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buffer/volumizer cupboard space eater, do i need one?

 
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Air Source Heat Pumps (ASHPs)
Last Post by SUNandAIR 12 months ago
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 AdamK
(@adamk)
Honorable Member Member
Joined: 12 months ago
Posts: 210
Topic starter 23/02/2025 1:20 am  

if i have about 13 rads, long runs of 28mm into 22mm, min 15mm to rads 4 bed house with a heat loss of 5.58-6.8kw why do at least 2 of the companies ive had round want to put these in? after watching Heat Geek vids it seems better to run direct?



   
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Toodles
 Toodles
(@toodles)
Illustrious Member Contributor
Joined: 3 years ago
Posts: 2576
23/02/2025 3:59 pm  

It rather suggests to me that any company that is wanting to install a buffer in that size of system is placing themselves on the rejected companies list! (Sticking to KISS is best!) Regards, Toodles.


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


   
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JamesPa
 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 3 years ago
Posts: 4292
23/02/2025 5:01 pm  

Posted by: @adamk
↑

if i have about 13 rads, long runs of 28mm into 22mm, min 15mm to rads 4 bed house with a heat loss of 5.58-6.8kw why do at least 2 of the companies ive had round want to put these in? after watching Heat Geek vids it seems better to run direct?

I agree with @toodles.  By way of a direct answer to your question, so you have a good reason to reject them and look elsewhere.

I adopted the approach of telling prospective installers before they quoted that I wouldn't accept a buffer, llh or phe for system separation, although I was prepared to accept a 2 port volumiser.

 


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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 JohnR
(@johnr)
Estimable Member Member
Joined: 1 year ago
Posts: 106
23/02/2025 9:57 pm  

I'll rock the boat as I've got a buffer tank as part of an installation of similar size to what is proposed and reckon that it's one factor in getting an average COP of well over 5 since October (Vaillant's numbers say a COP of 6 but I feel that's optimistic, perhaps due to accuracy of measurement). The buffer tank lets the heat pump do what it thinks is best while I can adjust the flow around the house to what seems to work most efficiently. The extra water capacity provided by the buffer also helps with defrosting and avoids short cycling.



   
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Mars
 Mars
(@editor)
Illustrious Member Admin
Joined: 5 years ago
Posts: 4241
23/02/2025 10:00 pm  

@johnr what’s the size of the buffer tank? With a COP like that it’s likely very well balanced pre and post buffer.


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 JohnR
(@johnr)
Estimable Member Member
Joined: 1 year ago
Posts: 106
23/02/2025 10:22 pm  

Posted by: @editor
↑

@johnr what’s the size of the buffer tank? With a COP like that it’s likely very well balanced pre and post buffer.

50 litres https://www.telford-group.com/product/tempest-heat-pump-indirect-50l-buffer-copy

But a flow temperature of less than 35C must be a major contributor to the good COP.

 



   
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JamesPa
 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 3 years ago
Posts: 4292
24/02/2025 8:11 am  

Posted by: @johnr
↑

Posted by: @editor
↑

@johnr what’s the size of the buffer tank? With a COP like that it’s likely very well balanced pre and post buffer.

50 litres https://www.telford-group.com/product/tempest-heat-pump-indirect-50l-buffer-copy

But a flow temperature of less than 35C must be a major contributor to the good COP.

 

You are right, the very low flow temp is the major contributor to good COP.  Also, at low flow temperatures, the potential penalty of a buffer is less.

There is nothing wrong in principle with buffers, but many, perhaps most, are it seems, badly installed/badly controlled.  This being the case they are best avoided in almost all domestic situations.

 


This post was modified 12 months 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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 AdamK
(@adamk)
Honorable Member Member
Joined: 12 months ago
Posts: 210
Topic starter 24/02/2025 11:07 am  

one installer said the extra tank was for the defrost cycles. could that be a volumizer then?



   
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 Judith
(@judith)
Noble Member Member
Joined: 2 years ago
Posts: 485
24/02/2025 12:55 pm  

We have a 26L volumiser. I invited a potential installer to leave (to save his time!) if he was proposing a 4 port buffer. They came back (having talked to the boss) offering just a volumiser (aka. 2 port buffer). Our system works well! COP of ~6 in warm weather like now, ~3 in the cold snap.

Volumisers can be smaller than low loss headers (aka 4 port buffers often shortened to buffer), so take up less space with no apparent performance penalty.

 


2kW + Growatt & 4kW +Sunnyboy PV on south-facing roof Solar thermal. 9.5kWh Givenergy battery with AC3. MVHR. Vaillant 7kW ASHP (very pleased with SCOP 4.7) open system operating on WC


   
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JamesPa
 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 3 years ago
Posts: 4292
24/02/2025 1:13 pm  

Posted by: @adamk
↑

one installer said the extra tank was for the defrost cycles. could that be a volumizer then?

Absolutely could be a volumiser and thats what I would recommend if the extra system volume is needed to meet the minimum requirement.

 


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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 Old_Scientist
(@old_scientist)
Prominent Member Member
Joined: 2 years ago
Posts: 346
24/02/2025 6:06 pm  

Posted by: @jamespa
↑

Posted by: @adamk
↑

one installer said the extra tank was for the defrost cycles. could that be a volumizer then?

Absolutely could be a volumiser and thats what I would recommend if the extra system volume is needed to meet the minimum requirement.

 

Agreed, but I would say we need to do far more than just meet the minimum recommended requirement. We have a similarly sized system to @adamk (12 large radiators, 28mm primaries, 22mm CH pipework dropping to 15mm into the rads) and we also have a 50L volumiser installed. This gives us a total system volume of around 200L. The minimum recommended system volume for our ASHP is 50L (but that is a minimum recommended volume) so we would have been 3 times over this without the volumiser yet they still installed one. The extra volume certainly helps during defrost cycles if the heat pump takes heat from the system to defrost itself.

Take a look at this recent video:

 


This post was modified 12 months 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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SUNandAIR
 SUNandAIR
(@sunandair)
Noble Member Member
Joined: 3 years ago
Posts: 542
27/02/2025 9:49 am  

So... where would you install the volimiser? See choices in the attached sketch. I think this will help to resolve some nagging inconsistencies in previous statements on various forums.

From my perspective I think there is only one correct answer with justifiable reasoning. However what do you think......?

@toodles @jamespa @judith @old_scientist @editor @adamk @johnr

@anyone

 

CF741B7C D9F9 4145 BBDD 642392CB2293

There are 7 options in the sketch labelled A to G. The layout has been elongated to give some space and is a simple heating and DHW system with 28mm primaries dropping to 22mm after the first tee.it is supposed to show a three port valve for the DHW. It shows only 6 radiators but could be 12 or 15 radiators. Assume existing volume to be 150 litres.

Would be interesting to see reasoning for your choice/choices.

 


This post was modified 12 months ago by SUNandAIR

   
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