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How to balance a Central Heating System

If some of your rads are not getting hot you will have to balance the system.

Failure to correctly balance a system can lead to complaints of inefficient operation and criticism of the operational abilities of thermostatic radiator valves, when in fact the valves are not the source of the problem.

Common complaints include:

Rooms are slow to reach temperature even though radiators are sized correctly

Some rooms with correctly sized radiators never reach temperature during cold periods

Room temperatures fluctuate particularly if the TRV is on a low setting

Balancing procedure

You will need either two clip on pipe thermometers (£12 each) or an infrared thermometer (about £30 from Maplin). I have used both and prefer the infrared as it has a faster response, is more accurate and can be used for other things, e.g checking the temperature distribution over the surface of a rad. However, IR thermometers are sensitive to the surface they are pointed at, so you need to wrap some black insulating tape round all pipes and hold the thermometer close to the tape.

  1. Turn boiler off and allow to cool
  2. Make sure HW is turned off
  3. Open all LS valves fully; remove all TRV heads and open all manual rad valves fully.
  4. Turn the boiler temp to max and bring system up to temperature.
  5. Check, by feel, the order in which the rads warm up and make a list.
  6. Adjust the bypass (if fitted) as per boiler mfr instructions
  7. Measure the temperature differences between the flow and return pipes at the boiler. What it should be will depend on the boiler -refer to the installation manual. If the temperature is not right, you need to adjust you pump speed (higher speed gives smaller difference and vice versa). Set the speed to give as a difference as close to the mfrs recommendation as possible.
  8. Turn the boiler off.
  9. Close all lockshield valves. Leave TRV heads off.
  10. Restart boiler
  11. Go to first rad in list made in step 5
  12. Attach the pipe thermometers (if used) to the flow and return pipes near the rad valves and open the LS valve a quarter turn.
  13. Wait until temperature has stabilised on the two thermometers (or use the IR thermometer to measure flow and return temp in the pipes adjacent to the valves). If the difference is smaller than that at the boiler, close the LS valve a fraction, or vice versa. Wait until the temp has stabilised and check again.
  14. When you are happy with the first rad, move on to the next on the list.
  15. Repeat steps 12, 13 and 14 for each rad on the list.
  16. When you have reached the last rad, check the boiler temp difference and adjust pump speed if necessary.
  17. Go back to the first rad and test it is still giving the correct difference
  18. And so on, going round and checking the temperature difference until they are all as near the required drop as you can make them. Don’t expect perfection.
  19. Replace TRV heads and set to required temperature

A very small change in the amount a LS valve is open can have a considerable effect on the temperature difference. Because of the way LS valve are constructed, it is virtually fully open when it is one and a half turns open. So most adjustment will be within one turn from closed.

Lockshield valve which is adjusted to control the flow.

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