Forums

Search with Wattson
Forums
Renewable Heating
Air Source Heat Pum...
Why Buffer Tanks & ...
 
Notifications
Clear all

Joining the Renewable Heating Hub forums is completely free and only takes a minute. By registering you’ll be able to ask questions, join discussions, follow topics you’re interested in, bookmark useful threads and receive notifications when someone replies. Non-registered members also do not have access to our AI features. When choosing your username, please note that it cannot be changed later, so we recommend avoiding brand or product names. Before registering, please take a moment to read the Forum Rules & Terms of Use so we can keep the community helpful, respectful and informative for everyone. Thanks for joining!

Why Buffer Tanks & Low-Loss Headers Can Sabotage Your Heat Pump's Efficiency

 
Page 24 / 24
Air Source Heat Pumps (ASHPs)
Last Post by CH_18 2 months ago
279 Posts
32 Users
127 Reactions
39.7 K Views
RSS
Mars
 Mars
(@editor)
Illustrious Member Admin
Joined: 5 years ago
Posts: 4516
Topic starter 15/02/2026 11:22 am  

@ch_18, welcome to the forums. 

On the principle first: converting a 4-port buffer into a 2-pipe volumiser on the return is a logical step if your goal is to reduce mixing losses and improve emitter temperature consistency. 

On your question about port orientation, in a true volumiser configuration (not decoupled) you’re not trying to achieve stratification in the same way as a buffer. So keeping both the emitter return and the heat pump return on the lower ports can be acceptable, provided the flow path through the vessel is continuous and you’re not inadvertently short-circuiting the top section.

In practice, like @judith alluded to, this is the more conventional way of plumbing in a volumiser.

Volumiser with back ground

 

 

On the pump question, a 7 kW Vaillant’s internal pump is generally capable of handling a typical single open circuit system provided the hydraulic resistance is reasonable. The fact that it’s currently circulating 1200 litres (I assume you mean system volume, not flow rate) is less relevant than the required flow rate and head. What's important here is whether, once you remove the secondary pump and decoupling effect of the buffer, the internal pump can achieve the required design flow at the actual system resistance. If it can't, you'll drown in flow errors.


Get a copy of The Ultimate Guide to Heat Pumps

Subscribe and follow our YouTube channel!


   
ReplyQuote
JamesPa
 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 3 years ago
Posts: 4714
15/02/2026 11:35 am  

Posted by: @editor
↑

 So keeping both the emitter return and the heat pump return on the lower ports can be acceptable, provided the flow path through the vessel is continuous and you’re not inadvertently short-circuiting the top section.

Without doing some horrendous modelling, or fabricating some transparent tanks, I would think that there is quite a significant risk that the upper section is effectively short circuited if both connections are at the bottom, particularly if the erstwhile buffer tank was designed to encourage stratification.

There will be some turbulence of course, and a bit of a convection current (hopefully not much if the tank is well insulated) but not a lot really to stir it up.  So personally I would go in at the top and out at the bottom even if it makes the plumbing more difficult.


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.


   
👍
1
ReplyQuote
 CH_18
(@ch_18)
New Member Member
Joined: 2 months ago
Posts: 2
15/02/2026 8:07 pm  

@editor Thanks for the reply. Yeah I thought in at the top, out of the bottom would be the best solution for converting to volumiser but was hoping to minimise repiping, just incase it doesn't work for example if the internal heat pump circulation pump can't cope. 

If you look, I have a spare 2 ports on the left hand side of the buffer. One is about 2/3 of the way up. Would this be a better option? Or does it have to go to the top nozzle?

I have 2 circuits, both open loop. One is supplying approx 120m2 of UFH circuits and the other is supplying upstairs (2 floors) of rads? You think the pump would struggle supplying enough flow?

Thanks again



   
👍
1
ReplyQuote



Page 24 / 24
Forum Jump:
  Previous Topic
Next Topic  
Related Topics
Topic Tags:  buffer tank (2) , heat pump (27) , LLH (2) ,

Currently viewing this topic 2 guests.



Share:

Join Us!

Worth Watching

Latest Posts

Members Online

Click to access the login or register cheese
x Powerful Protection for WordPress, from Shield Security PRO
This Site Is Protected By
Shield Security PRO →