@editor Mars! Wash your mouth out! (I think we should regard the words “Buffer Tank” as ones that require a hefty contribution to the swear box.)😉 Toodles.

Glad to hear another mostly positive review of BOXT, as mine is being installed by them in 3 weeks. Experience so far has been really good, they clearly have some experienced people on the boxt side doing the heat losses and general designs.

I'll do a full write up of my experience when it's completed, as I got 4 quotes/heat losses, but the summary from boxt is:

6.5kW heat loss (this is higher than it actually is) 

They're fitting a 7kW arotherm+, 200L Unistor, 20L lagged volumiser, 1 additional rad + 1 replacement (I've already replaced most rads in my house)

They wanted everything open loop but I've asked for 3 bedrooms upstairs to remain on TRVs (downstairs is quite a bit larger and will remain open loop).

£4.25k

50 degree design temp at -2 (but in reality, according to my own heat loss and backed up by the heatgeek survey too, I'll be able to run it at 42 degrees at -2)

The plumber who's installing it came and did the heat loss survey (but boxt do the actual heat loss using spruce - the report was surprisingly detailed and impressive). The plumber was most definitely a traditional gas boiler installer, but he's got very good reviews on Google and seemed nice enough. His knowledge of heat pumps and low temperature systems wasn't great (and nothing compared to the heatgeek we had round), but he has fitted a few for boxt already and another company apparently. I've done that much research that I could happily fit one myself, so I've no concerns about snagging the job. He didn't really ask or look at pipe run sizing (probably just fed back that all rads are in 15mm copper), but I'd already been under the floors and checked where we have 22mm copper and done the calcs (thanks to heatgeek website/videos) on whether we have enough capacity.

I've had about 10 emails back and forth with boxt to iron out technical questions and get specifics on the installation (pipework sizes, runs, insulation (primary Pro) etc) and they've been able to answer alebe) everything.

Like I said, I'll do a full write up in a couple of months on the full process and how the install goes.

@ashfp Mine does have a buffer tank. That was my experience that they have a contractor who does not do the detailed design of the system but liaises with a contact at Boxt who does the detailed heat loss calculations etc. My report I also found was quite detailed with calculations of individual radiator output relative to heat loss of each room.

I understand that the larger 7kw Vaillant might be slightly more efficient for the hot water heating (as a larger heat pump is generally better for water heating) 

Some Initial Heating Data - Up to 22nd November 

The last week or so was the first real cold snap so a good test of how the heat pump performs in cold conditions.

Energy Used 

So far up to now up to and including 22nd November:

• Heating : 124.7kwh used at a cop of 3.73 for the month so far. Cost £8.72 @7.00p/pkwh - (Octopus Intelligent and 18.8kwh batteries)
• Hot Water : 83kwh at 2.59 for the month so far.  Cost - £5.81 @ 7p/pkwh

Combined total Cost so far this month - £14.53

Cop 

In terms of Cop for heating, I have found:

• around 12/13 degrees - cop is around 500%,
• 6- 7 degrees - 400%
• 0 - 1 degrees - 300%  

Max daily cop was 502% on 4th November 

Min daily cop was 298% on 20th November. 

Weather 

There have been broadly 3 periods in the month: 

Early November Mild weather up around 12-13 degrees - house was slightly overheating at 19 degrees compared to 18 target temperature with weather compensation at 0.1 and flow temperature at 21 degrees running constantly. I had to set the target temp inside to 15 degrees to drop the flow temp to 21 (which is the lowest it will run). I turned the heat pump off due to overheating (to only come on at 15 degrees internal temperature) on the 5th November. The heating was off until the 9th November when the house started to get cold and around 15/16 degrees. 

Early - Mid November Heat pump back on around target temperature around 18 degrees - running generally at 25 -29 degrees flow temperatures weather comp 0.1. 

Cold Snap Mid November - Heat Pump running at 29-30 flow degree temperature. House is around 17 during the day but at night after curtains are closed, it creeps up to 18 degrees temp. For a couple of nights, I increased the weather comp up to 0.2 to test it and it made it slightly warmer, (increase flow temperature to 35 degrees) but I had to turn in down as it was too warm to sleep. I then put it back to 0.1 weather comp curve. 

Summary 

Average temp for the month 17.9 degrees. House is 70sqm 1973 build with 300mm loft insulation but no cavity wall insulation. 

Overall, the house is much warmer than my previous house with a gas boiler. I went round my parents house on Wednesday night (who have a gas boiler) and it was still 14.5 degrees at 6pm (the heating had been on since 5) but was only just beginning to warm up. It felt cold going from my house that was 17 degrees. 

I would expect the heating cop to creep back up closer towards 4 given the milder weather for the rest of November. 

I am still looking to improve the building fabric, I have two blown glass window panes at ground floor that I am getting replaced and two new double glazed windows at first floor. I am still waiting for cavity wall insulation as part of the GBIS scheme and have had a survey done.