Have you got a buffer tank, low loss header or plate heat exchanger between ashp and manifold?  If so there could genuinely be different flow rates!

Posted by: @jamespa

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Have you got a buffer tank, low loss header or plate heat exchanger between ashp and manifold?  If so there could genuinely be different flow rates!

Completely open loop. No buffer or mixing of any kind.

Posted by: @ashrolls

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Posted by: @jamespa

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Have you got a buffer tank, low loss header or plate heat exchanger between ashp and manifold?  If so there could genuinely be different flow rates!

Completely open loop. No buffer or mixing of any kind.

 

Hmm.  What do sensors do if you close 1 or 2 loops down.  Does that give a clue.  Tbh, apart from faulty sensor, I can't see what this is.  Maybe others have an idea.

@ashrolls Air circulating in the system will confuse flow meters and create noise.

@bornagain I had considered that, but have air release valves at all the highpoints in the system piping and have run the air purging function on the pump. I can't hear any bubbles passing through the pump if I listen to it closely.

Posted by: @ashrolls

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@bornagain I had considered that, but have air release valves at all the highpoints in the system piping and have run the air purging function on the pump. I can't hear any bubbles passing through the pump if I listen to it closely.

Air release valves dont always work!

You are gonna have to do some experiments if you want to know more, like closing loops down and seeing if that tells you anything.  Probably it wont, but short of taking all the sensors out and testing them, you dont have too many options.

What about the possibility of some bypass in the UFH, eg through the manifold.  If the only pump is in the ASHP I cant see how that would account for the symptoms you observe though

Ok, I think I have got to the bottom of the problem... the installers have installed the wrong flow sensor also in a poor position.

- Wrong sensor installed. It's a Sika VVX20 sensor, for inner diameter 19mm pipework. My pipework is 32mm. Not only does this affect the sensor accuracy but it also introduces a pinch point of narrow diameter piping into the loop. A VVX32 sensor would be more appropriate.

- Bad sensor installation position. Looking at the data sheets the sensor requires a minimum amount of straight piping before (190mm for VVX20, 250mm for VVX32) and after (100mm) the sensor. There are isolation valves directly before and after the sensor in the system. The sensor counts vortices to determine flow rate, so without straight piping before/after the sensor it does not have a smooth flow to count vortices accurately.

- Vibrations. The data sheet also states not to install it near sources of vibration, as this can also affect accuracy. The circulation pump is right next to the sensor on the wall.

I've notified the installer, lets see how they intend to resolve it.