ReedRichards

That's your suggestion, I think, not something the OP said. How about a volumiser if you don't like buffers?

Yes, it would work but the heat pump is more likely to cycle if it is only heating part of your house.

A section of my central heating was badly designed so that return water from zone 2 can flow backwards through a radiator in zone 1. I tried a double check valve but that made a lot of noise whenever water was being pumped through it. So I moved on to a single check flap valve which is silent except when it closes; closure causes water hammer in some pipes buried in a wall. I think the problem is that these pipes were not secured so move and bang against a beam. I tried fitting a hammer arrestor (a flomasta bell-shaped type) but it only worked for a week. What I seem to need is a flap valve with a slow-close mechanism. These appear to be used in North America for basement sump pumps but they are hard to come by here and anyway they don't look to be up to the standards required for central heating with pressurised water. I can't find anything that seems to be appropriate for use in domestic plumbing. Any suggestions?

They are typically no more noisy that the flue from a condensing boiler but they are subject to restrictions on noise, unlike boilers.

If the water coming out the shower had dropped to 30 C I'm sure your kid would have let you know! The only logical explanation I can think of is that the sensor is right at the bottom of the tank, for some unknown reason. That's pretty much was @jothis saying.

No, it's terrible, but I am struggling to think how an installer could get this wrong.

I don't think any specialised equipment is needed, just a lot of time and patience. If you can't get your house up to the desired temperature then whatever output temperature is being called for is set too low (for that particular outside temperature). Eventually you should be able to map out what output water temperature is needed for the range of outside temperatures you encounter. Then you set your weather compensation ("Water Law") parameters to match that.

Well it looks like it's Load Compensation as well as Weather Compensation if it boosts the output to get the house up to temperature. Heat pump manufacturers seem to delight in inventing their own terminology for processes that are well-establish and named for gas boilers. And no two heat pump manufacturers use the same terminology, as far as I can see.

I remember the days when you got 1000 W multiplied my time (In hours) for your kWh. At only 100 W no wonder the energy companies are raking it in!

I switched from oil to a heat pump and up-sized the radiators at the same time. Almost all oil boilers are just on/off so if you just oversize the rads but stick with an oil burner you will heat the house more rapidly and increase the frequency at which your oil burner cycles. That's neither good for efficiency nor good for the longevity of your oil burner.

For some heat pump manufacturers, their 12 kW model is just a software limited version of their 16 kW unit. In such instances there is no disadvantage in opting for the larger output one - except that they may charge you more for it!

Your piece of what?

My calculated heat loss was 8.94 kW and I have a 12 kW heat pump. I live in a timber-framed bungalow built in 1980. The most the my heat pump has ever used in a day that I recorded is just under 60 kWh. By @JohnMo's figures that would have cost me £15 for that day. However my average usage is about 16 kWh per day for both heating and hot water. Something that you need to understand about heat pumps is that it's the seasonal average that hits you in the pocket; you shouldn't get hung-up on worst case scenarios and costs.

You should always take the average temperature of the radiator, so measure the inlet and outlet water temperatures and take the average of the two. The temperature of the water entering the bathroom radiator should be the same as the water temperature entering every other radiator. Your weather compensation curve would give you an output water temperature of 39 C when it's 10 C outside and 30 C when it's 20 C outside. The temperature of the water entering the rad should be almost the same as that of the water leaving the heat pump so the implication is that it was about 12 C outside when your plumber made his measurement. Does that sound right? If the plumber had come when the outside temperature was below 0 C then he should have fond that the water entering the rad was at about 48 C.

Thanks. I have room for an outside unit so the comment about in and out vents for the all-in-one makes me think that the outside unit is preferable. They are cheaper too. And quieter (inside the room) do you think?