Radiator Balancing

Inadequate radiator balancing is one of the commonest central heating problems. A little knowledge and a little effort can bring big improvements!

Radiator balancing isn’t scary

We’ve looked online to see what’s out there about radiator balancing. Most of it seems complicated and suggests that special tools are required. We don’t agree. For most central heating systems, it really is easy. It’s not quick but if you follow through methodically, most systems can be balanced.
Are the results perfect? No, since most central heating systems are far from perfect, but you can get 90% of the way there and almost anyone can do it. It takes a little time and a few simple tools and we’ll show you how.

If you’ve been told you need power flushing…

Customers are frequently advised to try power flushing. There are times when a central heating system is so full of muck (usually black iron oxides and rust) that it really needs flushing out. This is particularly true if it causes pump failure. However, power flushing is expensive. It brings some benefits but can cause problems too and in most cases it’s not necessary. It’s almost always better to try radiator balancing first and, if necessary, change the pump.
If only one or two radiators don’t get hot, you should check that the radiator valves are not faulty. Thermostatic radiator valves (TRVs) commonly stick in the closed position.

You need to know about radiator valves

Radiator balancing is done using radiator valves. If you already understand the differences between lockshield valves, wheelhead valves and thermostatic valves and how to set them up, you can move on. If you’re not sure, read our page about radiator valves first.

It helps to know how your central heating system works

Again, if you are confident about how your system works, you can move on. If you’re not sure, read our page about central heating first and learn about Flow and Return.

It’s not a radiator balancing problem if…

If all your radiators get hot, evenly, in around the same time, they don’t need balancing.
If they were all getting hot evenly but there’s been a sudden change, it’s probably not a radiator balancing issue. It’s more likely to be a pump problem. If your radiators upstairs are all hot but downstairs barely warm, that’s likely to be a pump problem too.
If your radiators are cold at the top but hot lower down they need bleeding.
You should always bleed all the air out of the central heating system before trying to balance the radiators.

Tools for Radiator Balancing

You should have some absorbent rags and old towels handy. Opening and closing radiator valves sometimes makes them leak from the gland nut (the first nut down the spindle) and you need to be able to contain any dribble until you can get it sorted out. Always make sure you have absorbent sheets or towels under the valves you open or close.

Red-handled adjustable pump pliers
Good quality pump pliers

You will need a small adjustable spanner or a small pair of pump pliers as you may need to tighten gland nuts. We prefer to use pump pliers for this as they grip better and are less likely to distort stubborn gland nuts.

If the radiator valve caps are screwed on you will need screwdrivers. You may need one plain slot tip screwdriver, with a tip about 4 or 5mm wide; one No.2 pozidrive tip and one No.1 Pozidrive tip. Pozidrive screwdrivers are similar to Phillips screwdrivers but not identical. No.2 and No.1 Phillips screwdrivers might work instead.

We use red and blue electrical insulating tape to mark the flow and return ends of a radiator, red for flow, blue for return.

Check and mark which pipe is Flow and which is Return…

As the central heating system heats up from cold, one end of each radiator will get hot before the other. The pipe connected at that end is the flow pipe. The other end is the return.
Large radiators clearly get hot at one end before the other. However, if you have a small radiator connected to a powerful boiler, you may need to hold both the flow and the return pipe to be sure which one gets hot first. If you don’t, the whole radiator may heat up too quickly for it to be obvious.
Mark the flow and return ends as you find them on each radiator. It’s better than trying to remember. We use a small piece of red insulating tape stuck to the radiator at the flow end and a small piece of blue insulating tape at the return end.

Start balancing

Radiator balancing is the process of getting the right amount of water to flow through each radiator so that they all get hot in about the same amount of time.
Make sure that the radiator system is turned on at the programmer or timer and that the room stat is turned up high, calling for heat.

If only 1 or 2 radiators are slower…

If only one or two radiators are slower than the rest, check that the valves on both ends are fully open. If one valve is a TRV, open it by turning it fully anticlockwise. Then open the lockshield valve (LSV) fully by taking off the plastic cap and turning the central spindle anticlockwise until it stops.

If both valves are manual valves,  open the LSV fully and then open the wheelhead valve (WHV) fully by turning the plastic cap anticlockwise until it stops. If it just turns and turns, and doesn’t stop, take off the cap and turn the central spindle of the valve anticlockwise using a spanner until it stops. Don’t force it. It doesn’t matter which valve you open first.
Some manual valves may turn only two complete turns between fully closed and fully open but others may turn 5 complete turns. It depends on the valve design.

If, within a few minutes, these colder radiators get hot like the others, you may not have to do anything else. If they don’t get properly hot and there is a TRV fitted to the radiator, check that it is not stuck closed.
If they don’t get properly hot even though both valves are fully open, there’s one more thing to try before you start to balance the whole system. Read our page about air locks.

Balancing the whole radiator system

We’re assuming you’ve already marked the flow and return end on each radiator.
Fully open both radiator valves on all radiators. They open anti-clockwise. Some may open by turning the cap, others will need to be opened using a spanner at described above.
Make sure the heating is turned on (in an “On” period on the programmer or timer) and the room stat is turned high so that it is calling for heat.
Turn off the programmer or timer for the hot water (cylinder) circuit. It’s easier to balance if only the radiators are on.
Turn the circulating pump up to its highest setting. If the pump is too noisy to be left at its highest setting, it may be possible to balance the system with the pump set to a lower speed but it’s easier if it is pumping faster.
Allow the system time to warm up. This could take up to 15 or 20 minutes.
If, miraculously, all the radiators get hot together, there’s nothing more to do. That would be very unlikely!

Restricting the flow through the hottest radiators

In almost every heating system, some radiators will be hotter than others. What we’re aiming for with radiator balancing is to restrict the flow through the hottest radiators so that more of the hot water is sent to the cooler radiators. We do this by progressively closing down the return valves on the hot radiators. This is at the blue-taped end of the radiator.

TRV balancing screw

Numbered balancing screw in a Honeywell thermostatic radiator valve
Screwdriver slot in adjustable Honeywell TRV balancing screw. The valve head has been removed

Honeywell thermostatic radiator valves have a balancing adjustment screw built in. A small slotted screwdriver is used to turn the numbered black plastic adjustment screw which surrounds the central pin. Turning clockwise reduces the flow through the valve.
If the TRV is fitted on the return pipe we would use the built-in adjustment screw for radiator 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.
If there is no balancing screw built into a TRV, you will have to use the opposite, lockshield valve for balancing, even if it has been fitted to the flow end.

Note which radiators are hottest and restrict them

Allow the system to heat up (maybe 20 minutes) and note which radiators are really hot. We do this by touch and don’t bother with thermometers. It doesn’t matter which hot radiator you restrict first. Go to the return end of the radiator (the blue-taped end). If it has a wheelhead cap and there is a matching lockshield cap on the other end, first swap the caps over.
Next, take off the cap on the return valve. Then, using a small spanner, turn the central spindle of the valve clockwise from fully open to fully closed. Note how many complete and part turns it takes. Now open it again to half way. For example, if the valve travels 4 complete turns from open to closed, open it back up 2 complete turns. That should be half way.
Do the same to all the other hot radiators but remember, different patterns of valve will travel a different number of turns. You’re aiming for the valve to be half open.
If the valve at the return end is a TRV it may have an internal balancing adjustment screw. If it does, adjust it to the half closed position. If the TRV has no internal balancing screw, you will have to balance the radiator using the lockshield valve at the other end of the radiator, even though it is on the flow end.

If a radiator valve gland nut leaks

Red arrow marking the gland nut on a radiator valve
The arrow shows the gland nut which can be tightened to prevent water leaking past

When radiator valves are adjusted they may leak dirty water from the gland nut. This is the first nut down the central spindle of the valve. If a valve leaks water from this point you may be able to tighten the gland nut slightly (turning it clockwise) until the water leak stops.
Some gland nuts are locked down and cannot be tightened. If these glands leak you may be able to temporarily reduce or stop the leak by turning the central spindle of the valve to its fully open or fully closed position. There’s more about gland nuts on our radiator valves page.

Allow the radiator temperatures to settle then check again

When all the really hot radiators have been restricted, allow about 15 minutes for the radiator temperatures to settle. Now go round and check their temperatures again, by touch.
Unless all the radiators are now equally hot, go to the hotter radiators and restrict them as you did before. If they are radiators which have become newly hot, restrict them on the return end by half the travel of the valve. If they are radiators you restricted in the first round, restrict the return end a little further.
The amount you restrict each time doesn’t have to be exact. It’s trial and error. After each round of restriction, allow the radiators 15 minutes for the temperatures to settle again.

If a previously hot radiator becomes cooler

Make sure that the programmer is continuously on for heating and that the room stat is turned up really high so it doesn’t get hot enough to switch off. Also make sure that all TRVs are set to maximum on the externally visible number setting on the valve (this is different from the internal balance adjustment).
If a radiator which had been hot gets cooler, it has been restricted too far. Go back to the return valve and open it up a little. Don’t open it too much at first. It may only take a little. Now give the radiators time to equilibrate (settle) again.
It may take 3 or 4 rounds of adjustment before you get the system balanced. Provided you have the time, it doesn’t matter. If you close a radiator return valve too far, the radiator will go cool. Just open it up a little and give it time to equilibrate before checking it again.
If the colder rads are badly piped or the pipes are significantly sludged up you may find that you have to restrict the hotter rads a very long way. Sometimes it’s necessary to restrict the valve by more than 80% of its travel. This isn’t unusual. If you go too far, remember that it’s totally reversible.

If some rads simply won’t get hot

If you’re unlucky, some rads just will not get hot. Radiator balancing just won’t sort it out. You will then need good professional help. There may be a blockage in the pipework, particularly if it is 8mm or 10mm microbore. There may be a valve which is broken internally. You may need to have the pump replaced. In most cases though, you will achieve a marked improvement in the balance of your central heating system.

If it worked, and you’re happy with the result, well done!


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