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Additional ASHP kW for DHW cylinder?

 
Air Source Heat Pumps (ASHPs)
Last Post by Mars 2 years ago
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 TheBinMan
(@thebinman)
Eminent Member Member
Joined: 2 years ago
Posts: 18
Topic starter 05/05/2024 6:54 pm   [#1525]

Hi everyone.

My heat loss calculation comes in at 8.1kW.  I’ll need a minimum 250 litre cylinder (would take a 300 but not enough headroom) and was wondering what are the guidelines for additional ASHP capacity?   Is there a rule of thumb for extra kW?

Are all ASHP systems DHW priority or can they manage a bit of both space heating and DHW?

 

Thanks.

 



   
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Mars
 Mars
(@editor)
Illustrious Member Admin
Joined: 5 years ago
Posts: 4763
05/05/2024 9:50 pm  

Welcome to the forums @project_electrify.

While there's no fixed rule, I’ve heard some installers say that adding 1.5kW to your heat loss is a common practice to ensure sufficient heating capacity during peak periods, but I’m pretty sure that this figure will vary from heat pump to pump.

On the subject of DHW and space heating, the ASHP does both. Ours prioritises DHW. So if it’s cold outside, and the heating is on, and someone has a shower, the heating temporarily goes off while the hot water is reheated. As soon as it is, the system returns to central heating.


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 TheBinMan
(@thebinman)
Eminent Member Member
Joined: 2 years ago
Posts: 18
Topic starter 05/05/2024 10:10 pm  

Thanks for your reply.

Perhaps the forum could help me with another question!

My current system has 10mm copper microbore tapped from a 22mm main run.

One installer wants all the microbore gone and replaced with 15mm from the main pipes.  In general I don’t have an issue with this but the new pipe runs for 3 of the 16 radiators will require significant redecoration and I’d prefer it if they used the existing 10mm copper in the wall.

One of the radiators has a heat demand of 820w.  The other two are only 170w each.  In theory, would it be possible to run a system with mixed diameter pipes?  There would be 13 other rads on the larger pipe size.



   
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bontwoody
 bontwoody
(@bontwoody)
Noble Member Contributor
Joined: 4 years ago
Posts: 1055
06/05/2024 7:39 am  

@project_electrify 

Yes you can have different pipe sizes. The 10mm copper can move over 1100 Watts at a flow of 0.9 m/s and DT5 (the usual parameters). 

I assume they are replacing part of the 22mm copper main run? As that can only move about 6kW 

image

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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 TheBinMan
(@thebinman)
Eminent Member Member
Joined: 2 years ago
Posts: 18
Topic starter 06/05/2024 7:50 am  

@bontwoody Thanks for your reply.  Looking at your table they must be replacing all or part of the main run too.  

So for a heat loss of 8.1kW with the main backbone in say 28mm copper and radiators on 10mm requiring no more than 1.1kW that would be acceptable?   



   
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bontwoody
 bontwoody
(@bontwoody)
Noble Member Contributor
Joined: 4 years ago
Posts: 1055
06/05/2024 8:03 am  

@project_electrify 

Im not an installer but I would imagine they could just replace the first section of the 22mm with 28mm so that say 3kW of radiators come off that leaving the other 5kW on the 22mm. 

My bathroom radiator is fine on 10mm copper and my other bigger radiators are on 15mm plastic as it was easier to replace the existing 10mm copper with it. 


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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Mars
 Mars
(@editor)
Illustrious Member Admin
Joined: 5 years ago
Posts: 4763
06/05/2024 10:19 am  

Posted by: @thebinman
↑

My current system has 10mm copper microbore tapped from a 22mm main run.

We're in a similar boat to you with a hodgepodge of different pipe sizes, which was briefly covered in this recent video:

 

On the back of this, like you, we have no intention of ripping up our house to repipe. As a follow up to the video above, the plan is to increase the heating pipework in the cupboard to 28mm going up to the tees and to install balancing valves on each 22mm circuit. In addition to this, we are going to repipe the heating circuits as open loop, using the existing buffer as a volumiser instead. An additional pump may then be required to run the heating to the index circuit (our TV room). This, according to Ultimate Renewables, should give us sufficient flow through microbore.


New to heat pumps? Take the Heat Pump Fundamentals course and use INTRO50 at checkout for 50% off.
Rewritten and updated for 2027
📘 From Zero to Heat Pump Hero
🛠️ Bodge Buster
🎧 Prefer to listen? Heat Pumps Explained


   
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