JamesPa

The law is the law. Government 'guidance' is often an imprecise approximation (during COVID it was sometimes just plain wrong!). In this case building control and planning, which are two totally separate branches of law dealing with different aspects of development, are being wrongly conflated. Its best to keep a clear head and separate them. So to unpick this "Thanks, IIRC the Planning Portal says that any ASHP installation can fall under Permitted Development if the installation complies with MCS 020 and part of this standard requires that the installation must be carried out by an MCS approved installer" 'Can' is not the same as 'does' The actual law on permitted development (in England), which is definitive, is here. See specifically Schedule 2 part 14G. There are quite a few requirements beyond MCS-020. "Without a PD exemption, normal planning and building control applies. " Without a PD exemption, normal planning applies, however building control is a separate and parallel branch of legislation to planning. It applies whether or not the development is done under PD. Building Control doesn't say a lot about ASHPs but what it does say is totally independent of the planning regime under which the development takes place. Building Control rules do, however, depend on whether it is a retrofit or part of a new building/major refit/change of use. "This is all a lot simpler if the installation is done as part of the initial build and part of this planning and building control approval, but in our case that was over 6 years ago." Yes, because in this case the Local Planning Authority (LPA )focusses on the bigger picture not the minutiae of the ASHP! The Building Control regulations in this case are, however, much tighter, but in most cases easier to achieve because you are not constrained by what is already there. "As a post sign-off install, this control itself introduces a shed load of bureaucracy and costs. In the case where the proposed ASHP is to the rear and within the property curtilage, you would need to demonstrate to BControl that the installation complies with gas-safe, positioning and noise regs." No. Gas safe regs are irrelevant because there is no gas. Building control doesn't care about positioning and noise regs, that's a matter for planning. " In this last case it would be practically impossible to get BC sign-off of your calcs unless the ASHP unit is an MCS approved model." If its done as a retrofit BC has very little say about efficiency or anything else of relevance to swapping fossil fuel heating for ASHP (other than G3). BC doesn't care about MCS, its not even mentioned in the BC rules. If its a newbuild then BC does have quite a lot to say about efficiency, but it says nothing about MCS or any of the MCS rules that appear to be troubling you..

I was wondering that as well. If they did 4kW, which is my actual normal load for most of the heating season it would be very tempting indeed!

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