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Why are my flow and return temperatures reading the same?

 
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Air Source Heat Pumps (ASHPs)
Last Post by Bob77 1 year ago
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 iancalderbank
(@iancalderbank)
Noble Member Contributor
3665 kWhs
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Posts: 643
22/11/2023 2:20 pm  

so the most likely explanation is that there is another circulating pump in your system, and you also have a buffer tank or low loss header (LLH) in your system. 

the other pump will either be in the outdoor unit or near to the buffer tank / LLH, on the heat pump side of the piping. that's what's called the primary pump - its responsible for the flow rate you are seeing. the temperature sensors you are seeing are those of the primary flow and return, in / out of  the heat pump.

when your system is in CH mode, the HP supplies warm water to top the buffer tank(or LLH), that water being moved by the primary side circulation pump. and it takes cool(er) water from the bottom of the buffer tank. The UFH takes warmer water fromt he top, uses it to heat the house, and puts it back into the bottom. That called the secondary side. Thus there are two separate circulating loops that join in the middle.

when your system goes into HW mode it'll switch a valve for the water flow so that instead of going from the HP into the LLH, it goes direct to the cylinder. So in that case, the flow rate, and flow/return temps, that you see , are those of water flow directly between cylinder and HP. So the HP is seeing the true DT and flow rate of the cylinder.

But in CH mode, your HP doesn't "see" the heating system. It simply "sees" a buffer/LLH that keeps on losing some heat, and keeps topping it up.

Note in CH mode via the buffer/LLH, that the flow rates of the two loops are completely separate, and if the flow rates differ significantly then the DT will also. Read this about how a system with LLH/buffers works (HeatGeek is always a good place to start) . 

 

edit - I missed your info in previous posts. seems like you have an indoor unit with a PHE (Plate Heat Exchanger) and a separate primary side pump. The PHE is doing similar job to LLH as I mentioned above, separating the two sides. your temperature readings and flow rate that your system reports will still be on the primary side, you will never see the UFH side DT or flow rate without additional sensors.

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 Bob77
(@bob77)
Trusted Member Member
292 kWhs
Joined: 2 years ago
Posts: 36
Topic starter 28/11/2023 12:09 pm  

@iancalderbank Thanks Ian for that explanation.

I have noticed that in recent days with colder weather and a higher LWT temperature, there is more of a difference between leaving and returning water temperatures. Anyway the house is warm and the electricity consumption has been reasonable, so that's really all that matters!


   
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