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Another 17kw Grant Heat Pump - Advice Needed!

 
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Air Source Heat Pumps (ASHPs)
Last Post by AllyFish 3 years ago
26 Posts
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 bretix
(@bretix)
Estimable Member Member
Joined: 4 years ago
Posts: 85
20/12/2022 5:37 pm  

@allyfish Thank you, I've learnt so much more I'm the last week than the time the ashp were installed.

I'm assuming both pump adjustments would have to be like for like?

On a side note  since the milder weather I'm finding the radiators warmer than ever.


2 10kw Grant Aerona3
Heat loss calc 16.5 kw @ -2.8 degrees
4.32 PV


   
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 AllyFish
(@allyfish)
Prominent Member Contributor
Joined: 4 years ago
Posts: 520
20/12/2022 6:01 pm  

@bretix we're all learning on these systems 🙂 I would set the secondary pump to full flow, always. If the pipe distance between your low loss header (white rectangular box in photos above) and the outdoor unit is quite short, and is larger 28mm pipe, a lower speed setting on the pump inside the ASHP might better balance. Most of the system restriction is on the secondary (radiator and house side) of the system, there's normally much less pressure loss between the low loss header and the ASHP. ASHPs need more flow than fired boilers, because they have a lower deltaT temperature difference between flow and return. For half the delta T you need to double the flow to transfer the same heat energy in kW.

There's not too much to worry about having too high a flow on the primary side, but best to balance primary and secondary if possible. What happens with the low loss header is that water can pass down through it, short circuiting the secondary flow and return. They are designed to do this for installations where the primary ASHP side flow is constant, such as a fixed speed primary pump, like as used in Grant's units, but the secondary side flow might vary due to on/off zoning controls or zone modulating controls like TRVs or UFH control valves. The ASHP sees a slightly higher return water temperature then, but it just modulates down in response. The opposite, having higher flow on the secondary side than the primary, is more of a problem, as the secondary pump starts recirculating return water up through the header, into the secondary flow pipe, and you see a temperature drop - the flow to the radiators on the secondary side is colder than the flow from the ASHP on the primary side. That's not so good, as all the heat energy generated by the ASHP is then not flowing into the emitters.

Check that weather compensation is enabled and set to something sensible, the Grant default settings are a good starting point.



   
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