JamesPa

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That's sailing a bit close to the wind IMHO. See regulations 17 - 24 for extensive inspection and enforcement powers and note that regulation 2 says that the heat pump is eligible if 'it replaces the heat generating components of the original heating system installed in that property (where applicable). I do grant that this does not require removal, but equally Im not sure a heat pump could be said to 'replace' existing kit if the existing kit is still used and one might even argue that replace includes physical replacement as well as functional replacement. I'm normally happy to sail close to the wind when interpreting legislation, but if I were an installer (which I'm not, I hasten to add) I would absolutely refuse to get involved in anything where I could reasonably be assumed to know that there is a likelihood of reversion.

That's interesting. I wonder if it's a function of where on the body they hit you. A2W units tend to be at ceiling height thus aimed at head and upper arms, even if you are sitting down. I would guess these are where most people would be most sensitive to light wind. I would also guess that air hitting the torso (when clothed) would be less annoying to many. It's entirely understandable why they are normally mounted near the ceiling, but is this form over function?

Hopefully there are a few doing this. The range is currently limited and the price is currently high, other than for very crude models. It's difficult to believe that it's not possible to do better.

That was meant to read multisplit. The joys of predictive text!

It's a fair question. The answer is flexibility. A2W allows a combination of fancoil and conventional radiators. Also I don't know of a multispectral which handles >6 outlets.

is this documented anywhere. Its common knowledge that they have 'dropped' the MCS requirement, what seems more obscure is what they require instead.

Slightly off-topic (but only slightly) I've been giving some thought to a related topic recently. Although A2A is not a good match to my house, air to water + fancoils (which have many similarities - they heat air quickly, shift it around causing dust issues, and make a noise) have specific advantages for heating, which nothing else matches. In particular they can adjust the amount of energy they deliver without adjusting the flow temperature, so for example can respond quickly and deliver more energy than 'normal' without raising the flow temperature , by turning up the fan speed. If you want a perception of fast recovery after a night time set back, this is a distinct plus. I have a hunch that, for the common pattern described by @Crofter above (in during the evening, happy to cool down during the night, want a quick blast around breakfast, out during the day) they might be the ideal emitter if, as many do, you want to economise a bit when not in, whilst keeping overall efficiency high and maintaining comfort. Fancoils in bedrooms are particularly unpopular, but consider a fancoil which has sufficient emission without the fan to meet night time requirements, then blasts on (but with a slow start up) just before the alarm goes to warm up the air in the bedroom and bathroom for the couple of hours (if that) before everyone goes out to work/school. Our UK aversion to air-borne heating is understandable, but perhaps not entirely justifiable. I sense that there is more in this hot air heating thing than first meets the eye.

I can't recount it but you can sort of work it out from the cross section on on of the websites, I can't remember which one. There are a variety of constant flow/constant pressure/pressure limiting plumbing devices working on similar principles, basically a bypass acting against a spring or similar, the compression on the spring adjusting the flow/pressure. I can't quite convince myself these are what we need however as they depend on rads being correctly sized, which they certainly aren't. I think heat geek are correct in their view that balancing for same room temperature is the right approach.

Something like this, where the design is done by someone who specialises in design and the plumbing/electrics are done your local tradesmen, is the way forward. Imho. We need to scale up rapidly to 1.4M heat pumps per year (that being the number of gas boiler retrofits each year in the UK). The only workforce of sufficient scale is the workforce of plumbers and electricians we already have. Of course we need to get MCS out of the loop too, at least in their current form, but the design element will inevitably remain. The design people could also have specialist tools/software to do system sizing properly (ie by measurement) instead of the GIGO pseudo-science we currently are forced to employ.

Isn't that what the Romans did? Just saying!

If I could work out how to do a smiley face emoji with my tablet, that would be my response. But I can't, so it's this instead.

That would have been good, morally defensible, and consistent with the purpose of planning law. But MCS wrote themselves in by defining 'the mcs planning standards' to include things that have no place in planning law (which deals with the effect on others, not detailed installation standards) but conveniently create a closed shop. Not so easy. My lpa demands impossible to meet noise requirements (their policy is that it must cause 'no harm to neighbouring properties' (nb not 'no material harm') so in practice unachievable in many cases Furthermore they require a full noise assessment to bs 4142 which costs. From a quick Web search my lpa is not alone! + what @johnmo says!

Maybe some a2a HPs with the cooling function disabled in would be a sales opportunity in the UK. The PD requirement is 'used for heating' not 'incapable of cooling' so the fact it's capable of cooling is, as far as I can see, irrelevant, so long as it's not USED for cooling. However, to be pd, it must also be installed by an MCS contractor to MCS standards or equivalent standards. Not sure if MCS standards permit A2A, would need to reread them (possibly more than once). There are currently no recognised equivalent standards.

I made that comment in the context of radiators which self balance for equal room temperature not equal deltaT. They need to be proportional control, no shut off (except if the room temp is way too high) and crucially operate in slow time so they settle to the correct degree of opening and just stay there. Not sure that's what the reference you give does, but certainly something that a mod of electronic trv firmware could do. The challenge would be to avoid all the control loops fighting each other, leading to oscillatory behaviour. This might need an element of coordination, alternatively an element of randomisation in the time constants might do the trick. It would be a fun piece of analysis for a control theory nerd!

On the basis that both are just heat sinks, with the correct plumbing then yes in principle but: You will be dividing the total output between the two so both may struggle The efficiency for space heating will be awful because it's running at a much higher ft than necessary Not sure why you would want to do this, perhaps if you told us we could collectively suggest a solution to whatever problem you are trying to solve.

Thanks. Just to understand precisely, in this suggestion the insulated water pipe runs loose in the duct, surrounded by air (or water if water somehow gets in, which presumably needs to be avoided). It does sound simpler than the prefabricated twinpipe.

Would you be willing to share more about your experience with this stuff? It (or something similar) remains an option for me if I can't convince myself that the more obvious location next to the house won't annoy the neighbours.

See comment by @JohnMoupthread about minimum 'recommended' size. Also fitting a 3sq m coil (the 'safe' size accepted by almost all hp manufacturers, although some specify smaller) into a small tank means that the tank is disproportionately coil not stored water so reducing too far is a case of diminishing returns. A plate heat exchanger loaded tank solves this problem.

Good and I really hope you succeed. The current position is unsustainable in the long term, of that I am certain. However, whilst demand exceeds supply and the government continues to subsidise the indefensible whilst failing to scale up the supply, there is a risk that nothing will change because the industry doesn't need it to change. Hopefully someone, manufacturer or large installer (eg octopus) will break ranks sooner rather than later and deliver the kick up the rear that is urgently needed. The one which does break ranks will, for a while, enjoy a significant competitive advantage. Obviously they need to be able to achieve the volume to exploit this. In my dreams the other dhw problem, namely the uvc vent, will also be solved by someone breaking ranks. A dhw cylinder heated by a hp cant boil. If you don't put an immersion in the system then boiling is impossible. If you need immersion back up, then locate it outside the cylinder with alternative means to ensure boiling water never reaches the cylinder. Then no need for the troublesome vent, or at worst you open up the possibility of locating it somewhere more convenient than where the cylinder is. £500 or thereabouts and major disruption saved.

Exactly. And if it is a HT capable pump even a small coil will transfer sufficient heat to avoid excessive cycling. There is a cop penalty for running at a higher temperature for dhw, but the relatively small extra running cost is very unlikely to be sufficient to make the business case for replacing a newish cylinder stack up. And if it does cycle excessively (which it won't if it's a 6kW unit with a half decent modulation capability), add a plate heat exchanger and pump rather than ripping out (in some cases) lots of pipework in addition to a perfectly good dhw tank. This obsession with replacing perfectly good dhw systems in almost all cases is an old-thinking based myth, which it is convenient for the industry to perpetuate because it de-risks installations and makes the whole thing look more difficult than just replacing a boiler. In fairness the industry is aided by the modern obsession with 20l per minute showers for the whole family in quick succession. As I said upthread, the question op poses is not a technical one in practice, it's more a question of what the industry will allow you to do in your own home.

Extract from the BUS requirement taken from source legislation. There is some potential ambiguity but I think that @sharpener is most probably correct because of clause b (b)it is capable of meeting the full space heating and hot water heating demands of that property, and (c)it replaces the heat generating components of the original heating system installed in that property (where applicable), other than any— (i)supplementary electric heater, including any immersion heater, (ii)circulation pump, or (iii)solar thermal collector.

Also true. In summary it's not a technical question, it's a question about what the industry will accept.

Lots of discussion on this here from time to time and no definitive answer. Technically the answer is very probably that you don't _need_ a heat pump cylinder, provided you end up with a heat pump capable of a reasonably high temp (eg 65) and modulation ratio (eg3:1). Many modern heat pumps achieve this. Historically the 'problem', as @JohnMosays, is that at a lowish flow temp the coil doesn't transfer enough energy to the water to keep the heat pump happy, particularly if it doesn't modulate down well. But with higher flow temps and decent modulation depth, this is not necessarily any longer the case. Furthermore an add on plate heat exchanger and pump can convert more or less any 'standard' cylinder to one with plenty of coil area. Mixergy sell this but only for their cylinders. There is at least one installer who is developing such an add on, but the idea does not yet have mainstream traction. At least 2 heat pump manufacturers sell cylinders with a PHE instead of a coil. Alternatively @JohnMos suggestion simply to top up with the immersion is sensible, but again not mainstream so many installers won't accept it. In practice however the industry is completely obsessed with swapping out dhw cylinders. Heat pump manufacturers often specify coil sizes which are larger than makes sense and installers are bound to follow their recommendations. So you may not find an installer that will reuse your cylinder, even through it may well work. So for now it's an unsafe assumption that you will be able to reuse a cylinder without a hp (ie large) coil, despite the fact that it's technically feasible in many cases. Sadly the recently announcement of an increased grant and delay in the boiler ban will reduce the pressure on the industry to get it's act together and become more innovative in how it approaches this problem.