My heat pump was due to be installed in January after I had completed installing my underfloor heating and had got the floor tiling done. It all came off the rails when my tiler did his back in and rather than finishing before Christmas, he hadn't laid a single tile. By then the next available installation was the 3rd week of March and I thought I would get bored waiting, but no, my tiler didn't finish until the week before, leaving me with one week in which to do about 5 weeks' work. It was impossible so the heat pump was installed with 3 underfloor heating loops out of 14 not yet laid. This has turned out to be a blessing in disguise because it has left me with access to instrument the system to the nth degree.

I was planning to install Home Assistant anyway, but as soon as the installer left I sensed things were not right, and so it is.  On Friday this week my heat pump hit a new low with COP of about 0.7 for water heating. I have installed 6 temperature probes and 3 CT clamps to measure the performance of the system, and it bears little relation to what the heat pump reports itself. The pump only reports whole kWh of electricity used, whenever a full kWh is accounted for, whereas I need energy to be accounted for at the tariff in effect at the time it is used. That's evidently too much to expect!

Setting aside the contractual issues, warranty issues, and failure to honour the payment terms, the elephant in the room is that it was agreed that the buffer tank built in to the pre-plumbed hot water cylinder that was used would be configured as a volumiser on the heating return, but it has definitely been plumbed in as a buffer tank, and one small circulating pump is expected to supply my 14 underfloor heating loops. With only 11 loops installed, the highest achievable flow rate on my longest loop is 4 litres per minute, and the pump is noisy because it is working flat out and is running for many hours after the heat pump unit has stopped providing heat. My manifolds came with their own circulating pumps so just by using those pumps instead, the total flow could have been roughly doubled, but they were taken off on the understanding that the heat pump unit's pump would supply the whole system and I was pleased with the promise of the pump being outside and thus having virtually silent heating.

This is what today's disinfection cycle looked like. I've worked out that the heat pump cuts out when the water cylinder reaches 53C and it then uses the immersion heater to get to the "comfort" setting of 55C, before going on to use it again to get to the 60C disinfection temperature. Obviously the COP drops to 1.0 as soon as direct heating is used. Moreover, note that even though the central heating did not come on, the circulating pump came on and ran for hours.

The COP on Friday was terrible because the circulating pump ran for the rest of the day. As you can see, it continued running until Saturday afternoon, and it only stopped then because I'd had enough and reset everything by turning it off and on again.

The water heating recorded 1 kWh and the circulating pump racked up 1.2 kWh. In terms of cost it was far worse: the heat pump cost 5p to run to do the actual heating, the circulating pump cost 33p, making the total cost 38p for a 6C increase in the hot water temperature. I'd have been far better off with either just an immersion heater or the gas boiler!

It's not heating season, but my observation when forcing the heating to run is that the flow temperature to the underfloor heating is always less than the return temperature to the heat pump. How is such inefficiency introduced by a buffer tank?

I'm going to add to post as my laptop has ground to a halt and needs a reboot…