It was the one unexpected component in our ASHP installation; an LLH was fitted in the airing cupboard with a secondary pump. I have spent some considerable amount of time and a little expense to ‘optimise’ the running. With temperature probes on all four ports of the LLH, I have endeavoured to adjust the secondary pump speed so as to minimise any temperature difference between the flow in to the LLH and out to the pump.

I found this setup somewhat delicate and I am never sure if I could improve matters or not - partly due to the fact that the primary pump varies in speed to control the DT at 5 degrees C. I have assumed that as the average primary pump speed is approx. 15 lpm, then this is also the target flow for the secondary pump. The ASHP (according to the Daikin MMI kit anyway) achieves a COP of approx. ~2.5 in very low OAT’s and ~5.5 in milder weather (10-12 degrees C). I often wonder whether the system would be better without the LLH and secondary pump? Leaving well alone has its’ atractions though.😳 Toodles.

@toodles Thanks for this. 

I was working on a 'how to fix an LLH/buffer tank without removing it' script and more or less got, in my thought process, to what you have described.  As I don't have either a BT or LLH I cant test it, but you have done so for me.  Is there anything you can add in terms of how to go about it.  Did you need to calibrate the temp sensors first.

I'm in the process of writing up a piece (for this forum) on common problems and fixes so it is for that.

@jamespa I purchased a set of 4 temperature probes from that ‘Big River Company’ and, having strapped all 4 probes together, observed them over several days and noted the variation in readings. I have strapped them onto the inlet and outlet pipes and covered them with several layers of aluminiumised bubble wrap heat deflecting material as a collar and taped it in place. I have then adjusted the Wilo Pico pump speed to bring the (compensated) readings for inlet from flow into the LLH and outlet to the secondary pump and most of the time, the readings are within 1 degree C (or closer) though I still see the occasional excursion with short term deviation of ~2 - 2.5 degrees. I put this down to the Daikin ASHP pump having a dynamic rate of flow up to and exceeding 30 lpm. The average is 15 lpm though I have seen much lower on the Homely Dashboard plots.

There does seem to be a ‘sweet spot’ on the speed setting of the Wilo pump where it runs very quietly and this seems to be very close to the equilibrium spot I aim for. The Wilo display indicated `0.7 to 0.8 m3 and this equates to 12.5 lpm.

The Daikin controls inform Homely that at current daily temperature range, (8 - 18 degrees) we are enjoying a COP of 5.5+ on good days and perhaps just 4.9 on others this last week. Allowing for a goodly pinch of salt, these figure still look fairly respectable to me so I think the losses from the LLH are fairly small. Regards, Toodles.

@toodles  Thanks, very helpful.