TheScullster wrote:

Downstairs main runs are 22mm, but upstairs is all 15mm.
Not sure if it is significant, but the new part that hardly works is piped
in plastic - see link for plastic/copper joint location.

Plastic ought to make no difference...

Another factor that may affect the towel rail operation is the height.
Somewhere I seem to remember reading that a certain distance was required
between the top of a towel rail and the underside of the loft CH header
tank. In my case this distance is approx 650mm.

So long as when you bleed it you get water out of the top eventually
then you ought to be ok.

I have tried major throttling of the downstairs rads to improve flow
upstairs, but do not seem to be able to get acceptable performance of
upstairs and downstairs rads to include items 6 and 7.

It does sound like a balancing problem (or perhaps lack of flow from the
pump in general - have you tried a faster pump speed (watch you don't
cause it to start pumping over)))

It may be you need to go back and rebalance the whole system from scratch.

So the question:

Looking at the pipe runs and radiator sizes involved upstairs, are 15mm
mains simply not up to the job?

15mm is good for upto 6kW of total rad output and you are going to be
nowhere near that...

--
Cheers,

John.

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