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2 heat pumps - 1 as a slave

 
Air Source Heat Pumps (ASHPs)
Last Post by CatherineHH 3 years ago
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 CatherineHH
(@catherinehh)
New Member Member
Joined: 3 years ago
Posts: 3
Topic starter 17/04/2023 2:34 pm   [#938]

Hi

Im really hoping might be able to help and explain in really simple terms as I’m struggling to get my head around our system

Background - we had 2 grant aerona  air source heat pumps installed under eco4 grant which turned out to be a bit of a disaster so we’re trying to unravel everything.

The heat loss survey shows we need 24kw at -3

We had two heat pumps - 10 & 15 kw set up in tandem. Trying to reduce our outrageously high bills we were recommended that these should be set with the smaller as a Slave so had an EP002 installed  but then it seemed to keep kicking the slave on after 45 mins so I phoned Grants to find out why.

They stated that we will always need them both to come in as we have a need for 24kw, but it doesn’t make sense in my head that we need that when the outside temp is 13 degrees and we are only trying to raise the internal temp by 2 degrees. I thought the slave would kick in just when it was minus outside.

Does anyone have any thoughts on this please. 



   
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Grants Air source heat pumps slave
 Derek M
(@derek-m)
Illustrious Member Member
Joined: 5 years ago
Posts: 4425
17/04/2023 3:30 pm  

You are correct.

If your total calculated heat loss is 24kW, then at an outside temperature of 13C, the heat loss will probably be in the region of 8kW, so in theory either of the two heat pumps should be capable of supplying the heating demand. 

 



   
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 CatherineHH
(@catherinehh)
New Member Member
Joined: 3 years ago
Posts: 3
Topic starter 17/04/2023 3:40 pm  

@derek-m thanks, I thought it didn’t make sense. I phoned Grants back and spoke to someone else who talked me through lots of settings. I think the outdoor ambient temp was set too high so I’ll be checking tonight to see if they still both kick in.



   
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 robl
(@robl)
Reputable Member Member
Joined: 4 years ago
Posts: 216
17/04/2023 4:08 pm  

@catherinehh 

Is weather compensation active?  While you require on average a lower heating power when it is say 10C outdoors, without weather compensation both heatpumps would be needed to cycle the radiators on and off to a high setpoint.  That is, while the average power is low, the peak would be the same as ever, so both may be needed.

Weather compensation generally requires the use of the heatpump manufacturers own controllers, and is a way to match the power out of the heatpump to the expected power requirement.  In doing this, the heatpump can operate more efficiently.



   
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 CatherineHH
(@catherinehh)
New Member Member
Joined: 3 years ago
Posts: 3
Topic starter 17/04/2023 4:30 pm  

@robl so the initial installers didn’t set the weather compensation even tho they ticked to say they had on the paperwork.
We had a heating engineer out to look at the system and he set it. Not sure what to or where to find this though.



   
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