ReedRichards

I find it rather primitive compared to a standard central heating controller. For example there is no Holiday Mode so in order to turn off the hot water heating when away I had to remove all timed programming of the hot water. Otherwise when I tried to turn it off it would only stay off for the duration of the current timed period.

So similar to my LG Therma V (which I mentioned earlier). This has two fans, maybe it can behave as @JohnMo's 2 small heat pumps?

I have a second pump inside my house for the central heating because that is the way it was fitted for me. The pump itself is not particularly noisy although audible outside the airing cupboard where it lives. But noise from water circulating in the pipes close to the pump is more of a problem.

My heat pump has been in "standby" for the last hour. The reading on the meter that monitors its electricity usage has increased by 1 digit in the last unit; that's 0.01 kWh in an hour or roughly 10 W. Of course there is a high degree of uncertainty associated with this.

LG Therma V monobloc 12 kW model. Minimum power it maintains for any length of time is about 2.5 kW (according readings provided by my solar panel inverter).

Not exactly. Dimplex and Daikin ASHPs are stated to have a vampire load. LG's are stated not to have perceptible vampire load (BY ME!). I happen to know that the Daikin ASHP referred to is about 10 years old so what is stated might not be true for contemporary units.

My LG Therma V monobloc was installed with its own electricity meter. When the heat pump is idle the LED light on the meter is solid on, which I understand indicates zero (or negligible) power usage.

Or strike Mitsubishi split units off the list if it turns out only that type has this issue.

My heat pump is not in use overnight (unless it gets very cold) and consumes no power that I can perceive (the inverter on my solar panels logs power consumption). Occasionally there is a little blip which could be the heat pump but on most nights that does not happen.

Remember to factor-in the cost of the DHW cylinder with the large surface area coil that a heat pump needs. If heat pump prices are falling then the relative cost of the DHW tank is increasing.

I actually don't know the size of my buffer tank. It is integrated into the base of my DHW cylinder with no partition visible externally. Nor does the buffer tank have a temperature sensor, the sensors are on the flow and return pipes on the heat pump.

I have a third party controller. If a room thermostat on either of my two zones is below the set temperature then the circulation pump on the central heating is activated and the heat pump is powered on. The water circulates through the heat pump for a few minutes and ordinarily the heat pump will then activate. But if the return water is too hot then the flow water will be too hot also so nothing will happen externally until my central heating has dissipated enough of the stored heat in the buffer. I think.

I already use spare solar power to heat my DHW cylinder but is it worth adding a switch that would enable me to heat the buffer tank also (it's in the same place)? If so, what temperature should I set it to? Would the heat pump be confused by return water that is unexpectedly hot? At present the immersion heater in the buffer tank is not connected at all.

I made a mistake in my original response. I take it you mean a UVC using the normal DHW connections?

Whereas I don't sleep so well if the room is too warm - but when I get out of bed in the morning I like to wander round in a dressing gown and slippers so the house needs to have warmed up a bit by then.

And I would not want to. I don't want to live in a house that's the same temperature 24x7 (although admittedly I have never tried it). But I forgot to make the points about the influence of wind and sun in my earlier post.

There is some "magical thinking" going on here. If you want the same temperature inside your house 24/7 then in principle you should be able to tweak your heat pump/boiler to supply water at just the right temperature to achieve your desired temperature when it is, for example, 0 C outside and then with accurate Weather Compensation the heat pump/boiler can adjust the water flow temperature to maintain your desired indoor temperature as the outside temperature varies. If you light the log burner then I guess in principle the extra internal heat source will cause the return water temperature to increase "unexpectedly" and the boiler/heat pump could infer that something else is heating the inside of the house and reduce its output to compensate. I'm not aware of any boiler/heat pump that actually does that but I guess in principle it is possible and perhaps some do. The above only works with a lot of initial tweaking. If you can tell your controller that you are too hot or too cold then it may be capable of doing this. If you tell your controller a desired internal temperature then there must be an internal temperature sensor, even if you are not aware of it. You also have to sacrifice the use of controls like TRVs and you must want your dwelling to be at the same temperature 24/7 or, if you are lucky, the controller might offer you a set-back temperature option. Since I got my first programmable room thermostat in 1998 I have found that what works for me is a nighttime set-back and a daytime temperature that increases as the day goes by. I tend to become more sedentary towards the evening and need the house warmer than in the morning. So the type of control I have described above does not work for me. A better option is Load Compensation where the boiler/heat pump modulates its output according to the difference between the desired room temperature and the actual room temperature. That gives you much more direct control over what is going on. Many gas boilers and a few heat pumps have this capability.

There's some good sense mixed with a fair bit of tosh on Heatgeek.com. What they call "advanced weather compensation" is Weather Compensation combined with Load Compensation. Load Compensation modulates the heat output according to the difference between the set temperature on the room thermostat and the actual temperature in the room. But this requires that your boiler or heat pump knows what the room temperature is. You could never infer the internal temperature from the return water temperature unless you got rid of all heating zones, TRVs (or the UFH equivalent), never cooked, never lit a fire, never opened a window and never left the building.

I live in a 1980 timber-framed bungalow with 4 bedrooms, 16 radiators. I have had a monobloc 12 kW Therma V for 16 months. My heat pump used 5330 kWh in the 12 months from 31/3/21 to 31/3/22. Hitherto I had an oil combi boiler, which used 1936 litres of oil in a 367 day period. I calculate that would be 17935 kW h per year if my boiler (installed in 2012) achieved 90% efficiency. Since I had the heat pump I have had a hot water cylinder and that has enabled me to heat it via electricity from my solar panels supplied to the immersion heater on sunny days for about 7 months of the year. Power to the immersion heater is not metered. It's impossible for me to know what CoP I achieve but it must be in the vicinity of 3.

For me the issue is not the ASHP min load but the ASHP max load. To heat my water to the required 50 C my heat pump works its way up to using 6 kW of power for around 10 minutes. My PV array can maintain over 4 kW of output for a few hours during the middle of a sunny day but that is not enough to prevent the heat pump drawing power from the grid also. But my immersion heater draws only 3 kW so its power requirement is better matched to the output from my solar array. I can also use my immersion heater to heat the cylinder to beyond the 50 C achieved by my heat pump alone.

Heat pumps run most efficiently and therefore most cost-effectively when the leaving water temperature is as low as possible. UFH generally requires a lower water temperature than radiators so with a mix of UFH and radiators you may not be able to run the heat pump as efficiently as if you had only UFH. So use UFH on both floors.

I'm curious; if you use a heat pump to heat your water but not your house, how are you heating your house and why is that better than using a heat pump to heat both your water and your house?

The CoP depends critically on the temperature of the water leaving the heat pump. So if removing insulation and triple glazing but increasing the radiator surface area and the size of the heat pump gets you back to the same leaving water temperature as you had before then your CoP will be the same. But your house will cost more to heat because you have taken out all that insulation. And for most efficient operation of the heat pump, the leaving water temperature will vary according to the outside temperature ("Weather Compensation"). So yes, what you wrote is accurate.

Whatever you use to heat your house, adding insulation will reduce your running costs. Obviously at some point the cost of the extra insulation will not be justified by the benefit you achieve but it doesn't sound as if you are at that point yet. You don't need to have a well-insulated house to have a heat pump. The bigger the heat requirement the bigger the heat pump will need to be and that adds expense but it's a myth that you have to have a well-insulated house in order to have a heat pump.

I have an LG Therma V Heat Pump with an LG controller for the immersion heater for my hot water fixed to the side of the DHW cylinder. After my heat pump had been in operation for about 24 hours it went wrong and I discovered that the error state prevented me from using the immersion heater as a back-up to heat the hot water. That was unacceptable so I had it re-wired so there was a switch that could turn the immersion heater on and off wired in parallel with the LG control box. I have been using this for about 15 months no with no problems whatsoever. Just do as I did and wire the iBoost in parallel.