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Balancing radiators

Checked against its sources: Updated

To balance radiators, note how fast each one heats from cold, then close the lockshield valve on each and reopen it a little, opening those furthest from the boiler more.12 Viessmann says the job is usually done by an engineer, as adjusting valves can cause leaks.2

Covers
Water-filled radiators on boiler central heating, UK
You
Turn lockshield valves only, using a key or spanner, after bleeding. No draining and no removing radiators.
A professional
Proper hydronic balancing with calculations, leaking or seized valves, and balancing after system work if you prefer.

1 Balance or bleed?

Bleeding removes air from a radiator. Balancing shares water flow across the house, so it means checking every radiator1. Vaillant's signs for balancing are big differences between rooms, certain radiators failing to heat, and long heat-up times1. Viessmann says to try bleeding first2.

2 Steps from Vaillant and Viessmann

These come from the makers' web guides, not a user manual. Viessmann calls its version general guidance for consumers and says an engineer will use a more technical method2. No maker page read gives a target temperature for homeowners, so none is given here.

  1. Let the radiators cool completely and bleed them (Viessmann)2
  2. Turn the heating off so that you start from cold (Vaillant)1
  3. Open both valves on every radiator by turning anticlockwise, taking the plastic cap off each lockshield (Vaillant)1
  4. Turn the heating on and write down the order in which radiators reach full heat (Vaillant). Those near the boiler are usually first1
  5. Turn the system off and let it cool again, then turn it back on (Vaillant, Viessmann)12
  6. In that order, close each lockshield clockwise, then reopen it. Vaillant says roughly a quarter-turn, with far radiators opened a small amount more1. Viessmann says only to open it slightly2
  7. Test again from cold and repeat until heat-up times are close together (Vaillant)1

Step 6 is done with the heating on, so pipes and radiators will be hot. Baxi group's Main Eco Compact manual says radiators may reach 85 °C3. If a valve weeps, leaks or will not move, stop and call a heating engineer.

3 Other maker advice

Viessmann offers a smaller step for downstairs radiators that stay cold while upstairs ones get hot. It says to close each upstairs lockshield, then open it a quarter turn4. Its combi page gives a different method, opening each closed lockshield slowly until you can feel a temperature difference between the two pipes, starting near the boiler. It gives no figure for that difference5.

Viessmann's FAQ goes further than its own guide and says it is not advisable to carry out hydronic balancing yourself, as it needs calculations6. Ideal also suggests calling out an engineer7.

4 When it is worth doing

Vaillant recommends balancing at least at installation or after maintenance work1. Viessmann says to do it if heat is very uneven, or after parts have been replaced, and not as a precaution6. Worcester Bosch's Greenstar 4000 manual says the system must be balanced correctly so that a condensing boiler can condense as much as possible8.

Ideal and Viessmann each quote an efficiency gain of up to 15%57. Those are the makers' own figures, not official ones, and neither page explains how they were measured. For the wider picture on running temperatures, see flow temperature.

Sources

  1. Balancing guide, Vaillant read 3 November 2026.
  2. Balancing guide, Viessmann UK read 3 November 2026.
  3. Main Eco Compact Combi manual, 7716571-04 p5, read 3 November 2026.
  4. Cold radiators, Viessmann UK read 3 November 2026.
  5. Adding a radiator to a combi, Viessmann UK read 3 November 2026.
  6. Viessmann UK heating FAQ read 3 November 2026.
  7. Eight energy tips, Ideal Heating read 3 November 2026.
  8. Greenstar 4000 booklet, 6720891170 (2020/09) p19, read 3 November 2026.