Balancing a heating system, and why one room is always freezing

Why systems need balancing at all

A heating system is a set of parallel circuits sharing one pump. Water flows preferentially through the path of least resistance, which is the shortest and widest one – typically the radiator nearest the boiler.

Without intervention, that near radiator takes a disproportionate share of the flow, gets very hot quickly, and satisfies its own thermostatic valve. Meanwhile the radiator at the far end of a long circuit, with more pipe and more bends between it and the pump, receives very little flow and never properly warms up.

The house is then warm at one end and cold at the other, the room thermostat is probably in the warm end so the boiler shuts off early, and everybody concludes the boiler is too small.

What the two valves do

Every radiator has two valves and they do completely different jobs, which is worth knowing because people adjust the wrong one.

The lockshield normally sits under a plain cap and is not meant to be adjusted casually. It is set during commissioning to restrict the near radiators enough that flow reaches the far ones.

The two most common mistakes: opening all the lockshields fully, which unbalances the system completely; and trying to balance with the TRVs, which cannot work because they open and close on their own.

How it is actually done

Balancing is done by measuring the temperature drop across each radiator – the difference between the flow inlet and the return outlet. A radiator with too much flow has a small temperature drop, because the water passes through too fast to give up its heat. A radiator with too little flow has a large drop.

1. Start from a known stateSystem cold. Open every lockshield fully and every TRV fully. Note the number of turns each lockshield takes to close, because they are not all the same.
2. Close all the lockshieldsThen open each one by a small, equal amount – typically a quarter to half a turn – as a starting point.
3. Run the system up to temperatureWith all TRVs fully open, so nothing is closing on its own during the process.
4. Measure each radiatorA clamp thermometer or an infrared thermometer on the pipe at each end. Note the flow and return temperatures and the difference.
5. Adjust to a target dropOpen or close each lockshield to bring every radiator to a similar temperature drop. Start with the radiator nearest the boiler – it will need restricting most – and work outwards.
6. IterateAdjusting one radiator changes the others, because they share a pump. Two or three passes are normal, and this is why it takes a couple of hours rather than twenty minutes.
7. Record the settingsNumber of turns open for each lockshield, written down. Invaluable next time anybody takes a radiator off.

The target temperature drop depends on the system design and the flow temperature it runs at. What matters more than the absolute number is that every radiator is achieving a similar drop, because that means each is taking its proportionate share.

What balancing will and will not fix

It will fix

• Far radiators that never get properly hot while near ones roast.
• One consistently cold room in an otherwise warm house.
• Upstairs too hot and downstairs too cold, or the reverse.
• A system that takes a very long time to heat the whole house.
• A boiler that short-cycles because the near radiators satisfy quickly.

It will not fix

• A radiator that is cold at the bottom – that is sludge. See radiators cold at the bottom.
• A radiator that is cold at the top – that is air. Bleed it.
• A seized TRV, which needs replacing.
• A radiator genuinely too small for the room – that is a sizing problem and needs a bigger emitter.
• A failing pump.
• A room with very high heat loss – an uninsulated extension, say – where the answer is fabric or a larger radiator.

So the sequence is: bleed, check the valves, confirm it is not sludge, then balance. Balancing a system full of magnetite is doing the right job in the wrong order.

Why it is so rarely done

Honestly: because it takes time, it is invisible, and nobody sees it on a quote.

A boiler installation quoted competitively has to find its margin somewhere, and two hours of methodical measurement with a thermometer at the end of a long day is the easiest thing to omit. The system works – the house gets warm – and the customer does not know what they did not get.

It is worth asking about explicitly. If you are getting quotes for a new boiler or a new system, ask whether commissioning includes balancing and how it will be done. The answer is genuinely informative about the quality of the rest of the work.

It is also worth having done on an existing system that has never had it. It is a couple of hours, it needs no parts, and on a system that has never been balanced the difference is frequently striking.

Things that unbalance a system afterwards

1. Taking a radiator off for decorating and reopening the lockshield fully afterwards. Count the turns when you close it – see isolating heating and hot water safely.
2. Adding a radiator, which takes flow from somewhere else. Always rebalance afterwards.
3. Replacing a radiator with a different size or connection arrangement.
4. Changing the pump, or changing its speed setting.
5. Changing the boiler, particularly to one with different flow characteristics or a built-in pump.
6. Fitting TRVs where there were none.
7. Someone helpfully opening all the lockshields because a radiator seemed cold.