ReedRichards

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.

The Sunamp product is a thermal store. A tank of water can be configured as a thermal store. By comparison, the Sunamp unit is more expensive to buy but more compact and better insulated. They are no more efficient than a tank of water except for the better insulation. I am suspicious of heat loss ratings for thermal stores and hot water cylinders because they don't take into account heat loss down the connecting pipes which I suspect can be very significant. H stands for Henry, the S.I. unit of inductance, not capacitance.

Yes but your heat pump does not operate at one fixed temperature (unless it is extremely primitive). When it needs to heat the hot water it will ramp up the temperature to 55 C and you will get a COP in the range 2.5 to 3.0. The rest of the time it can give you 34 C (or less on a warm day) and hopefully you will get that COP of 4 that you aspire to. So you have a choice between heating your hot water with a COP of 1 or a COP of 2.5 to 3.0. I cannot understand why you want to do the former.

But this might be about 30p a day if you were using a heat pump on that cheap tariff, saving 45p a day about £165 per year.

Yes, I read that but is it the right thing to do? You would have to run the immersion heater solely on some cheap night-rate tariff to achieve a comparable running cost to using an ASHP to heat the water.

Me 2100 litres of oil per year. @pacemaker1000 2500 litres of oil per year. Me 12 kW ASHP. @pacemaker100010 kW ASHP (suggested) Me MCS heat loss calc. @pacemaker1000 Installer uses rule of thumb Me New DHW cylinder @pacemaker1000 Not mentioned so presumably uses old cylinder with immersion heater.

Any heating system will cycle unless: It can modulate its output (to as little as necessary) It knows the both the desired room temperature and the actual room temperature ("Load Compensation"). Oil boilers do not modulate so will come on until either the room thermostat is satisfied or the return water temperature gets too high and causes a cut-out. ASHPs can modulate (but not as much as gas boilers). Some can do Load Compensation; other cannot. Short cycling is when the normal cycles are so short in duration that wastage of energy that occurs when the heating system is cranking itself up has a significant impact on overall efficiency. With an oil boiler there is not much that you can do about it. Heat pumps generally use weather compensation to reduce the amount of short cycling but if you oversize your heat pump for the demands of your house then it will run in shorter cycles.

I replaced an oil boiler with an ASHP; one company I approached (Grant) initially suggested a hybrid boiler but it wasn't necessary, not remotely as I ended up with a 12 kW heat pump. Don't be put off by nay-sayers. However don't confuse your seasonal average energy use with the amount you will use on a cold winters day. My average oil usage was 59 kWh per day (2100 l of oil per year). My average ASHP usage (all-radiators) is about 19 kWh per day but the biggest 1-day usage (that I noticed) was 54 kWh. A COP of 4 seems a tad optimistic; you would have to operate the radiators at the same low temperature (35 C ???) that your UFH requires so the ones you have would need to be replaced by radiators with a very much larger surface area. Most people with UFH use it for long periods because it is very slow to heat up and cool down (so if the interior cools it takes a long time to warm up again). Be guided by the mode with which you operate your oil boiler at present. If you can run it for 4 hours then turn it off for the rest of the day you'll be able to do the same with your heat pump. I doubt this will work and it's too warm outside now to do the experiment. If you want your UFH to take-in and store a lot of heat you will need to wear thick-soled shoes much of the time as your floor will be too hot for bare feet (I suppose you could use a huge buffer tank but my system won't heat the buffer tank alone, there has to be a thermostat demanding heat). And heat pumps run less efficiently at higher output water temperatures so you can only dump a lot of heat quickly by sacrificing performance. All this considered, my vote is for small pump flat out, except most of the time it won't be. Your heat pump will only need its full output capacity on cold winters days; most of the time it will be under-employed. Heat pumps generally can't modulate down to a small fraction of their full capacity so the MCS accredited approach is to match the size of the heat pump to the 99.6% requirement of the heating with margin for the hot water and defrosting. If you are looking to get the £5k grant you may well be guided along this direction. On the other hand people on this forum who do a DIY install seem to favour over-capacity. If you get an MCS-accredited installer they should do a heat loss calculation and work out the size of heat pump you need; just check whether the calculation is based on intermittent or continuous use

Anything less than 1 m from your boundary and the heat pump is no longer a permitted development and Planning Permission would be required.

Q1) There is nothing magical about ASHPs; they are as capable of heating your house as a gas, oil or electric boiler. However retrofitting an ASHP without changing the radiators is a recipe for a cold house with a high running cost. Q2) Typically you would heat your water to 50 C or less with an ASHP. At 48 C you would suffer second degree burns after 15 minutes; given that the water in the bath would cool you might just get away with a 48 C fill. Q3) That electric stove will be heating your room at least 3x the cost of using the heat pump. But if you put a TRV on the radiator in that room and locate any control room thermostat somewhere else then it should not interfere.

Do you mean something like this https://www.telford-group.com/product/lg-air-source-heat-pump-packages/ (which is what I have)?

Well you need to allow a certain amount of margin for when your heat pump is heating your hot water and not your house. And when it is defrosting in cold weather. Also consider whether you want to turn it off at night or keep it running 24/7.

So that would be: 1.625 * 0.97 - 0.259 MWh = 1.31725 MWh (assuming the readings you gave are actually in MWh and kWh)

My LG Therma V monobloc consumed 60 Wh in 7 hours overnight, according to its dedicated meter. It should have been on "standby" the whole time.

If you are doing the MCS heat loss calculation then the outdoor temperature is whatever it is warmer than for 99.6% of the time.

We never established if the problem was confined to split stage Ecodans (and possibly other makes). There was some suggestion that it was. We don't know what type @joth's friend has or @JamesPais thinking of getting and this could be rather important.

I use a night time set back on my thermostat but unless it's very cold out this just has the effect of turning the heat pump off completely; there would not be any need to set back the flow temperature and keep the heat pump running. During the day I ramp up the set temperature on my thermostat; something I have been doing for the last 25 years and long before I had a heat pump.

I looked at my MCS certificate and that specifies the outside temperature for the heat loss calculation so I used that outside temperature and the specified flow temperature as the lower limits of weather compensation. Then I assumed the heat loss from my house depended linearly on the difference between my room temperature and the outside temperature and calculated the required radiator temperatures to match the heat loss, then made a linear approximation to that curve. That gave me the upper limits. Written like this it sounds complicated but actually it's very easy. Possibly this is still giving me warmer leaving water than I need - but I'll look at that next winter. I have a room stat which I set back a few degrees at night then ramp up the set temperature as the day progresses. My house actually has two zones which makes things more complicated. My installer says "air + water" gives you load compensation but I haven't yet tried to verify this. It's not documented, that I can find.

The weather compensation is linear and the four parameters are set in the installer settings. Thereafter you can vary the target leaving water temperature by + or - 5 C but I have never needed to do that. The controller might also be capable of Load Compensation, but it's not a documented feature.

0.4 Wh per minute = 60*0.4 =24 Wminutes per minute= 24 W .