ReedRichards

My hot water is off overnight then comes on at 6:30 at 50 C for showers. At 10:00 I drop the required temperature to 40 C and keep it there until 16:00 when it is set back up to 50 C. In both cases the "hysteresis" is set to 5 C so the water is not heated until it drops more than 5 C below the set temperature. All this is done using the scheduling function of my Therma V controller; you can choose your own times and values to suit If you wanted to wire-in another controller for DHW you would need a mains voltage to logic converter; somebody on this forum has done this. The main benefit of using another controller is that the Therma V controller does not have a 'Holiday' function and you need to delete (or modify) all programs in order to stop the DHW heating when away. At this time of year I use spare electricity from my solar PV to heat the water via my immersion heater. This is set to cut-out at about 72 C so on a sunny day I get enough hot water to last quite a while.

Only if your boiler is 100% efficient. It would be nearer 12.5p per kWh if the boiler were 80% efficient, perhaps a more realistic value. In which case if your ASHP could achieve an SCOP of 3 then electricity would need to cost 37.5 p per kWh to match the cost of oil.

An ASHP is NOT a drop-in replacement for a gas (or oil) boiler. Because their output temperature is (typically) 50 C or less you will probably need to replace most of your radiators, possibly add a few and replace your hot water cylinder (if you already have one).

At the end of 2020 I paid roughly the same for a 12 kW ASHP (LG), controllers, expansion vessels, buffer tank, 300l DHW tank, 12 radiators, radiators completely replumbed via the loft (instead of underfloor pipes). The vast majority of the time taken (2 people for over a week) was fitting the new pipes for all the radiators. My local plumber quoted more than this!

I would have hoped there was not much to go wrong; what happened?

I checked a few years ago and nobody nearby had a smart meter that actually worked. If I try to apply for one online from my electricity supplier (Eon.next) the link dumps me back at the overview page so I really don't think I can.

@SteamyTea A heat battery is cheaper than an electrical battery, kWh for kWh, so if the Sunamp product, or something similar, was cheap enough I would be tempted to get one. But I don't have a night rate tariff or a meter capable of running one so I could only use a Sunamp in conjunction with my solar PVs and it would only benefit me on a cloudy day or two following a day of sunshine (like today, as it happens). If there were 50 such days a year and i could save myself £1 each day then for me the payback period would still be too long.

Sorry, i should have written "order" rather than "specify". What i mean is, how do you ensure you get the right Phase Change Material when Sunamp seem reluctant to acknowledge there are different options?

So how do you specify one of those (instead of the standard product which has a transition temperature of about 56 C)?

Possibly they do but you have to search their literature quite hard to even find out what the transition temperature of their standard PCM is. You would think that his page https://sunamp.com/savings-add-up-with-economy-10-ashp-sunamp/ would mention the need for a different PCM with a lower transition temperature, but it doesn't.

I don't think so. Their standard Phase Change Material has a transition temperature of around 56 C which is higher than you would want for a heat pump. Also unlike a tank of water, which you can heat by gradually raising the temperature (as the efficiency of the heat pump drops), with a PCM you have to charge it at its transition temperature which means you will be operating the heat pump at near-minimum efficiency for much longer.

Another minor con is that by load-shifting to the middle of the night you will be running the ASHP when outside temperatures are coldest so you will achieve the minimum efficiency over the 24 hours when you might have run the heat pump. On a Time-of-Use tariff you should still be getting the electricity much cheaper so it's only a minor con. But if you had a big battery you might achieve a lower running cost by charging the battery in the middle of the night then using it to power your heat pump to charge the thermal store in the middle of the day.

My comment was meant as a general observation about winter (you're right about the reason why). In the UK there are mitigating factors due to our proximity to the Gulf Stream; otherwise winters would be even colder!

Ask yourself "Why is it cold in winter?". The answer is because we get very much less solar energy in winter than we do in summer. This makes solar PVs a very bad fit to any form of electrical heating; you get the least amount of solar electricity just when you need it the most.

I'm not convinced that is significantly true. Once the water reaches its set temperature the speed with which the building warms up is entirely down to the heat emitters, the radiators and/or UFH. In my case this takes about 15 minutes from cold; I might be able to shave 5 minutes off that with a bigger heat pump but what's 5 minutes? Whilst the building is much colder than usual then delta T will be a bit higher than usual but with a nighttime set-back it probably won't be more than 20% so I suppose you could size your heat pump 20% higher than you think you need. After you have reached temperature if you want a building that warns up more quickly you oversize the heat emitters (radiators) and/or you set the water temperature higher than it would be in its steady state. The former would make your heating system more prone to cycling and the latter would hit your efficiency. And the latter you could only do if you have Load Compensation or you intervene manually.