How to balance radiators, step by step

What balancing actually does

Hot water from the boiler takes the easiest route round the pipework. Radiators on short pipe runs near the boiler get a strong flow, while the ones at the far end of the house, often upstairs, in an extension or at the end of a long run of pipe, get whatever is left. The result is a house where the kitchen is hot within ten minutes and the back bedroom is barely warm after an hour.

Each radiator has two valves. The one you turn day to day is the thermostatic radiator valve (TRV) or a manual valve. The other, under a plain plastic cap, is the lockshield. Balancing means partly closing the lockshields on the radiators that get too much flow, so more water is pushed on to the ones that get too little. Once it is done, you leave the lockshields alone and control each room with the TRV as normal.

Check it is a balancing problem first

Balancing is often tried as a cure for every cold radiator, and it only solves one kind of fault. Spend five minutes checking the radiators before you start, because the steps below will make no difference to the other problems and can hide them.

What you need

You do not need special tools for a basic balance, but a thermometer makes it far more accurate and saves a lot of guesswork.

• A lockshield key or a small adjustable spanner to turn the lockshield spindles
• A radiator key to bleed any radiators with air in them
• A notepad, or a sketch of the house with each radiator marked
• Ideally, a digital thermometer with two clip-on pipe probes, or a basic infrared thermometer
• About two hours with the heating in use, because radiators respond slowly

Step 1: prepare the system

Bleed any radiator that is cold at the top, and check the boiler pressure gauge afterwards. On a sealed system with a combi or system boiler, it should normally read around 1 to 1.5 bar with the heating cold. Top it up through the filling loop if bleeding has brought it down.

Turn the heating off and let the radiators cool completely. Then open every TRV to its highest number, or take the heads off, and open every lockshield fully anticlockwise, counting the turns on each one as you go and writing them down. Those numbers are your way back if you need to start again. If a lockshield will not move, stop rather than force it. Old brass spindles snap, and then the radiator cannot be isolated.

Step 2: find the order the radiators warm up

Set the room thermostat high enough that it will not switch the heating off during the test and turn the heating on. Go round the house every few minutes and feel the pipe going into each radiator, or read it with the thermometer. Write down the order in which each one gets warm.

The order is not always what you expect. A radiator upstairs can come before one in the next room if it sits on a short branch straight off the main pipe. Trust the list, not the floor plan.

Step 3: set the lockshields, nearest first

Let the system cool again, then turn it back on and work down your list. The aim is to restrict the radiators that warm quickly so the flow is shared out.

• First radiator on the list: close the lockshield fully, then open it about a quarter of a turn.
• Second and third: close fully, then open about a third to half a turn.
• Middle of the list: open around half a turn to one turn.
• Towards the end of the list: open one to two turns.
• Last radiator: leave the lockshield fully open.

These turns are a starting point, not a rule. Pipe sizes, radiator sizes, the pump setting and the valve make all change the numbers. Adjust in small steps and wait at least 15 to 20 minutes after each change before judging the result.

Step 4: fine-tune with a thermometer

Heating engineers balance by temperature drop. Clip one probe on the pipe going into the radiator and one on the pipe coming out, give it time to settle, and compare the two readings. On a conventional gas boiler system the drop across each radiator is often set around 10 to 12°C. If a radiator shows a much smaller drop than the others, water is rushing through it, so close its lockshield slightly. If the drop is much larger, open it a little.

Radiators on heat pumps and on low flow temperature systems are designed for a smaller drop, so follow the installer's figures rather than the numbers above. When you have finished, put the TRVs back to their normal settings, refit the lockshield caps and note each final setting for next time.

If rooms are still cold after balancing

If you have balanced carefully and some rooms still lag, the flow is being lost somewhere else. The usual causes are a TRV pin stuck in the closed position, a lockshield that has broken internally and no longer opens, sludge narrowing the pipes, or microbore pipework that cannot carry enough water to a large radiator. On some systems the pump speed is set too low, and on a few the radiator in that room is simply too small for it. If a whole floor stays cold rather than a few rooms, read our guide to radiators not working upstairs or downstairs, because a zone valve or air is more likely than balancing.

Black water when you bleed a radiator, cold patches along the bottom, or a boiler that has started to bang or kettle are all signs of sludge. At that point the answer is cleaning the system, not more turns on a lockshield, and it is worth adding the right strength of inhibitor afterwards so it does not come back.

Getting the heating balanced for you

Balancing is a slow job, and it is easy to lose an afternoon on a larger house. If lockshields are seized, rooms stay cold after you have balanced, or the water is black, our engineers can free or replace the valves, balance the whole system with pipe thermometers and tell you honestly whether the system needs cleaning.

Loyal Plumbers has 10 vans working across London and everywhere inside the M25, 24 hours a day, and gas work on boilers is carried out by Gas Safe registered engineers. Our call-out charge is £60. When you call or send an enquiry, tell us how many radiators you have, which rooms are slow to heat, and whether you have a combi, system or regular boiler.