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Air source heat pumps and radiators

 
Air Source Heat Pumps (ASHPs)
Last Post by Broadsman 2 years ago
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 Broadsman
(@broadsman)
Estimable Member Member
Joined: 2 years ago
Posts: 77
Topic starter 12/10/2024 4:42 pm   [#1756]

I have recently had a heat pump installed with all new radiators. I have noticed that when set to Weather compensation, ie at low flow temperatures, the front of the radiators take forever to warm up even though the backs are "warm" and so heat the wall.  When set to 'Heat" mode at 40C, the whole radiator heats up more quickly.  This seems to indicate that the radiators are made in such a way as for the reverse side to heat before the front. If that is the case, WHY? surely it would be sensible for them to be designed the other way around. 

Has anyone else noticed this, or am I being stupid here?



   
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bontwoody
 bontwoody
(@bontwoody)
Noble Member Contributor
Joined: 4 years ago
Posts: 1055
12/10/2024 5:11 pm  

I dont know if this is the answer, but could it be that the heat at the back of the radiator is trapped close to and in the wall whereas at the front the heat dissipates more easily leaving the front cooler and so more heat flows from the front of the radiator than the back, cooling the front of the radiator?


House-3 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
10kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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 AllyFish
(@allyfish)
Prominent Member Contributor
Joined: 4 years ago
Posts: 520
12/10/2024 5:15 pm  

Hi @broadsman, what a good observation. I've seen this with one Stelrad K3 triple, the only triple rad I have, in our conservatory. We don't always heat the conservatory, and the radiator pipework dates from when the conservatory was built. It is T'd off the lounge flow and return rather than being put in series. That does means it has a rather slow velocity. The flow favours the lounge radiators - which is how we prefer it as the lounge is always heated.

If I turn on the conservatory smart rad valve it warms up steadily from the back to the front. It heats the conservatory fine, but it takes a good 15 minutes or so to warm up. Other rads with better flow rate are much more responsive. I think the conservatory radiator is over-sized, it's massive, but that compensates for the poor flow. Most rads have a delta T temperature drop of only 3 or 4 degC, implying their flow is more than adequate, the delta T on the conservatory radiator is a few degC higher.

It's one of the reasons I have a secondary pump on a high speed setting - the house has 22 rads in total, with some added on as afterthoughts in the conservatory and garages by the previous owner. The circuit doesn't naturally balance well - it needs quite high velocities to get adequate hot water round all the rads. I've found the end of line and T-branch rads run cool when I've tried lowering the secondary pump speed. There's no way the primary pump alone in the ASHP would do the job without extensive and costly pipework recircuiting + larger flow and return pipe diameters, to lower the system hydraulic resistance - something we're not minded to do for the disruption and cost involved.



   
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Toodles
 Toodles
(@toodles)
Famed Member Contributor
Joined: 4 years ago
Posts: 2860
12/10/2024 5:45 pm  

@allyfish ‘22 rads’, that’s a lot of emitters innit? Another possible theory for the 2 and 3 panel radiators; the rear is normally close to a wall and may radiate the heat into that fabric but have very restricted flow for convected flow, the front panel has unrestricted flow for convected heat and thus dissipates the energy most readily? Just a thought. Toodles.


Toodles, heats his home with cold draughts and cooks food with magnets.


   
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 Broadsman
(@broadsman)
Estimable Member Member
Joined: 2 years ago
Posts: 77
Topic starter 13/10/2024 10:50 am  

Bontwoody and Toodles may have the answer. Thanks



   
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