TheScullster wrote:

The inserts on plastic pipe reduce the 15mm to 12mm. This may contribute.


Not sure if inserts have been used!

I think that is a bit of a red herring anyway... yes the inserts will
reduce the diameter a little, but flow resistance is a function of not
only the diameter of the pipe, but also the length - and inserts are
short. So a reduction to 12mm for a couple of cm every now and then will
make a negligable difference.

Also note that your total heating load should still be satisfied even if
you had a complete run of 12mm pipe - you only have a couple of not huge
rads on this pipe run IIUC.

Have you increased the pump speed?


Reluctant to do this as I have had pump over problems in the past!
Is it likely that the two issues are related?

Could be...

It might be you need to tackle the problem with more pump speed and then
sorting any pump over issues.

What make/model of boiler is it?


--
Cheers,

John.

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