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Oversized 10.5kW Grant Aerona Heat Pump on Microbore Pipes and Undersized Rads

 
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Air Source Heat Pumps (ASHPs)
Last Post by JamesPa 6 hours ago
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 Old_Scientist
(@old_scientist)
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Joined: 2 years ago
Posts: 346
19/02/2026 5:38 pm  

Posted by: @jamespa
↑

@uknick   Some very useful real world data from @old_scientist very relevant to your case.  If your loss is really 3.5kW I think its less than 50% probability you need to re-pipe.  Many installers re-pipe at the slightest provocation and I have recently been accused on this forum of 'having a thing against re-piping'.  I stand guilty as charged, but whats the point of taking up floorboards and removing perfectly good copper (or plastic) if you don't need to.  Its massively disruptive and, unless there is an identifiable problem, unlikely to deliver any material benefit.

I'm torn on this. The issue I see is that when one performs a retrofit (heat pump installation onto existing heating system), the installer often has no real idea of the specification and condition of the pipework as it's often buried under floors and in walls. If it's microbore and/or in a poor condition (clogged up with 30 years of sludge), then you aren't going to know you have an issue until you commission the system and find you can't achieve the required flow rates. From that perspective, I can fully understand why an installer wants to just rip everything out and start from scratch in the knowledge that everything is right and will work when they leave site on Friday evening. Just putting the other view for a balanced discussion.

 

 


Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.


   
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 uknick
(@uknick)
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Posts: 13
Topic starter 19/02/2026 5:50 pm  

@bontwoody Thanks I'll investigate this to check sense of installing UFH - fancy getting rid of the rads TBH



   
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 Old_Scientist
(@old_scientist)
Prominent Member Member
Joined: 2 years ago
Posts: 346
19/02/2026 5:53 pm  

Posted by: @uknick
↑

@old_scientist Thanks for the messages - yes I have watched Glyns install video (more then once actually 🙂 and have also followed his adventures over the years with his family in his mini electric camper - great guy and very knowledgeable.... I also liked the look of the R290 Grant - still not quite sure what advantage R290 has over R32 but like you if I was starting again I'd consider the R290 units. How do you monitor performance of your system - do you have Open Energy Monitoring installed? 

it's good to hear your experience and the fact that you get a very good SCOP with an oversized heat pump and that your rads would in theory be sufficient even on micropore. I guess I need to be careful not to over engineer and/or waste time and money unnecessarily.... 

@uknick No Open Energy Monitoring here I'm afraid. I do have a MID electricity meter fitted that records ASHP electrical usage (a bit like those used for solar generation on solar installs), but otherwise I'm just going by the data reported on my Samsung control panel which seems reasonably accurate.

I think the main advantage R290 has over R32 is that R290 heat pumps can reach higher temperatures, which may be useful if you want to heat your hot water tank hotter, but for me that's not really an issue as the heating uses 32C flow temps and we heat the hot water tank to 45C so my older R32 heat pump is fine for my purposes. R290 heat pumps also make it easier for installers to just slap them onto existing systems replacing a high temperate boiler without doing any radiator upgrades, at the expense of very high flow temperatures. For me, the limiting factor with any heat pump is finding one that has sufficient maximum output to meet your heating needs on the coldest days in winter but can also modulate it's output low enough so that it doesn't cycle excessively in milder weather. It often proves to be a bit of a goldilocks thing trying to get that balance just right - not too oversized but not too small. Glyn touches on sizing in his video.

As @bontwoody touched upon, in an ideal world with UFH on the ground floor, you would be able to run the UFH at a low temperature such as 30C, and have large enough radiators upstairs to also run at the same low flow temperature, so no blending is required. Heat will also naturally flow from downstairs to upstairs if there are open staircases, and design for 21C in the downstairs living spaces and 18C in the upstairs bedrooms. This to me seems like an ideal setup if it can be achieved and would be very efficient given the low flow temps, otherwise you start introducing additional complexity to solve the issue of requiring hotter flow temps in the radiators than in the UFH.

 

 


This post was modified 10 hours ago 3 times by Old_Scientist

Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.


   
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 uknick
(@uknick)
Eminent Member Member
Joined: 2 days ago
Posts: 13
Topic starter 19/02/2026 5:57 pm  

@old_scientist @jamespa Thanks both I can see both sides - I think as I already have some of the upstairs floor up, and still more to do over next few months I will consider upgrading the 10mm copper tails. From the table posted above, I feel I should consider 22mm plastic as tails are fairly short and the plastic is easier to bend under the floor 22mm potentially provides 4.63 vs only 1.89 for 15mm plastic with inserts which is only a bit better than my existing 1.14 copper. 



   
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JamesPa
 JamesPa
(@jamespa)
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Joined: 3 years ago
Posts: 4290
19/02/2026 9:18 pm  

@uknick 

Posted by: @uknick
↑

think as I already have some of the upstairs floor up, and still more to do over next few months I will consider upgrading the 10mm copper tails.

 

TBH if I were taking the floor up anyway then I would upgrade.  Its quite possibly not necessary but (apart from the cost of the pipe) wont do any harm.

 


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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