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Grant Aerona3 10Kw cooler flow during cold weather

 
Air Source Heat Pumps (ASHPs)
Last Post by JamesPa 4 weeks ago
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 gilesthedog
(@gilesthedog)
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Topic starter 23/01/2026 4:18 pm  

I moved into a new build early october 2025, epc rating of B, detached 1.5 storey, underfloor heating downstairs with radiators upstairs. After several attempts by a heating engineer sent by the developer, the pump is still unable to increase or maintain room temp during cold weather, it's OK when 8 degrees outside, but as soon as temps drop the rooms begin to cool down. My own attempt to understand the system led me to put a thermometer on the underfloor manifold to see what is coming in from the pump, it doesn't seem easy to get a flow temp at the pump or before the manifold. The temps at the manifold max at 33 degrees, and go down when it gets colder outside. The heating guys are supposed to have verified all settings with Grant and say "it's working", which obviously is either not true or the pump/system is not fit for the property. During the cold weather the pump is almost constantly running and so my elec usage is easily 40 to 60kwH per day.

Should the pump not be outputting nearer 40 degrees, especially during zero degrees temps ?

Hopefully a different engineer is attending next week, should I expect him to be able to verify flow temps from the pump, or anything I can ask him to check ?

Thanks. Tom



   
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JamesPa
 JamesPa
(@jamespa)
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23/01/2026 4:22 pm  

Posted by: @gilesthedog
↑

Should the pump not be outputting nearer 40 degrees, especially during zero degrees temps ?

This depends on the system design so cant be certain without further details but UFH often has a max FT of ~35.

Before we dive too deeply can I ask, are you operating it 24x8 with thermostats set well above the required temperature and TRVs (if any) ditto?  If not then it may just be you need to heat for more time and adjust the weather compensation curve accordingly.

 

 


This post was modified 1 month ago by JamesPa

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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Mars
 Mars
(@editor)
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23/01/2026 4:41 pm  

@gilesthedog, you’re right to question this, because what you’re describing doesn’t line up with a system that’s been properly designed and commissioned, particularly in a new-build.

A flow temperature of around 33C at the UFH manifold isn’t automatically wrong in isolation, but it only works if the house can actually maintain temperature at that level. Yours might not. The giveaway is that the rooms start to cool as soon as outdoor temperatures drop. That tells us the system either can’t deliver enough heat to meet the property’s heat loss in colder weather (which is insanely common in the UK) or the control strategy is actively reducing flow temperatures at exactly the point they should be rising.

In a new build with UFH downstairs and radiators upstairs, once you get towards 0C outside it would be entirely normal to see UFH flow temperatures closer to the mid-to-high 30s, sometimes nudging 40C depending on pipe spacing and floor construction. If your manifold is topping out at 33C and then dropping further as it gets colder outside that either shows a system imbalance in distributing heat or your heat pump can;t generate heat at lower temps because it's undersized.

The “Grant have checked it and it’s working” line is, unfortunately, something a lot of homeowners hear. A heat pump that runs almost constantly, uses 40-60 kWh per day, and still allows the house to cool is not “working” in any meaningful sense from a homeowner’s point of view. It may be running, but it’s clearly not doing the job it was installed to do. I'll be publishing a video on this, this coming Wednesday.

When the next engineer attends, it’s reasonable to expect them to show you the actual flow and return temperatures at the heat pump itself, not just what’s happening at the manifold. They should also be able to explain, in plain terms, what flow temperature the system is targeting at 0C outside and why. If they can’t clearly articulate that, or simply defer to “factory settings”, that’s a concern. Your house should be comfortably warm to your area's design temperature.

It’s also worth asking whether a proper heat loss calculation exists for the property. If it hasn’t, or if the upstairs radiators were sized optimistically, the system may simply be under-delivering when demand increases or the heat pump is too small as mentioned earlier. Mixed systems with UFH downstairs and radiators upstairs can work very well, but only if the hydraulics, controls and emitter sizing are properly thought through.

To your direct question, yes, at or around freezing outside temperatures it would be entirely reasonable to expect the system to be capable of delivering nearer 38-40C (most heat pumps will be comfortable to even deliver 45-50C) if that’s what’s required to hold indoor temperatures. If it can’t, or won’t, then either the setup is wrong, the commissioning is wrong or the system isn’t fit for the property as installed.


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 gilesthedog
(@gilesthedog)
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Joined: 1 month ago
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Topic starter 03/02/2026 10:45 am  

Thankfully we managed to get a Grant approved installer to attend the house and diagnose the setup, the HP looks fine and settings correct, but the heat loss from the pipework to the house is about 6 degrees, which means cooler water into the manifold/rads but also how the pump reclaims heat from the return flow, adversely affecting operation such that the pump stops operating despite heat demand from the house. The insulation itself was not tightly fitted above ground, below ground was only 10cm deep with waterlogged foam around the pipes. 

Estimated costs for resolution are around £1800, which the developer should pay for directly or I will have to take to small claims if he tries to wriggle out.



   
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JamesPa
 JamesPa
(@jamespa)
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03/02/2026 11:02 am  

Posted by: @gilesthedog
↑

t, but the heat loss from the pipework to the house is about 6 degrees,

Wow, you are lucky you get any heating at all. 

6 degrees is more than the 'normal' design DT for emitters on heat pump systems (which is 5C) so this is doubling the energy requirement!

Posted by: @gilesthedog
↑

Estimated costs for resolution are around £1800, which the developer should pay for directly or I will have to take to small claims if he tries to wriggle out.

Absolutely right!


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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