JamesPa

Interesting Assuming they really are 3kW then 3 will do 9kW which is more than sufficient for most houses and gives at least a 3 to 1 turn down ratio. It's almost tempting given my ongoing battle over planning consent. Interesting that the auxiliary heater (if fitted) heats the incoming air, not the outgoing water.

It's worth unpacking that. BUS requires MCS and both space and water heating by ashp Building regs do not require MCS or that dhw is heated by ashp Planning under express consent does not require MCS unless your LPA make it a condition which is unlikely. Under permitted development MCS is required (there is a possible argument that if you install to equivalent standards it's ok, but if you wish to deploy this I'd get a certificate of lawful development in advance because this argument is untested (and in my view shaky) and, more importantly, the burden of proof if your LPA disagree lies with you). Either way planning doesn't require dhw by ashp I'm not sure whether the MCS standard actually requires dhw to be by ashp (it's worth checking MIS 3005-d), but many MCS installers will insist anyway on ripping out whatever functional dhw you have so their rookie plumbers can fit a pre-plumbed cylinder. However there are some that are more sensible. Depending on your situation you may be able to circumvent the grant chasing vampires, but most can't. And yes it's crazy but it's what you get if you don't have the necessary skills in the civil service, have politicians almost none of which have any engineering or science qualifications, and thus are entirely reliant on the industry to advise on regulation without any meaningful ability robustly to interrogate them. Of course I'm not suggesting that is what we have!

It's a requirement for the BUS grant. If you aren't taking the grant then you are right in some cases, mine included, that the payback time vs just using an immersion heater can be rather long and the money might well be better spent elsewhere. This is particularly the case if you already have a functioning dhw system that the 'industry' insists on replacing.

One might argue that it's the same problem that Ofsted has, ie dumbing down the assessment of something to a level that readers of (insert your choice of 'newspaper') can readily understand and/or make for good headlines. Unfortunately almost nothing in the world isn't as simple as a one word, one dimensional, rating. But if we want/need such simplicity we have to accept (and preferably be educated about) it's limitations.

I'm thinking of the air at the top of the tank (if any). Agree it's unlikely unless (I suppose) the expansion vessel is over pressurised, pushing the air bubble down into the tank.

The argument for the opposite I think is to avoid the possibility of sucking air out. Not sure if this is a real risk or not.

Thanks. I've already got 3.68kW PV under g98. I was thinking that this ac coupled battery solution was a low cost simple add on but I think the dno regards it as a separate generator thus requiring G99, although it's clear that both would not be generating simultaneously. Does anyone know whether I'm right or not?

Yes. Nominal capacities are a bit meaningless. Heat pump capacity varies with oat and ft and you need to make sure it works at your design oat/ft (and also that it will go low enough to cover the OAT that is more common) There are detailed tables for most heat pumps, including this one, which tell you the actual output at a variety of ft/oat combos

How did you determine this James? The tables I found in the user/installation instructions showed (on page 50 something) something like 8kW at +7 and 4.5kW at -10, with no intermediate figures (so what happens?). Note I am reading the docs on a phone so it's not easy and possible I have missed something

That good value and simple (in a good way). It looks like it needs a dedicated 32A connection. Did you get a sparky to do that and self install the rest. I can't remember if a battery needs dno permission

Makes sense. So in a system with pumped mixing similar would probably happen, unless you take a shower during a timed re-heat period or a second shower shortly after the first.

I think so, but I'm thinking of the DIY assemblies with a pump for mixing, such as @sharpener has spoken of (and may be planning to implement), or any of the cylinders that use a phe instead of a coil (ideal, Mitsubishi, maybe others)

Mixergy have a blog on this and claim so. I read the argument, it's plausible but I haven't examined it critically. Keeping the tank mixed means you can heat more to target temp, but the top of the tank presumably takes longer to warm so maybe it seems like the recovery time is a bit longer. I have been wondering what happens when you call for water during a heating cycle. There is possibly a risk of getting cold water. I'd be interested in views/experiences of this. Presumably it's dependent on pipe layout

Others may have some more quantitative answers but I haven't personally seen much by way of concrete numerical evidence here or elsewhere, other than for some extreme cases. However there are several very sound reasons to believe excessive cycling should be avoided if possible. It's reasonable to suppose your heating system will spend most of its time at around half the load at the design oat, so a reasonable design approach, it seems to me, is to be reasonably sure it won't cycle at half load (bearing in mind that, at half load, OAT is higher and FT lower, which tends to increase both the max and min output from the pump relative to the values at design oat). If you can do that its probably around the best achievable with current technology, but it requires knowing the load at design oat to within perhaps 20% (which is why I've been watching my smart meter carefully for two years). It's inevitable that heat pumps (and boilers) will cycle when the load is lower than the minimum. You can increase the cycle period but adding system volume, but you can't change the on off ratio as this is governed by the ratio of min output to load. Unfortunately that doesn't answer your 7 vs 5 kW question because of the lack of quantitative data. The best I can suggest it to decide how accurate your load figures are, and weigh that up against how much you care if for a few days per year you need a little bit of auxiliary heating. For what it's worth I have decided (if I ever get planning consent) to err, if at all, on the low side, but I'm pretty certain of my demand figures because they are based on measurement reconciled to a spreadsheet calculation.

For the simple reason that many, possibly most, heat pump installers will insist on replacing a cylinder with a coil area less than 3sq m. They are, in many cases, wrong to do so but sadly its a case of Resistance is Useless (well almost). So if you are installing a cylinder and planning later to install a heat pump, it's the 'safe' choice. I'm not sure how kW capacities of coils are measured, but I suspect its at a higher flow temperature than is typical of a heat pump (noting that ht heat pumps can reach 75) so you probably can't make a direct comparison. Nevertheless the frequent insistence on 3sq m remains unjustified in many cases. Of course for the grant chasers it provides an excuse to spec a new pre plumbed cylinder which means rookie plumbers are less likely to make a mistake.

You are of course right. Many people don't appreciate that the house still loses energy when the heating is off and that energy has to be replaced. There have been several big debates about this recently on the Renewable Heating Forum with people claiming savings of 20% or more (based imho on dodgy assumptions), which prima facie violate the laws of thermodynamics. There are one or two scenarios where there is a material saving to be had (say >10%) but they rely on a heat pump that is poorly set up (eg way oversized) in the first place or hefty standing loads due to pumps etc ie they are scenarios that don't apply to a well designed system. @mk1_man a very simple calculation to do is to work out the average temperature of the house with setback, over 24 hours, assuming linear cool down and reheat. So if the heating is on at 20 for 12 hours, and off for 12hrs during which time the house cools to 16C, the average temp of the house is 19C, 1C cooler. Now if the OAT is say 7C (a reasonable value for the heating season) the reduction in energy lost from the house (and therefore in the amount which must be supplied to the house by the heating system) is 1/13, just under 8%. With resistance electric heating this will translate to an 8% saving in electricity supplied. With gas or a heat pump, but particularly with a heat pump, it may be less because the heat pump has to work harder during the reheat period ( equally the saving might be a bit more if the reheat period occurs when it is significantly warmer outside than the setback period). Either way its around 8% so not the game changer much advertising about the advantages of smart controls would have us believe. Savings from zoning are similarly well overstated by those who sell radiator valves, and with a heat pump zoning can easily lead to increased consumption (depending on the shape of the house). This is why those who have taken the trouble to understand this stuff advocate avoiding complex control systems and (in most cases) zoning.

I looked at the full spec for the first link only. Performance tables pretty useless and show large fall off below 7C (data given only at 7 and -10). Noise figure is almost certainly noise pressure at unspecified distance not noise power, so useless for calculations. Caveat emptor.

In addition to (or possibly another way of looking at) what @JohnMo says you can't usefully extract all the stored energy from a thermal store because, once all of it is below the desired output temp, the remaining energy can't be transferred to the outgoing stream (unless of course you were to fit a small water to water heat pump). Effectively useful deltat is less, unless the thermal store is very hot.

That and 16kW. Based on the stories posted here and my own personal experience I would say don't, whatever you do, trust it. The thing about ASHPs is that the installers interest, when it comes to sizing, is almost diametrically opposed to the householder interest. For the installer oversizing increases price for little additional effort, justifies a buffer tank and almost guarantees no call outs due to 'its cold'. For the householder it increases cost unnecessarily and makes poor efficiency highly likely, with a large barrier to fixing it. Plumbers/heating 'engineers' are used to shoving a 28kW boiler into a 6-8kW house, jacking up the flow temperature so it doesn't condense, and letting trvs sort it all out. There is an efficiency penalty for this approach, but we are used to it. With an ashp you need to match the size to the demand more closely otherwise you risk a much larger efficiency penalty and having to add a buffer tank (which, unless correctly set up introduces a further penalty) unnecessarily. People who have been through this, or have spent time on this forum, now understand this. Sadly there is a proportion of people in the grant chasing (aka installation) industry who either don't, or can't be bothered. You really do need either to do your own heat loss calculations or have someone you trust do them. Sizing by wet finger wont do and you will come to regret it.

Similar build to ours, but we are 195m2 and 3 bed, 210L cylinder and a 6kW ASHP, although max heating load is 3kW. I would be your own heat loss calculations. May have seen big house made up the numbers, you can afford it quote. 16kW will likely be an expensive disaster due to gross oversizing. Do as @JohnMosays and make your own calculations (or pay someone independent to do them).

That must be ok because draining to a suitable internal soil stack is a recognised solution.

Same as an immersion in cylinder just moved so it doesn't sound as bad - same effect on UVC if it goes wrong. Not necessarily. By separating it from the tank you have the option, in a fault scenario, to dump or divert the hot water it produces before it even reaches the tank.

All I am trying to do is solve a real problem. What's wrong with that? And why is it never going to happen. No laws of physics are violated so is it 'never going to happen' because the industry either doesn't want it to or can't think out of the box? I'm not sure why it's necessary to be so dismissive, there is a problem to solve, let's try at least to postulate a solution. Obviously for new build there is no problem, but we have 1.4M gas boilers in existing buildings to replace with ASHPs each year for probably 20 years, and uvc minus the difficult discharge requirements would make a good proportion a lot easier and a lot cheaper.

I've said it many times, but I am convinced there must be a better solution to ensure the safety of a uvc than the current one, particularly when used with an ashp which is incapable of boiling the water. The discharge pipework is a major hurdle for retrofit situations. If we dispense with it then fitting a uvc is a no brainer. If only the industry could be persuaded to think outside the box on this one! In an ashp system, if we dispense with the in-cylinder immersion, (use a Willis heater located elsewhere as a backup) then any number of safety features could be added to ensure water reaching the DHW heating coil never goes above say 75C. What's the actual residual risk if we do this? I know someone will quote the D1,D2, tundish... guidance (the actual mandatory regs don't mandate a solution, only a requirement that it is safe), but the problem is significant enough to justify rewriting the guidance if there is an alternative safe solution.

It's the termination that gets me, it always seems to take much longer than it should. Mind you I've only ever done half a dozen so I don't have a lot of practice in this!