Underfloor heating issue

21 Feb 2022
Reaction score
United Kingdom
Hi all, we need your help.
We have live in our property for over three years now, had a new Grant Vortex oil combi boiler fitted about 2 years ago. Half of the downstairs is heated by polypipe underfloor heating system which consists of two zones, 2 thermostats & 3 loops, we control the heating with Google Nest app. Ever since we have lived here the underfloor heating has never been 100%, sometimes only one zone would warm up. Now when we put the underfloor heating on after a while the boiler overheats & shuts down. We have been advised that the underfloor heating has been plumbed wrong.
I have attached pics, the flow is the top pipe.
Any suggestions would be much appreciated.
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Hi Murdochat,
Firstly thanks for replying.
We have one Google Nest stat in the lounge (for the central heating) & 2 stats in the lounge/diner for the underfloor heating.
I've been advised that the actuators are working.
Hi Roy,

You were told it is plumbed incorrectly, that should be relatively easy to establish.

Is the arrow on the pump body pointing downwards.

On the TMV (mixing valve) above the pump I can see a H on the top and I think O on the left hand side, can you confirm this and there should/may be a C on the right hand side.
Can you post its (TMV) make/model.

The three flow meters on the bottom will have a indexed scale from 0 to 5 (or maybe 6) LPM, is the 0 at the top of each meter or on the bottom?
they look very discoloured, do they register a flow and if so what are the flow rates?

What temperature is the boiler running at.

What is the TMV setting.

What temperature does the temperature gauge show on the top manifold with UFH on.
Last edited:
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Hi Johntheo5,
Thanks for reading my thread, the answers to your questions.
(1) the arrow on the pump is pointing down.
(2) looks like a C both sides, I have attached a pic of the same blending valve which I found online which shows this more clearly. I know it's Polypipe, not sure on the model, it could be a PB970035 got this number off Google images pic attached.
(3) 0 top 6 bottom. Not sure on the flow rates.
(4) can't find a gauge to see what temperature the boiler is running at.
(5)TMV is set at 8 maximum.
(6)temperature on the UFH temperature gauge is 22.

Ive slightly mislead you, I thought the top pipe coming out of side of the boiler was the flow, I've now taken off the boiler covers and the top pipe is the return and the bottom pipe is the flow. So the return pipe has been plumbed to the H port of the blending valve.

Hope this helps, thanks again.



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Well the boiler flow/return should be swapped, the flow should go to the mixing valve H port and the return to the mixing valve left hand port.

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