SteamyTea

That must be because it is not Ronseal

I would expect 1,500 kWh a year more than you are getting. And 11 kWh in 6 weeks recently points to something wrong. Have you checked it against PV-GIS estimates?

Is 0202 a passcode Just need to find out where to punch it in.

Does weather compensation work best when the ASHP is in constant use? Or does it just use predetermined parameters to try and deliver the designated amount of energy i.e. -5°C outside, Flow Temp 40°C, -2°C outside, Flow Temp 37°C (or whatever)

I am finding all this fascinating. Seems that the biggest problems people have (apart from incorrect sizing), is setting up the flow temperatures. This is probably non-intuitive as a number of things happen in a heat pump at the same time, and it is all a balancing act. Just remember that temperature is not energy delivered. I may have to sleep in this and try and sketch what is happening in the morning (just had my busiest night since October and feeling tired).

is that when you want your heating on? May be useful for charging an EV, just don't do to many miles.

Just before I go off to work, thought I would have a look at your local temperatures. Got some data from Whitchurch, which is fairly close to you I think. Looked at the highest air temperatures on any given day, and the highest dew point temperatures on the same day. Then plotted them. This will give the best case. Time period is 28/12/2020 to 15/01/2021 As the majority of the data points were between 0°C and 4°C, this is about the worse point for an ASHP to be working (there has been a few comments about them freezing up)

If you have a 3 kW immersion heater on permanently, they you will use. 3 [kW] * 24 [hours in day] * 7 [days in week] = 504 kWh Not far of what you have used. Some heat pumps have a built in resistance heater, as does the storage cylinder.

By working out the heat loss for your construction type. If you had a SAP done, it should be o there. Or just work out the numbers from the areas i.e. floor, walls, windows, doors, roof, hove much insulation they have, that will give you the U-Value (eventually), then find a local weather station to get the last weeks figures, then work out the temperature differences, multiply that by your overall U-Value, multiply by hours in the week, and hey presto, you have your theoretical losses. So say you have a 10m2 wall that has a U-Value of 0.2 W.m-2.K-1, and the first two hours of the week had a mean temperature of -1°C, and your internal temperature was 18°C. Then: 0.2 [W.m-2.K-1] x (18 - -1) x 10 [m2] x [ΔT] x 2 [time in hours] = 76 Wh. Tedious, but useful, it is why we have spreadsheets.

How do you heat the rest?

So your house is pulling about 3.5 kW. What are your heating load figures for the type of external temperatures you had last week? May be worth doing some detection work. Then you may be able to see if you have a large load on permanently. Just a case of turning things off and looking at the remote meter display if you have one.

If, with the UK's climate, we could make use of stored solar energy for use later, we would be doing it. As would the rest of the world. The numbers just do not stack up unless you have a huge area of land to collect the energy. And then not loose any while it is stored. This is why growing trees and then burning the timber seems so attractive.

You will need a structural engineer to assess the foundation. At 300mm thick you may well have to dig it up. But then you can dig deeper and get some insulation in. Do you need a land contamination survey?

As long as the adhesive can take the temperature and RH, and you still need to put a lid on it. Acrylic can usually take the temperature, why they use it for bath tubs. Just use cast acrylic sheet, not extruded. Why not just fit a sauna. Easy wooden box with an electrical heater, not messing about with water (other than cleaning down). Make sure the unit can handle all the water that will pass though it.

They need to withstand temperatures of 55°C and 100% RH for prolonged periods of time. Similar numbers are used when 'rapid aging' paints and adhesives. And you will need some sort of extraction in the bath room, but not in the actual cabinet.