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What suggestions to give to someone with a buffer tank - how best to set an unnecessary buffer tank up

 
Air Source Heat Pumps (ASHPs)
Last Post by JamesPa 2 months ago
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JamesPa
 JamesPa
(@jamespa)
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Topic starter 16/10/2025 8:36 am  

We get a lot of reports here from people who are having problems with their systems, many of who have a buffer tank.  I think its clear that very best advice, in most cases, would be not to have the buffer tank in the first place or to replumb it as a volumiser, however that generally isn't an option (without compromising warranty) by the time the problem is reported.  The most typical scenario we see here (sadly) is a 50l dumpy tank with absolutely no control that attempts to maintain stratification.

Obviously the suggestions in any particular circumstance depends on the circumstance, but lets presume for the sake of this argument that its reasonably clear that the buffer tank is causing a loss of efficiency, which is commonly the case from what we see here.  Lets also presume that the installer has, one way or another, disconnected so the householder is effectively on their own.  What suggestions can we give?

Obviously one option is 'do nothing and get a professional in' but sadly that is often not realistic.  Can we offer any practical suggestions to someone who will have no specialist tools?

 

In the circumstance above here is an outline strawman (detail can be added later) and I am inviting people who may have experienced (and maybe solved) the problem to tear it apart and rebuild in the spirit of being constructive

 

1. Dont panic!

take things slowly and calmly, make notes of any changes you make

2. Assess the problem

get at least 2 digital thermometers, eg from Amazon.  Measure the temperature difference between flow from heat pump and flow to emitters.  Be sure that the probes are well in contact with the pipes and put some insulation over them, also swap the probes over and note all four readings, so we can eliminate probe variability.  Do this when its cold, or at least fairly cold, outside (say 10C or less) and the heat pump is running.

For every degree that the temperature of the flow to emitters is less than the temperature of the flow from the heat pump you are losing about 3% in efficiency.

3.  If the loss above is significant, you can attempt to mitigate it

There are two possible ways to reduce this. 

 (a) you can reduce the deltaT across the emitters by speeding up the pump on the secondary side

 (b) you can reduce mixing by ensuring that the volume flow rate on the primary side is equal to, (or, in practice, slightly more because equal to is impossible), than the volume flow rate on the secondary side, thus pushing the thermocline down the tank.

4.  Here are some outline guidelines on a process

If you have more than one pump on the secondary side care will be needed to keep them in balance.

You will need to work out what adjustments are available to you.  It may be possible to change the primary pump speed either in the installer menu or on the pump itself.  It is very likely possible to change the pump speed on the secondary side.  

At this point you should note whether the secondary pump is controlled by the heat pump or by the room stats (often via a wiring centre).  If it is controlled by the heat pump note if there are two cables or one cable from heat pump to secondary pump.  If the latter then its probably a PWM pump the speed of which is controlled by the heat pump in which case it is likely best to leave well alone.

Unless the above indicates leave well alone you can then try to adjust the pump speeds to improve matters.  Before you start note all current settings so you can put them back.  You should ideally look to maximise the secondary pump (volumetric) speed and adjust the primary to be slightly higher - you can check by minimising the temp diff flow to flow across the buffer.  If you reach the max speed of the primary before it matches the secondary, turn down the secondary until you can match them.  You may need to iterate.

The above will need to be done with all zone valves/TRVs fully open (which is how you should be operating anyway).  It may not be possible with heat pumps (eg Daikin) which attempt to maintain a fixed deltaT flow to return by modulating the pump. 

The result should be a significantly smaller difference between flow temp from heat pump and flow temp to emitters from buffer.  If it is then good, if not then reset to initial conditions.

 

 

Comments from all welcome, @ashp-bobba, @pirate-rich and other professionals your comments would be particularly welcome as would @toodles who, if my memory serves me, has done this himself, but all are welcome to make constructive comments.


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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16/10/2025 9:35 am  

In our installation, we actually have an LLH but any difference is subtle I know; our secondary pump (Wilo Pico) is switched on/off from the Daikin control but the flow rate has separate control via the Pico’s own flow controller.

I placed sensors (from Amazon) on all four ports to / from the LLH and covered them with insulation. I had previously ‘calibrated’ all four sensors by bundling them and observing the difference in readings. By much tweaking I have been able to adjust the pump so that the difference between flow in to LLH and flow out to emitters is <0.2 degrees and similarly, the temperature from the emitters into the LLH and the out port for return flow is equally close. There are minor variations due I think to the Daikin pump ramping between ~28 lpm and 15 lpm (with occasional excursions as low as ~10 lpm), I see Wilo readings that indicate that the flow rate hovers around the 15 lpm mark with very little variation. Now I know that the temperature sensors may not be accurate themselves but I am using a relative-to-the-other probes method to compare the readings on the four ports.

As an aside, having recently replaced our late 1980’s double glazing with argon gas filled triple glazing, I note that the Homely controlled flow temperature has dropped some degrees to provide the same level of comfort. Time and winter temperatures will no doubt make any differences clearer! Regards, Toodles.


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


   
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 jet4o
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28/12/2025 12:51 pm  

Posted by: @jamespa
↑

In the circumstance above here is an outline strawman (detail can be added later) and I am inviting people who may have experienced (and maybe solved) the problem to tear it apart and rebuild in the spirit of being constructive

Thanks for putting all the effort to help so many people out.

Wouldn't increasing flow rates at both sides of the buffer increase fluid velocity and promote even higher mixing of cold and hot water? This, combined with a temperature difference of just a few degrees between top and bottom would practically eliminate any sort of stratification inside a small buffer tank.

I have been struggling myself with a 4-port 100L buffer. Even with matching dT at both sides of the buffer I still get ~3 degrees difference, which is close to the dT:

_dT on HP side: 33.6 - 29.9 = 3.7 oC
_dT on UFH side: 30.4 - 26.4 = 4.0 oC 
Discrepancy between buffer input and output: 3.2 oC

 



   
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Toodles
 Toodles
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28/12/2025 2:34 pm  

@jet4o Our system contains a low loss header which with its’ four ports is just a smaller unit than a buffer tank might be generally. The secondary pump is a Wilo Pico with a variable speed option; I have (painstakingly) matched the flow from the primary pipework so that most of the time, the difference between flow into the LLH and the flow out of the LLH are within ~0.2 degrees of eachother. The same applies to the return flow into the LLH and the return to the primary circuit. My scheme occasionally falls awry briefly as the Daikin pump flow rate will commence a run at ~28 lpm but slows down ~14 thence to ~7 lpm within minutes and will then continue to flow at ~7 lpm for several hours or more. I would prefer that there was no LLH at all but had Daikin had their way, the installation might have had a buffer tank too as they seem to feel them to be a good idea! Regards, Toodles.


This post was modified 2 months ago 2 times by Toodles

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


   
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JamesPa
 JamesPa
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Topic starter 28/12/2025 2:46 pm  

Posted by: @jet4o
↑

Wouldn't increasing flow rates at both sides of the buffer increase fluid velocity and promote even higher mixing of cold and hot water? This, combined with a temperature difference of just a few degrees between top and bottom would practically eliminate any sort of stratification inside a small buffer tank.

Possibly, but it would also reduce DeltaT across the emitters which means that mixing matters less.  With a small buffer tank (such as 50l commonly fitted)  there is unlikely to be stratification anyway, so the principal effect is likely to be to reduce deltaT. 

 

Posted by: @jet4o
↑

I have been struggling myself with a 4-port 100L buffer. Even with matching dT at both sides of the buffer I still get ~3 degrees difference, which is close to the dT:

_dT on HP side: 33.6 - 29.9 = 3.7 oC
_dT on UFH side: 30.4 - 26.4 = 4.0 oC 
Discrepancy between buffer input and output: 3.2 oC

 

Not surprised TBH, I would guess that anything < about 200l, unless exceptionally well designed and set up (ie primary and secondary pumps running at the same rate),  is quite likely to be too small to stratify materially.  Your results indicate more or less complete mixing.

If you can increase the pump speed on the HP side you might just push such thermocline as there is to the bottom of the tank reducing the delta across the buffer on the flow, which is costing you about 6-9% efficiency penalty.  However no guarantee that this will work.  Reducing the pump speed on the secondary would in principle have the same effect, but of course will also increase the deltaT in the secondary which will act the other way.

 


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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