Looks like the attachment has been removed.  Can you upload a copy, or paste the key points?

cheers

Here you go.

Posted by: @kev-m

An interesting paper with some detail on buffer tanks on pages 61,62 and 68 in particular.

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Hi Kev & Mars,

Very interesting paper, and seems to answer many of the questions that are becoming apparent from peoples experiences with their heat pumps.

I suppose one thing to note is that the document is dated 2013, and that heat pump technology has improved since the tests were carried out, in particular the stated COP values are much lower than those that can be achieved with the latest heat pumps that are now available.

I have recently been assisting AnnG to improve the overall efficiency of their heating system, and one of the areas that we have been investigating is the water flow in and around the installed 4 pipe buffer tank. Measurements appear to show that the 35C temperature water coming from the heat pump is experiencing some mixing within the 50L buffer tank, such that the temperature of the water going out of the buffer tank to the radiators and UFH is often in the 28C to 30C range. Although we cannot guarantee the accuracy of the measurements, it is obvious that the present buffer tank arrangement, would appear to have a detrimental effect on overall system efficiency.

The designers, in their wisdom, decided that three water pumps, one within the ASHP, one in the flow from the buffer tank to the heating system and one in the return from the heating system to the buffer tank was a 'good' design. From the tests carried out it would appear that rather than working together, the pumps may actually be working against each other. This is obviously an area for further investigation.

@derek-m 

that doesn't sound good.

Regarding the buffer tank article, the paper mentioned that ASHPs that could modulate may not need buffer tanks as much as those that couldn't.  I think they all can do this now. 

My ASHP cycles when its output drops but by that time its consumption is so low it's not a big factor.  It can drop its output from its nominal 14kW to about 4kW.   If anng's is similar do they even need a buffer tank? 50l sounds small compared to the ones in the paper anyway.

As usual your thinking is probably correct. The problem, as identified in the paper is where the active water volume of the system becomes reduced by the closing of TRV's, which of course will become more frequent during milder weather conditions. But as you rightly point out, that is when the heat pump needs to run less often. I think I have read that some controllers have built in timers that prevent frequent starting of the heat pump, which if correctly set would help to alleviate the problem.

As far as AnnG's system is concerned, the heat pump is now supplying the heat demand with greater efficiency, and Ann is in the process of balancing the radiators to match the heat demand in each room. I am now looking at the overall heat balance and emitter sizing to see where further improvements may be made. In the longer term a slight redesign of the system may be required to fully optimise its operation.