The answer
How can you tell which radiators need changing?
Ask for a room-by-room list showing how much heat each room needs and how much its current radiator can provide at the proposed water temperature. Any replacement should show the improvement, not just appear as an unexplained cost.
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A radiator gives less heat with cooler water
Radiator ratings often assume hotter water than a heat pump uses. Ask for the output at the water temperature proposed for your system, not just the figure in a catalogue.
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There is more than one way to warm a room
A larger radiator is one option. Better insulation, a different radiator type or underfloor heating may also help. Space, noise, appearance and cost all matter in the choice.
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The pipes need to carry enough hot water
A bigger radiator will not help if its pipes and valves cannot deliver enough water. Ask whether any pipework, valves or system balancing are included in the quote.
Which radiators can stay and which need changing?
Radiators give out less heat when the water running through them is cooler. Your installer should list the size of each existing radiator, how much heat it can give at the proposed heat-pump temperature and how much that room needs on a cold day. That shows which ones can stay.
The pipes also have to move enough water. A bigger radiator will not solve the problem if its pipework cannot supply it. Be wary of a quote that replaces every radiator without showing why, or keeps them all without checking they can heat the rooms.
What to check next
Your next steps
Where we checked the facts
Check the original information.
These official pages back up the rules and technical details above. They can change, so check them again before spending money or starting work.
Apply this to your own property
Could a heat pump work at your property?
Every property is different. A free survey checks heat loss, hot water and radiators to see whether a heat pump may fit before a system is priced.