ReedRichards

According to my inverter in 2021 I used 10714 kWh of which I generated 2862 kWh, about 26.7% of the total. My meter says I imported about 8200 kWh in 2021, my inverter thinks it was 7852 kWh so that's broadly correct. The heat pump used 6026 kWh. In summer I use spare solar electricity to heat my water but I use the immersion heater because its power requirement, 3 kW, is better matched to my solar output.

Oops yes @SteamyTea , of course I meant to write 25.6 kWh. It seems to be too late to edit the original but I have corrected the quote

I have a 4.8 kWp solar array and a 6.5 kWh storage battery. Yesterday was sunny but the peak output from my array was about 3.4 kW. My battery was fully charged by about 15:00 and fully discharged (15% charge) by 18:30. And that's on a really good day, sunny days are rare in November and even rarer in December. This is what my inverter reports; about 30% of my 25.6 kW of electricity usage was supplied by my solar panels on a really good day for solar power.

I wonder if your HE is just touting for business? I have both solar PV and a heat pump and the two are very ill-matched. I have more solar electricity than I need in summer when I don't need heating and not nearly enough in winter to make much of a dent in the demand for electricity of my heat pump. By all means get both but don't expect them to play nicely together.

This feature is a deliberate part of the design of many fan-assisted heaters (that I looked at). It makes them ill-suited for use with a heat pump. I wonder if there is an override option? You would think the manufacturers would clue-in and provide one.

That's useful information for @Ronnybut it strikes me as very poor that the best air temperature hysteresis setting you can select has a 2 C temperature variation. It's not rocket science or anything new to make a thermostatic control that is much more precise. I'm sure mine (Drayton Wiser) is but at the same time it does not allow more than 3/4 switch-ons per hour in order to avoid short cycling if the thermostat is in a draughty location where the temperature fluctuates.

So that means you have to have a hot water cylinder; you can at least eliminate the option of a combi boiler. I have a heating loop which I run for 5 minutes every waking hour. This keeps the water out of the hot taps at least warm and reduces the running cost. The pipes in the loop are very well insulated but even so it's a bit of a luxury as I was traumatised by the overly long pipe runs before I refurbished the heating system. So your choice is between a low temperature gas boiler (since you have UFH throughout) or a heat pump and with a hot water cylinder in either case. I would have thought you could achieve a good enough COP with a heat pump and UFH to make the running cost competitive with that of a gas boiler. At the very least you should design a system where the gas boiler could be easily replaced by a heat pump when it comes to the end of its life or if gas becomes prohibitively expensive.

In a large house you need to consider the length of your hot water pipe runs. If your combi boiler is situated at one end of the house it will take a long time to get hot water out of taps at the other end. A cylinder situated near the centre of the house would minimise the pipe run lengths. If you want greater luxury, a cylinder supplying a hot water recirculation loop would give you the fastest hot water out of hot taps.

I had no idea it was possible to set AI to be different at different times of the day. There is a super-geeky way to control your heating where you set your Weather Compensation to exactly match the needs of your house and abandon the use of room and radiator thermostats. If you have managed to achieve this state of perfection and balance then you control everything based on the LWT and you would reduce this by a few degrees overnight to make the house a bit cooler. But unless you know exactly what you are doing I would steer well clear of this type of control and just set the room temperature to be what you want it to be at the different times of day and night. I also have no idea why your room temperature shows so much hysteresis; my third party controller would not allow this. Remind me if you have radiators or Underfloor Heating (or both).

I have a third party thermostat so I use Water control rather than Air + Water. This just gives me Weather Compensation. With your settings and AI=0 this would mean that the LWT is 47 (C) when the Outdoor temp is -5 or less and 30 when the outdoor temperature is 27 or greater. If the outdoor temperature was halfway between the two limits, 11, then the LWT would be halfway between the two limits, 38.5. At any outdoor temp between the two set values there is a linear relationship between LWT and outdoor temp. This is shown towards the end of Part 2 of the document I posted (on page 85). Because you use AI=1 I believe this means the LWT is 1 degree hotter than otherwise, equivalent to changing the LWT values to 31 48 I have no idea how the Indoor temp auto mode settings affect matters in your case (Air + Water) and if it is documented I have not found it.

If the settings you are using succeed in keeping your house at the temperature(s) you want it to be then I see no reason to change them. You are using all the sophisticated control features that your heat pump is capable of. Load Compensation (Air and Water rather than Water) may cause your system to use more energy when the house needs to be heated quickly but less energy to maintain a steady temperature so that's swings and roundabouts. And you should be able to increase the temperature quickly if you need to (a capability for which heat pumps generally have a bad reputation). I like to experiment so I would see if AI+1 is necessary over the full range of outdoor temperatures you encounter but there is no great need for you to do that.

I have been told that Air + Water gives you Load Compensation. If your heating is off at night or set back to a lower temperature then when the target becomes 21.5 C the Load Compensation may decide that it needs to override the Weather Compensation setting in order to bring your room up to temperature as quickly as possible. This could be the reason your LWT is hotter than you would expect it to be early in the day.

Assuming you get the AC-Thor rather than the AC-Thor 9s then the immersion heater supplied to go with it consumes the standard 3 kW. I'm not sure what "very low thermal surface loads" actually means but if you make the immersion heater larger than usual then the surface will get less hot than a standard element. Regardless, an immersion heater should be easy to replace if one ever did fail.

46 C is (very likely to be) too high for heating with Weather Compensation (AI) at this time of year but might occur if your were heating your hot water tank to 40 C.

After the heat pump turns off the water in the radiators takes a while to cool back to room temperature so the house is still being heated. Meanwhile it is warming up outside so the rate of heat loss from the house decreases. And possibly the sun was shining in through the windows so there was solar gain.

The cycles are 20 minutes long, approximately, so not particularly short. I think the main reason for the cycles is that it is a 12 kW heat pump and incapable of consistently modulating down to less than 2 kW of output. All heat pumps will cycle if the average power required from them is below the limit of their modulation range so I don't think anything is actually wrong here, it's just autumn rather than winter.

Not necessarily a good thing. This is the power drawn by my house whilst I was away and I asked the system to maintain a constant temperature with AI/Weather Compensation engaged. The house gets cold at night so consumes a lot of power when it is coldest outside so at the minimum efficiency (but it wasn't very cold out so the heat pump cycles).

LG Therma V Product and Installation Manual - the first part. 1988709679_ProductandInstallationTrainingMaterial_1-43.pdf

LG Therma V Product and Installation Manual - the middle part. 934772090_ProductandInstallationTrainingMaterial_44-89.pdf

I have had a request for the Product and Installation Manual so I have split it into 3 parts to enable me to upload it. LG Therma V Product and Installation Manual - the final part covering settings 93321262_ProductandInstallationTrainingMaterial_90-end.pdf

My installer tells me 'Air + Water' gives you Load Compensation as well as Weather Compensation, although I have never found this documented anywhere. As such you must use the LG controller/thermostat, not one supplied by a third party which will only allow on/off control.

You always have to heat all the metal in the system, this always metal loses some heat and that wastes energy if it is located outside the heated fabric of the building but not if it is inside (which is why the pipes feeding radiators are not insulated). Actually the main reason short cycling is inefficient is because there is the potential to supply the same amount of heat using a continuous supply of lower temperature water and running a heat pump to supply lower temperature water is a lot more efficient. At the moment my heat pump spends the day cycling but the peak power in the cycle is the minimum power the heat pump can provide (which is about 20% of its maximum output). If my heat pump could modulate down more it has the potential to be more efficient when it's not too cold outside. But dems da breaks, heat pumps typically don't have the modulation range of a gas boiler.

This is the Glyn Hudson who is "definitely not an expert"? If the coil area is too small the cylinder will take a long time to reach temperature and the heating will be off during that time. So if he/you have got it wrong then you won't know until it's a cold day in winter. However the volume seems okay if your old cylinder worked for you.

Bear in mind that you will want to keep DHW cylinder at 50 C or less. That's probably cooler than you are used to so when you run a bath you will use more hot water and less cold. So you may want to use a larger volume cylinder. You will also need one with a larger surface area coil than a conventional cylinder; that doesn't appear in your materials budget.

34/10.3 = 3.30 so your heat pump would need to achieve a SCOP of better than 3.3 to make it cheaper to run than a gas boiler that is 100% efficient.