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Flow rate discrepancy between ASHP and manifold

 
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Air Source Heat Pumps (ASHPs)
Last Post by AshRolls 3 weeks ago
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 JamesPa
(@jamespa)
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Joined: 3 years ago
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01/12/2025 8:37 am  

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!


4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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 AshRolls
(@ashrolls)
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Joined: 7 months ago
Posts: 8
Topic starter 01/12/2025 9:07 am  

Posted by: @jamespa
↑

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.

 



   
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 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 3 years ago
Posts: 3754
01/12/2025 9:21 am  

Posted by: @ashrolls
↑

Posted by: @jamespa
↑

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.

 


4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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 bornagain
(@bornagain)
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Joined: 11 months ago
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01/12/2025 9:39 am  

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


3.68kw FiT AC coupled pv
5 kw of DC coupled pv
14 kWhr of battery
3kw A2A ASHP
5 kw Vaillant ASHP heating UFH & Thermal store


   
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 AshRolls
(@ashrolls)
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Joined: 7 months ago
Posts: 8
Topic starter 01/12/2025 5:35 pm  

@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.



   
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 JamesPa
(@jamespa)
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Joined: 3 years ago
Posts: 3754
01/12/2025 5:43 pm  

Posted by: @ashrolls
↑

@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

 


4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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 AshRolls
(@ashrolls)
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Joined: 7 months ago
Posts: 8
Topic starter 03/12/2025 10:43 am  

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.



   
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