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.

Posted by: @jamespa

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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

@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.

Posted by: @jet4o

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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

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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.