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

 
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Air Source Heat Pumps (ASHPs)
Last Post by CH_18 2 months ago
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 samiebon1
(@samiebon1)
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14/01/2025 12:02 pm  

Coat tailing on this thread. I'm having a 5kW Vaillant put in and our house is only 3 bed (for idea of size). The installer has said two pumps on the heating system may be needed. It doesn't sound right to me but any help appreciated!


This post was modified 1 year ago by Majordennisbloodnok

   
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JamesPa
 JamesPa
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14/01/2025 12:43 pm  

Posted by: @samiebon1
↑

Coat tailing on this thread. I'm having a 5kW Vaillant put in and our house is only 3 bed (for idea of size). The installer has said two pumps on the heating system may be needed. It doesn't sound right to me but any help appreciated!

It's unlikely you need 2 pumps, did he say why?

 


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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 samiebon1
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14/01/2025 12:55 pm  

@jamespa I think because they usually put buffers in and i've specified no buffers.


This post was modified 1 year ago by Mars

   
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JamesPa
 JamesPa
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14/01/2025 2:26 pm  

Posted by: @samiebon1
↑

@jamespa I think because they usually put buffers in and i've specified no buffers.

Well that's not really a reason.

If he had said "because the index circuit has a resistance that is too high for the water pump" that would be a reason.

Is there any reason to suspect the resistance in your system will be high like small diameter, circuitous or extra long pipework?  

 


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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 samiebon1
(@samiebon1)
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14/01/2025 2:59 pm  

I think i'll ak him about the index circuit. My own concern is the amount of 90deg elbows in my system! 


This post was modified 1 year ago by Mars

   
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JamesPa
 JamesPa
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14/01/2025 5:55 pm  

Posted by: @samiebon1
↑

I think i'll ak him about the index circuit. My own concern is the amount of 90deg elbows in my system! 

Maybe best not to sound too clever, it might upset him.  You might be better first to ask why he thinks you might need 2 pumps and then, if necessary, ask a follow up eg how he is going to ensure a second pump is fitted if and only if its actually needed.   

Personally I doubt many plumbers calculate the index circuit not least because it's often not visible.  I'm sure many if not most just assume the pump will be good enough which, in reality, it usually will.

Of course putting in loads of 90 degree elbows isn't best practice!

 


This post was modified 1 year ago 2 times 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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Mars
 Mars
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Topic starter 15/02/2025 3:05 pm  

For those that having the thread, here's the latest video from our podcast series on buffer tanks:


This post was modified 2 months ago by Mars

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 Tim441
(@tim441)
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Posts: 398
17/02/2025 2:01 pm  

I wonder how much efficiency it really costs - if there is unnecessary mixing it results in higher return temps so a "saving" on the next heating. Clearly there will be some heat & efficiency loss but perhaps not quite such a big problem?

I suspect in most cases it will not be worthwhile removing an existing buffer tank.

But on any future installation I would definitely question necessity if proposed as part of installation.

 


Listed Grade 2 building with large modern extension.
LG Therma V 16kw ASHP
Underfloor heating + Rads
8kw pv solar
3 x 8.2kw GivEnergy batteries
1 x GivEnergy Gen1 hybrid 5.0kw inverter
Manual changeover EPS
MG4 EV


   
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JamesPa
 JamesPa
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17/02/2025 2:16 pm  

Posted by: @tim441
↑

I wonder how much efficiency it really costs - if there is unnecessary mixing it results in higher return temps so a "saving" on the next heating. Clearly there will be some heat & efficiency loss but perhaps not quite such a big problem?

I suspect in most cases it will not be worthwhile removing an existing buffer tank.

But on any future installation I would definitely question necessity if proposed as part of installation.

 

15% or more typically

The problem is not loss of energy (which as you say is small) its reduction in flow temperature to the emitters (due to the mixing) which means that the heat pump must be operated at a higher flow temperature than would otherwise be the case.

 


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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Toodles
 Toodles
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17/02/2025 2:33 pm  

@jamespa There again, I have my secondary pump set to match the average flow rate from the heat pump system; this is 15 lpm. When the pump speeds up (as it does periodically) to as much as 30 lpm for a short time, my secondary pump connected to the output of the LLH does not change. Efficiency loss? Regards, Toodles.


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


   
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 Tim441
(@tim441)
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Joined: 3 years ago
Posts: 398
17/02/2025 2:48 pm  

I doubt I could get my weather compensation curve any lower so must conclude my buffer is not hurting me tooooo much. Certainly accept the main point that buffer tanks are likely unnecessary.

My settings:

Leaving Water temp 30/45

Outside temp -4/15

Currently outside temp 6 deg. LWT 36 deg. Indoor temp 21 deg

 


Listed Grade 2 building with large modern extension.
LG Therma V 16kw ASHP
Underfloor heating + Rads
8kw pv solar
3 x 8.2kw GivEnergy batteries
1 x GivEnergy Gen1 hybrid 5.0kw inverter
Manual changeover EPS
MG4 EV


   
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JamesPa
 JamesPa
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Joined: 3 years ago
Posts: 4634
17/02/2025 2:56 pm  

@tim441 if you want to you can work out roughly how much it's costing by measuring the temperature drop across the buffer flow to flow.  The cost is roughly 2-3 percent per C (more according to some, but I think the argument for more depends on deltaT flow to return being higher (than the typical 5C) on the emitter size.

It's entirely possible to set up buffer tanks not to mix, but unfortunately most installers appear not to do so, or fit a buffer tank that is to small to stratify.  Of course some do,  @toodles has for some while reported a buffer tank that doesn't mix to any great extent.

The real point is that they shouldn't be fitted in the first place because they are, in almost all cases, superfluous.  Obviously once they are there it's a different calculation.


This post was modified 1 year 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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