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Discuss ASHP radiator pipework requirements? in the Central Heating Forum area at PlumbersForums.net

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I am having an ASHP, radiators and pipework installed to replace storage heaters. How can I check whether the proposed pipework and pump can cope with the heat demand? The installer is proposing a single circuit with 14 radiators with a combined heat demand of around 8kW. I hear heat pumps getting bad press, but feel sure the problems arise through bad design in one or more of the heat loss calculation, heat pump size, radiator sizes and distribution system. The first three I feel more confident about, but not the distribution system design. Are there any rules of thumb or guidance anyone can offer? Thanks.
 
14 rads on one zone? What size is the floor area of the property? (Thinking of Part L).

The rads should be sized with the correct delta T (rad to room) and the pipework should be capable of delivering the flow rates required by those with a delta T (flow & Return) of 5 degrees. The pump obviously needs to suit all that.

I know that's not a lot of help and I am stating the bleeding obvious but it is more technical than I can go into on a Forum without a lot more information.

Your installer should be qualified in designing the system.

Why do you have doubts?
 
14 rads on one zone? What size is the floor area of the property? (Thinking of Part L).

The rads should be sized with the correct delta T (rad to room) and the pipework should be capable of delivering the flow rates required by those with a delta T (flow & Return) of 5 degrees. The pump obviously needs to suit all that.

I know that's not a lot of help and I am stating the bleeding obvious but it is more technical than I can go into on a Forum without a lot more information.

Your installer should be qualified in designing the system.

Why do you have doubts?
Yes proposing 14 rads on a single zone for the whole property which is about 150m2. I ma having some doubts because the installer sent a table of room by room heat loss and radiator sizes which each room having a different external temperature , ranging from -6 to -9. initially they used deltaT of 30 although they said the flow temp would be 45. When I pointed this out they changed the Delta T to 25 but have not explained why they would use a different external temp for each room.
I have used the MCS heat pump calculator myself, with an external design temp of -2.2C for my postcode. So wondering why they (MCS accredited) should use a different method. They have not explained yet. So this leaves me wondering whether the distribution system is appropriately designed. The adjusted radiator sizing seems to match my own so not overly concerned on that front.
We agreed on a single zone as this is a retrofit and would be simpler. Is it just a case of putting a big enough pump to supply 14 rads, or are the pipe size requirements and length limitations that I should be aware of?
 
It’s down to installer mostly installing incorrectly

8kw on water flow and return max length 16m with 28mm primary’s

With glycol this is 12m

15mm copper to each rad at a min

Also should be sizing at delta 12 eg 21 room temp and 35 flow 30 return your cop has half a chance of working / being efficient
 
It’s down to installer mostly installing incorrectly

8kw on water flow and return max length 16m with 28mm primary’s

With glycol this is 12m

15mm copper to each rad at a min

Also should be sizing at delta 12 eg 21 room temp and 35 flow 30 return your cop has half a chance of working / being efficient
So my calculations/drawings (Attached) show proposed rad positions main pipes running in ceiling with drops to ground floor rads, and assuming no diagonal runs through floors would involve 52m of pipework for flow and the same for return, assuming a dual loop separate flow and return arrangement. Over 3x to long?? I originally assumed three zones to limit the number of rads. Thanks I am not asking you to design my heating system, I just want to be sure I know what questions to ask the installer to give me confidence it is designed to be effective. OR pay someone who does know.
Screenshot 2021-11-25 160623.jpg
 
Will put this here so you can have a quick fiddle with your rad outputs so you can see roughly what size you need the branches / legs, these are the max for both f+r

Water

5kw 22mm copper 15m
6kw “” 10m
7kw “” 7m
8kw 28mm copper 16m
9kw “” 14m
10kw “” 10m

Glycol

5kw 22mm copper 10m
6 kw 28mm copper 20m
7 kw “” 15m
8 kw “” 12m
 

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