Radiators are the least glamorous part of a heating system and the part that decides whether it feels good. Get the sizes right and the flow shared correctly, and the house holds temperature with the boiler running gently. Get either wrong and you have a house that is simultaneously too hot in some rooms and too cold in others.
Two jobs sit behind this: sizing each radiator for its room, and balancing the whole circuit so every radiator receives the flow its size demands.
Sizing starts with the room, not the old radiator
Replacing like for like carries forward whatever error was there before. The correct approach is to calculate the room's heat loss and then choose an output that meets it at the flow temperature the system will actually run at — which is the step almost always skipped.
That last clause matters enormously. A radiator's published output is quoted at a high standard temperature difference. Run the same radiator at a lower flow temperature and its real output falls significantly. If you plan to run cooler water for efficiency, radiators need sizing for that condition, not for the brochure figure.
What changes radiator output
| Factor | Effect |
|---|---|
| Flow temperature | Large — lower flow temperature means noticeably less output from the same radiator |
| Type and panel count | Large — double panel double convector delivers far more than a single panel of the same size |
| Surface obstruction | Moderate — long curtains, shelves and furniture in front all reduce useful output |
| Air or sludge inside | Moderate to large — a partly blocked radiator has less effective surface area |
| Flow rate through it | Moderate — an unbalanced radiator starved of flow runs cool at the far end |
| Wall position | Small — reflective backing helps marginally on a cold external wall |
How balancing is actually done
- 1
Open everything up
All thermostatic valves fully open and all lockshields opened, so the starting state is known.
- 2
Run the system up to temperature
The whole circuit needs to be hot before any measurement means anything.
- 3
Measure across each radiator
Flow and return temperatures taken at the tails of every radiator, one at a time.
- 4
Close in the nearest radiators
Lockshields on radiators nearest the boiler are throttled to achieve the intended temperature drop across each one.
- 5
Work outwards to the index circuit
The furthest radiator usually stays fully open — it needs all the help it can get.
- 6
Recheck the whole circuit
Adjusting one radiator changes the others, so a second pass is normal rather than a sign of trouble.
When a radiator genuinely needs replacing
- Visible corrosion, weeping joints or staining down the panel
- Persistent cold bottom section after flushing, indicating internal sludge build-up
- Undersized for the room after insulation, glazing or an extension changed the heat loss
- Being kept while the rest of the system moves to lower flow temperatures
- Old imperial-sized units in a house being repiped anyway
- Damaged or seized valve tails that cannot be made good
Questions people ask
Will bigger radiators make the house cost more to heat?
No — they let the system reach the same room temperature with cooler water, which improves boiler efficiency. The heat delivered is set by the room's losses, not by the radiator's size.
Can I replace one radiator without draining the whole system?
Often yes, by isolating both valves on that radiator. Where the valves are seized or there are no isolation points, a partial drain-down is needed.
Does every radiator need a thermostatic valve?
Almost all should have one, with the exception of the room containing the main thermostat — leaving that one fully open stops the two controls fighting each other.
How often does a system need rebalancing?
It should hold for years once done properly. Rebalancing is needed after adding or removing radiators, changing the boiler, or altering the flow temperature strategy.
Before anyone suggests a bigger boiler for a cold room, ask whether the radiators have ever been sized for the rooms and whether the system has ever been balanced. In most houses, one of those two answers is no.