Posted by: @heatgeek

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As an example for a low energy house with UFH and grossly over-sized heat pump, a 300l independently heated (by HP) low temperature thermal store with 8C minimum DT low flow rate to thermostatically controlled rooms will achieve much better energy saving than any form of open-loop working,

Are you sure about that. I have done almost every configuration of heating possible including what you describe,it isn't the cheapest or most comfortable.

By far the lowest cost is batch charging direct to floor in cheap period, if without battery, so around 7 hours overnight. That is open loop direct to floor (you a single low hysterisis thermostat for that). Heat pump should run continuously for whole period and ASHP generally be well oversized.

If you have a battery run elevated flow temperature for 7 hours then normal WC (in a low energy house the ASHP should stay off for nearly the whole day, when really cold may start again early evening - again all open loop. 

Neither of the above get any advantage from the thermal store. Especially if using a dT of 8.

If you have battery and PV, just run heat pump at elevated temperature on excess PV once battery is nearly charged, then it's free heating. Most the shoulder seasons can be done this way. Then back to straight WC. When we did this in Feb and March, night temps were dropping to zero and the floor remained hot enough to keep heat pump off.

Running 8 dT, means heating your thermal store hotter than needed, compared to dT of 4, for the same mean flow temperature in to floor, so isn't the correct way to go. And thermostat in all rooms, been there also, not the cheapest for low energy house 

Are you sure you have a low energy house?

Posted by: @heatgeek

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Large solar gain

Are you sure you get large solar gains, we can get temperature close to 30 in the living room, if we let the solar gain run uncontrolled.

SCoP is pretty meaningless cost is all important. I am currently making my scope worse each day as I am now in cooling mode getting a negative CoP.

Posted by: @damon

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id use a buffer when the job would dictate.for example on a 13/16 Viessmann vitocal you don’t get much head left on the pump, so a buffer would allow separation and additional pump.

id usually go with a 3 pipe buffer on the flow, upsize the tee and entry point init the buffer.

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you also need inside your buffer correctly, generally you find issues with smaller buffers and header where low volume affects the temps more severely and also the set up, the little buffers under cylinders are useless.

Thanks for that thoughtful reply. 

Unfortunately we get far too many reports here where 50l 4 port buffers have been installed 'because that's what we do' often, so far as we collectively can tell (remotely), contributing significantly to poor performance which is why many on here will advise against them unless there is an identifiable reason.

It sounds, in contrast, like you 

• Only use a buffer when there is an identifiable reason
• If you do use a buffer, take several  careful steps to reduce system distortion

Is this a correct reading of your approach.

Posted by: @damon

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you could also use a buffer where you have different circuits and the customer wants control over rooms.

Is there any reason in not to use trvs (or zone valves) without a buffer in the rooms which need (and can sustain) control (usually a minority).  That's how my system is set up and it seems to work well. 

As ever I'm thinking about ordinary houses (typically 100sq. m in the UK, but my comment is intended to apply to houses up to perhaps 250-300sq m which are not very long and thin).   I can see that with a very extensive and complex system multiple pumps and a buffer might be useful.