JamesPa

Complex though and uses a lot of space. Personally I'd just amend my routine to be more reasonable in my demands, but I know that's an unpopular view, even though it's the only one consistent with any faint degree of sustainability. Four showers in quick succession (often, perhaps not in this case, for a long time at 20l/min) is the archetypal first world problem!

Nothing wrong in the logic. I pay 7p nighttime, 25p daytime, 16.5p export. The logic becomes beautifully simple, import as much leccy at night as you can (obviously assuming you can use it) even if it reduces self consumption of PV. I love the simplicity, it probably won't last forever, but while this sort of tariff is offered it works well for me.

Agreed. I think he implied that he was going to get a bigger tank anyway though (irrespective of the source of heat). Best approach would be waste heat recovery, but that requires access to the shower drain and some plumbing work. It would double the amount of showers he could get from any given size of tank though, and reduce his DHW bills by a corresponding amount.

Except that an electric shower, sofaik, will do max 10l/min. We still don't know if OP 'requires' 20l/min (some people do apparently, no idea why!). He needs to answer the questions I posed above about length and rate before a design can be done.

Based on some of what you say above I do wonder if this is truly the case, or whether (perhaps subconsciously) you are really trying to find evidence to support the decision you have already made (again perhaps subconsciously). Are you truly undecided? If you want an ashp to work for you it will, if you don't want it to work for you it probably won't!

4 showers at what rate (20l/min, 10l/min?) and for how long. The calculation isn't difficult once these parameters are known. Even with a boiler you aren't going to be doing much reheating during the time it takes to shower, so reheat time is of secondary importance what matters primarily is tank size and temperature

If you already have a hot water cylinder then you have information from which you can work out your needs. Unless your current hot water cylinder is 500l plus, which as it sits above the boiler is rather unlikely, you don't need to do this! Most likely because they believe the nonsense they read in a certain section of the press, which is pumping out misinformation (on a whole host of subjects) in support of vested commercial interests and a certain part of the political spectrum. Personally I view with scepticism anything I read in the press or hear in most of the 'media'. There are too many vested interests with too much influence to trust anything which isn't verifiable as a minimum by reference to an outlet with the opposite leanings.

A properly designed ashp system will heat your house and hot water at least as well as a boiler, probably better (more uniformly both in time and space). It will be about the same cost to run and will heat the planet 1/4 as much, reducing to zero as the carbon intensity of the grid reduces. Mine (1930 solid wall house with partial internal wall insulation) is about 20% cheaper to run than my gas boiler and much, much more comfortable; I did my research and the system is properly designed (which to first order, at least in the majority of cases, means heat pump correctly sized, no external controls/thermostats, no buffer tank (kiss!), heat pump run 24*7 on weather compensation). With a correctly sized dhw tank you won't run out of water and if you need a rapid boost you can flick on the immersion as well as the ashp. Really not a problem. However if the system is badly designed you will have issues That's true of a boiler based system as well, but boilers are more tolerant and the heating industry, with notable exceptions, has become lazy. As @JohnMosays you will also have issues if you try to operate it like a boiler. Incidentally neither of your placement options sound great, it is worth thinking out of the box on this. I would go along with get your boiler repaired and swap next year having done the research, or, if that isn't feasible, get an installer you can trust not a fly by night grant harvester. Heat geek is definitely a good start, but there are also many good installers who aren't heat geeks. Finally here is a great place to post quotes/questions for comment.

To give an example of the calculation: Radiator output is normally (but not universally) quoted for a average deltaT (rad to room) of 50C. Convector rad output is approximately proportional to deltaT^1.3 (manufacturers do quote exponents very slightly different to 1.3, but they are almost always close to 1.3). So a convector radiator operating at ft 35 and DT (flow to return) of 5, target room temp 20, will have an average deltaT of 35-5/2-20 = 12.5. The output will therefore be (12.5/50) ^ 1.3 = 0.16 times the output at deltaT 50. Thus a radiator quoted as 1kW at deltaT 50 will emit only 160W at deltaT 12.5. You can invert this to get the 'oversize factor', in this case 1/0.16 = 6.25, the figure by which you must multiply the calculated loss to get the radiator output (at deltaT50). For any given type of radiator (unless it is very short or very tall) the oversize factor is a constant so this is a very convenient way to work out which radiator from a range you need Column radiators may have a different exponent than 1.3, or even a different basic formula (although the latter is unlikely); you need to check with the manufacturer. There is, however, a fair likelihood that the same formula will apply. Incidentally if you could pump a bit harder and reduce the DT to say 3, the oversize factor reduces to 5.25.

Seems sensible. The only thing I would add is to do it on a day when the previous day, or better still two days, was the same temperature and you were heating the house. Houses have a lag of sometimes 24hrs and you don't want to be caught out by that. Also if it doesn't end up close to the same temperature as it started (say within a degree), you may need to apply a correction to account for the house heat capacity (my house, for example, appears to have a heat capacity of very roughly 20kWh/degree, so a 1 degree change in temperature over 1 day would result in roughly 1 kW error). (Ps the reason I am so conscious about sense checking retrofit surveys is that I had 2 done, both taking 3 hrs and both coming up with 16kW. Actual loss is 7kW. Difference due to ach, ignored invisible fabric upgrades - even though I made a big point of telling them about them - and double counting. I'm clearly not alone given what I read hear and on other forums, particularly rhh).

Fair enough, but this is 'tinkering with home made controls' which most ordinary people won't want to do. Most people expect to turn a dial calibrated in degrees C to change the temperature! Quite why some heat pump manufacturers haven't realised that is a mystery!

Your MCS installer applies. If you want to DIY then you need an MCS umbrella scheme. There are several around. No way round this if you want the grant sofaik. Also vat zero rating depends on having a 'supply and install' contract.

I would add to this: If it's a retrofit, particularly in a house which has had fabric upgrades at various times, try to 'sense check' the surveyed loss against historical consumption. Surveyors (even those who spend a few hours) often ignore invisible fabric upgrades, overestimate air change rate and even double-count some losses, which can result in a calculated loss more than double the actual, which can make a huge difference. As a very rough sense check, take annual consumption in kWh, divide by 2000. Do the same again but dividing by 3000. If the calculated loss in kW isn't between these two figures, look more carefully and try to reconcile. If you have half hourly meter readings for a few weeks or a season, you may be able to use these to get a much more accurate figure. The method and caveats are beyond the scope of this article, but it's not difficult and can, depending on the circumstances, be very informative or even decisive. If it seems likely that assumed air change rate is the culprit, get a measurement done. It costs a few hundred pounds only and will also tell you where you might be able to reduce draughts.

Excellent. Here is my contribution (I may add more) Controller: Generally use only the heat pump's own controller and under no circumstances use 'smart thermostats'. Alternatively use an add in controller specifically designed for heat pumps, (currently only Homely, Havenwise or Adia). These interact with the heat pump bus and so only work with some heat pumps, but do so in. A way specifically tailored to heat pumps. Anyone selling you a controller, or even just a thermostat, claiming to work with all heat pumps is (at least at time of writing) selling you one you shouldn't buy! If you are going to use only the heat pump's own controller and want to program different temperatures at different times of day, you need a heat pump that does this by shifting the weather compensation curve. Some (for example, but not limited to Vaillants and the R32 Ideal models, maybe also the R290 Ideals) do this, others (eg Samsung) don't and rely, in effect, on a crude on/off thermostat function if you want programmable temperatures, which compromises performance. That said it's worth asking whether you really need to vary temperature at all unless you are doing it to take full advantage of the more complex time of use electricity tarrifs such as agile or cosy. In this case you probably need Homely, Havenwise or Adia (or a battery) anyway unless you enjoy tinkering with home made control systems.

The converse is also true You don't say which model this is but given your comments about physical size im presuming it's either the 5kW or the 7. The 7 (which I have) will be inaudible even outside at 7m most of the time, but when the compressor ramps up to max, which for me only occurs during recovery from defrost and at very low temps like -4 or below, it will be audible outside, although with double glazing it will be barely audible (if at all) inside even at max. If you have a little spare capacity you can have invoke noise reduction mode at night to calm it down, obviously at the expense of output. Is there any possibility of a noise (and visual) barrier maybe disguised as something decorative.

Indeed. Yet people still ask why the actual output differs from the sticker figure. It's just not a simple question! Mitsubishi (and Samsung htq I think) solve this 'problem' by clamping their outputs, but I can't say I like this practice much.

That's an interesting debate. In one sense at least ours is one of the worst in Europe. In much of the UK we design for -2 or -3 and have a damp climate, which means that any reduction in output due to defrost really matters. If you are designing for -10 in a drier climate the penalty for defrost is of lesser consequence, arguably of no consequence at all, since it will be occuring most at temperatures higher than the design temperature (where the demand is less).

https://professional.vaillant.co.uk/downloads/aproducts/renewables-1/arotherm-plus/arotherm-plus-installer-quick-guide-2848532.pdf Has got a bit more (Vaillant UK) information, but only on maximum outputs for heating and cooling, not minimum.

Imho it's pathetic how little info some manufacturers publish. Vaillant isn't the worst by any means, or at least it wasn't two years ago when I did the bulk of my personal research. Quite why the most complete Vaillant info is on Czech is a mystery, perhaps that is where they are designed even though it's a German company.

Steady on, that goes head to head with the prevailing trend in politics, or at least politics of a certain kind. Facts may mean you cant justify doing what you want to do or deceive those you need to deceive to remain in power. What we need is the complete absence of facts.

So far as I know yes and none respectively, except for a relatively small price difference. There is detailed data (in Czech) around but I can't find the link currently. PS found it https://community.openenergymonitor.org/t/vaillant-arotherm-owners-thread/21891 Original here https://downloads.vaillant.cz/downloads/projek-n-podklady/kl-06-e2-verze-01-18012023-2564719.pdf The give away for same hardware (many manufacturers play this trick) is same dimensions, same weight and same refrigerant charge.

Which is the 12kW model firmware limited.

Not only that but the fact someone chooses to is no recommendation. They managed to choose a piss poor ashp system, so obviously their procurement/contract management processes/understanding of heating are piss poor. I wouldn't rate a recommendation from someone who evidently makes poor decisions!

The radiation is still absorbed by other materials, but less so. The point is, at least in my understanding, that water is preferentially heated. Actually I think it might be the o-h bond so fats also absorb preferentially.

I would ask for a simple explanation of what is and what isn't heated and how your system differentiates and thus delivers warm people more efficiently than another system. For example a microwave oven works because it generated radiation at the resonant frequency of water molecules, this preferentially exciting them. Organic matter has a high water content so is differentially heated. Something like this is what we need.