Eh? Whilst it's true that pipework that's too small will affect the total heat distributed, and be more restrictive, I think you've misunderstood the purpose of pipe sizing.macstevie said:This ensures adequate flow to all the radiators giving a faster heat up time on the rads throughout the property.If you use 15 mm piping as your flow and returns depending on your heating controls(thermostatic valves or room stat?)you may find some rads at start of flow loop heating quickly,whilst rads at end of flow loops heating up much more slowly.
For example: why would you not use 28mm (or greater) pipe for the flow and return?
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Edit: oops - this has overlapped with GasGuru's post; sorry.