers being cooler and higher numbers warmer. You choose a number that gives you the right temperature in the room. The numbers are not exact, so one room may be set on 3 while another room needs to be on 4. Once you have chosen a setting they are good at maintaining a consistent temperature.
Most TRVs also have a frost setting, if you want the radiator to open only when the room is at risk of freezing.
TRVs can normally be manually closed, by turning them fully clockwise, but they are usually set to the correct number and left alone.

Plastic topped Honeywell VT117 thermostatic radiator valve showing numbered temperature scale

Honeywell VT117 thermostatic radiator valve

Unfortunately, TRVs often become faulty as they get older. If they are left in the closed position over the summer they commonly stick in that position. When they are turned back up to a higher number in the autumn little or no heat comes into the radiator.

This is caused by the central pin sticking and maybe the washer sticking to the valve seating. Ideally, the TRVs are left open during the summer when the heating is turned off. When colder weather comes they can be closed and re-opened. If the pin had tended to stick, closing the valve forces the pin down, freeing it.

Unfortunately if the pin is stuck down, opening the valve does not pull it up. Removing the head of the valve exposes the pin. If the pin is then repeatedly sharply tapped downwards it may become free and spring back up. How much force to use is a matter of judgement!

With stuck thermostatic radiator valves some people advocate pulling the pin up. If you try this be careful not to pull the pin right out. We find tapping it downwards better, as it is not only the pin which sticks, but the closed valve seating below it.

If a TRV sticks repeatedly it should be replaced. Most newer TRVs now use a common size of back-nut (and both back-nuts on the valve body are the same) so you may be able to use a different brand of valve while keeping the same back-nuts. It makes the valves easier to change.

The thermostatic action of TRVs can also fail, so they don't close down the flow through the radiator when the room gets too warm. If the temperature calibration has slipped out of range, or the thermostatic action has failed completely, the valve will need to be changed.


You should never rely on thermostatic radiator valves to stay closed if you take radiators off the wall. The calibration of a valve may be faulty and it may open as the room cools, even if it is apparently turned right off.
If this happens in a sealed heating system you will probably only lose a bucketful or two of water but it is enough to take down a ceiling overnight. If the system is open vented, it will keep on filling and, overnight, you can flood the house!

Decorators' caps

New Honeywell TRV not yet fitted to a radiator. The grey plastic decorators' cap is present as part of the valve kit.

Honeywell VT200 TRV with a grey decorators' cap

Thermostatic radiator valves are supplied with decorators' caps. These are only fitted to the valve if it is necessary to take the radiator off the wall.
Decorator's caps are specific to the valve manufacturer and maybe even to the valve model. They don't necessarily fit other makes of radiator valves.
The main valve head is removed and the decorators' cap is screwed on in its place. When the cap is screwed right down it physically closes the valve.
When the radiator has been removed you should check the valve to make sure that it is not weeping slowly. This can happen if the valve seating has become contaminated and does not seal perfectly. In this case you would probably want to change the TRV for a new one.
If you have lost the decorators' cap you will need to physically and securely close off the open end of the valve to be sure it won't leak as the room gets colder.

Manual Radiator Valves

Manual radiator valves come in two forms, wheelhead and lockshield. They are often supplied in pairs. Unless your radiators have TRVs fitted, they will usually have a wheelhead and a lockshield valve, one on each end. A matched pair of wheelhead and lockshield valves are usually identical, except for the plastic caps.

Wheelhead valve

Altecnic wheelhead radiator valve with plastic cap which turns the valve

Altecnic wheelhead radiator valve

With a wheelhead valve, the plastic cap can actually turn the valve mechanism. You can use the plastic cap to open and close the valve. The wheelhead valve is used for turning an individual radiator on and off. ("Wheelhead valve" is sometimes shortened to WHV.)
Sometimes the inside of the valve cap has been chewed up so that it doesn't turn the valve spindle. If there's any doubt, take the plastic cap off and operate the valve directly with a small spanner. It's the central spindle that needs to turn.

Lockshield valve

Altecnic lockshield radiator valve with plastic cap which spins but does not turn the valve.

Altecnic lockshield radiator valve

A lockshield valve has a cap which does not turn the valve mechanism. The cap either spins without engaging the valve mechanism or is locked in place so that it can't be turned. Lockshield valves are used to balance the radiators.
With the cap removed, a small spanner is used to open and close the valve. Progressively closing the valve restricts the water flow through the radiator. The cap is then refitted.
The cap is deliberately designed not to operate the valve. This is to prevent the valve setting being accidentally altered.
If the lockshield valve is one of a pair (with the wheelhead valve), the cap from the wheelhead valve can be used to turn the lockshield valve instead of a spanner. ("Lockshield valve" is sometimes shortened to LSV.)

If a radiator valve gland nut leaks

gland nut marked with a white arrow on a manual radiator valve

Altecnic radiator valve gland nut

If a valve leaks water from the gland nut (the first nut down the spindle) you may be able to tighten it (clockwise) slightly until it stops. We find a small pair of pump pliers best for doing this but an adjustable spanner should do.
Tightening the gland nut compresses the gland packing which is designed to prevent the water passing. However, it may make the spindle stiff to turn. If it does, you may have to slacken it again (slightly) while you turn the spindle to make adjustments, then tighten it afterwards.
Some gland nuts do not tighten as they are already locked down. These tend to rely on an O ring seal made of a rubber material. In time, this can wear away and the valve may leak. In this case the radiator valve will need to be repaired or replaced (usually replaced). Setting the valve to its fully open or fully closed position may temporarily stop the leak.

Which valve goes on the Flow and which on the Return?

The water flow through a given radiator is always in the same direction but it may be in a different direction from the radiator next to it or in a different room. It depends on how the radiator was piped when it was installed. The pipe which brings water from the boiler to the radiator is called the Flow. The pipe which takes it back to the boiler to be re-heated is called the Return.
So, starting the system from cold, the Flow pipe is the pipe which gets hot first. If you're working on the heating system it is worth marking up the Flow and Return ends on each radiator. We use red electrical insulating tape to mark the Flow end and blue to mark the Return end. It's worth marking both in case one piece of tape falls off or is pulled off. The remaining pieces will still allow you to identify which end is which.

Flow and Return with Wheelhead and Lockshield

Conventionally, the wheelhead valve is fitted to the flow pipe and the lockshield valve is fitted to the return. There is a reason for this.
If air finds its way into the system and travels towards a radiator, it comes along the flow pipe. When it enters the radiator it rises to the top where it is trapped until you bleed it out. Very little air makes it as far as the return pipe.
To balance the system, the lockshield valve might need to be set very largely closed. It might only be open about 20% of its maximum travel. If the lockshield valve is connected to the flow pipe and set like this, an air bubble may become trapped in the valve, effectively stopping the passage of water through the radiator. That may sound strange but it happens.
The wheelhead valve stays fully open when the radiator is in use, so the wheelhead valve is connected to the flow pipe. Any air coming along the flow pipe passes easily through the fully open valve and rises to the top of the radiator where it stays safely out of the way.
Paired wheelhead and lockshield valves are often identical to each other, except for the plastic cap. This means that the caps can be swapped, effectively changing the valves from wheelhead to lockshield and vice versa. You can't assume that the pipe connected to the wheelhead valve is the flow pipe. Remember, the flow pipe is the one which gets hot first.

Flow and Return with TRV and Lockshield
Uni-directional TRVs

Single arrow marked on the body of an old Myson thermostatic radiator valve. These radiator valves were uni-directional and the water flow had to be in the direction of the arrow.

Old Myson uni-directional TRV body

Until about 1995 all thermostatic radiator valves were uni-directional. They were made with a single-headed arrow which marked the direction of flow. They had to be fitted so that the water passing through them followed the direction of the arrow. This meant that they were normally fitted to the flow pipe.
Because they had back-nuts which were interchangeable, TRVs from some manufacturers, like Danfoss and Honeywell, could be fitted on the return pipe, with the valve head lying horizontally. The flow of water through the valve would then still follow the direction of the arrow.
If they were fitted with the water flowing the wrong way through the valve, they could make a sharp, repetitive banging noise. This happened just as they were closing off under the thermostatic action of the valve. The noise was sometimes described as being like machine gunfire.
Until 1995 nearly all TRVs were fitted to the flow pipe.

Bi-directional TRVs

Honeywell thermostatic radiator valve body showing bi-directional arrows

Honeywell VT117 TRV bi-directional arrows

Around 1995 Honeywell modified their TRV design and marketed new valves which were bi-directional. These could accept water flow in either direction and were marked with a double-headed, bi-directional arrow.
Because they are sensing air temperature, TRVs work better when they are fitted to the end of the radiator nearer the middle of the room, rather than tucked into a corner.
Heating installers originally had to design pipework so that the flow pipe went to the end of the radiator nearer to the middle of the room. It was also important not to get the flow and return pipe ends muddled up.
With the new bi-directional TRV, installers didn't have to worry. The TRV could always be fitted to the end nearer the middle of the room. It was no longer necessary to remember which pipe end was flow and which was return . The installers loved the new valves and Honeywell grabbed a large share of the TRV market. Very quickly, the other manufacturers followed suit and since then almost all TRVs have been bi-directional.

TRV balancing screw

Honeywell thermostatic radiator valve with head removed and showing balancing screw

Numbered black plastic balancing screw

Honeywell TRVs have a balancing adjustment screw built in. You fit a small slotted screwdriver into the slot in the black plastic ring round the pin.
The numbered section rotates. As it screws in it draws the pin down and progressively reduces the amount the valve can open.
If the Honeywell TRV is fitted on the return pipe we would use the built-in adjustment screw for balancing. If, however, the TRV is fitted on the flow pipe we would use the lockshield valve on the return pipe to balance the system.
Remember that if you have to balance a radiator using the valve on the return pipe you could get a bubble of air trapped in the valve if the valve is largely closed. The radiator would then go cold.
If this happens, fully open the valve while the heating is running. Make sure you note how many turns and part turns are required to open it. When the air has cleared through, close the valve down again by the same number of turns.


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