Balancing is a free job most homeowners can do in an afternoon with a lockshield key and a bit of patience. You are adjusting one small valve on each radiator so the hot water shares itself out evenly instead of rushing through the radiators closest to the boiler.
What balancing means and why systems drift
Water in a central heating system follows the easiest path. Radiators on short pipe runs close to the boiler offer little resistance, so they take more than their share of the flow. Radiators at the end of long runs, upstairs or in an extension get what is left and stay lukewarm. Balancing adds a little resistance at the easy radiators so that every radiator gets enough.
A system that was balanced when it was fitted can drift out over time. Common reasons are a new radiator or extension, a replacement boiler with a different pump, valves knocked or fiddled with during decorating, radiators taken off the wall, and a system flush or powerflush that changed how freely water moves.
Signs you need to balance, and why you bleed first
- Radiators near the boiler get hot quickly while those furthest away stay lukewarm.
- Downstairs is warm long before upstairs, or the reverse.
- One room never reaches temperature even though its radiator is the right size.
- The return pipe back to the boiler gets hot very quickly.
Air trapped at the top of a radiator gives a similar cold patch, so always bleed before balancing. Our guide on how to bleed a radiator covers that part, including topping the boiler pressure back up afterwards. If a radiator is cold along the bottom rather than the top, that points to sludge, which balancing will not cure; see radiators cold at the bottom.
Lockshield valves, TRVs and wheelhead valves
Most radiators have a valve at each end, and only one of them is used for balancing.
| Valve | How to spot it | Use when balancing |
|---|---|---|
| Lockshield | Plain plastic cap with no numbers, often pulls or unscrews off to show a small square spindle | This is the one you adjust |
| TRV (thermostatic radiator valve) | Numbered head, usually 0 or * to 5 or 6 | Set fully open during balancing, then back to normal |
| Wheelhead | Round handle you turn by hand, no numbers | Open fully during balancing |
The lockshield sets how much water the radiator can take. The TRV or wheelhead is the everyday control. Balancing with the TRV instead of the lockshield does not work, because the TRV keeps opening and closing as the room warms.
You also need to know which pipe is which. The flow is the pipe bringing hot water in, and it gets warm first when the heating starts. The return carries cooler water back to the boiler. On a given radiator either side can be the flow, so feel both pipes a few minutes after the heating comes on.
What you need
- A radiator bleed key, for bleeding first.
- A lockshield key or a small adjustable spanner to turn the lockshield spindle.
- A clip-on pipe thermometer or a digital thermometer with a probe. Two clip-on thermometers make the job quicker.
- A notepad, to record the order radiators heat up and how many turns each valve is open.
- An old towel in case a valve weeps when it is turned.
Balancing step by step with a thermometer
- Cool down. Turn the heating off and let the radiators go cold. Bleed any that need it and check the boiler pressure.
- Open everything. Turn every TRV to its highest setting and open every wheelhead. Remove each lockshield cap and open the lockshield fully anticlockwise, counting the turns so you know where it started.
- Record the order. Switch the heating on and go round the house feeling the flow pipe of each radiator. Write down the order in which they start to warm. The first few are usually closest to the boiler, but not always.
- Start with the fastest radiator. Close its lockshield fully, then open it about a quarter of a turn. Let it settle for 10 to 15 minutes.
- Measure. Take the temperature of the flow pipe and the return pipe close to the valves. Open or close the lockshield in small steps until the return is roughly 12 degrees cooler than the flow.
- Move down the list. Repeat on each radiator in the order you recorded. Each one usually needs the lockshield opened a little more than the one before. The last radiator or two may end up fully open.
- Put things back. Refit the caps, return the TRVs to their normal settings and write down the final turns for each radiator for next time.
If the furthest radiator is still poor after a full round, go back to the first two or three and close them a fraction more. Two rounds is normal.
Balancing without a thermometer, and with TRVs, combis and towel rails
The touch method
You can balance by hand. Follow the same order, but instead of measuring, compare the two pipes by touch. Close the first radiator's lockshield until the return pipe feels clearly cooler than the flow but the whole radiator still warms. Open each later radiator a little more than the last. Be careful, as flow pipes can be too hot to hold. It is less precise, but it fixes most obvious cold rooms.
Systems with TRVs
TRVs control the room temperature, but they do not share water out between rooms, so a system full of TRVs still needs balancing. Some modern TRV bodies are pre-settable or limit flow automatically, which removes most of the manual work; a heating engineer can tell you if yours are that type.
Combi and system boilers
The method is the same whatever boiler you have. On a combi, avoid running hot taps while you take readings, because the boiler stops heating the radiators while it makes hot water.
Towel rails
A towel rail on the heating circuit has a lockshield like any radiator and goes into the order with the rest. Small rails near the boiler often take a lot of flow, so they may need closing down early.
Benefits, and what an engineer charges
A balanced system heats every room more evenly, so you are less tempted to turn the thermostat up to warm the coldest room. Cooler water returning to the boiler also helps a condensing boiler run in its more efficient condensing mode. Our guide to boiler temperature settings explains why return temperature matters.
If you would rather not do it, radiator balancing by a heating engineer typically costs £80 to £150 in the UK. Ask for it during an annual service. If a lockshield will not turn, leaks from the spindle or is seized, stop and get a plumber or heating engineer to look at it rather than forcing it.