Is it fairly open plan or subdivided into lots of small rooms with doors that are kept closed.  It might be worth posting a floorplan.  Depending on layout you may get away with fewer strategically placed units taking into account that the rooms with the party wall will not be losing heat through that wall.  If you can do that and reduce to one odu you shouldn't need pp unless you are in a conservation area.

Heat is heat.  If its in the right place it doesn't matter in terms of effectiveness how it was generated!

@jamespa thank you!

Don't have a floorplan to hand but will try to dig one out.

Unfortunately it isn't very open plan - the rooms that the six units would go in are fairly distinct; there may be good flow between them with doors open, but the doors would be closed a lot of the time that heating or cooling would be needed (wfh offices, children's bedrooms), so I don't think fewer than 6 indoor units would be realistic without compromising the comfort of one of the rooms.

Not in a conservation area, just a regular street in a London borough (Lewisham). It looks like the application would be a few hundred pounds - would that be something that I'd be able to handle between myself and the installer to apply for and submit the relevant paperwork? Are the plans DIYable?

If @ashp-bobba is around, he may be able to give you more technical advice and answers.

Posted by: @donciccio

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Not in a conservation area, just a regular street in a London borough (Lewisham). It looks like the application would be a few hundred pounds - would that be something that I'd be able to handle between myself and the installer to apply for and submit the relevant paperwork? Are the plans DIYable?

Many people, including me, prepare their own planning applications.  Whether you can in this case depends on a few things but mostly:

• how confident you are
• what documents your local planning authority (LOPA) requires beyond the basic drawings/location plan
• whether the development is likely to have an impact on the neighbourhood and thus how controversial it is

You need to check the requirements on your LPA website.  You will almost certainly need to do a noise analysis (use the MCA template for two ODUs or your installer may well be able to do this) and possibly a design and access statement.  If you are required to do the latter it is your opportunity to explain what a great job you have done to minimise any adverse impact and set out how it conforms to the policies in the local plan, which are the policies against which it will be judged*.  If you look at a few other planning applications you will get the idea.  If you are not required tro do a design and access statement you can anyway, and if the application is likely to be controversial then it would be best to do so.

If there is no material impact then its not going to matter how 'well' you do the above, provided it meets the requirements on documentation.  If the local amenity is impacted, visually or otherwise, then it will matter and showing that your development conforms to the policies in the local plan is key.

Hope that helps

* Planning applications are judged against 'the policies in  the local plan and other material considerations'.  'Other material considerations' include national policies, including but not limited to the National Planning Policy Framework (NPPF).  Mitigation of climate change and the fact that a single ODU would be permitted development (as would two on a detached property) are clearly material considerations.

Posted by: @donciccio

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The size of these seem like they would be more than enough to heat or cool the rooms that they are in, however the installer company themselves doesn’t recommend them as a whole house heating solution, suggesting that they’d just be a “top up” to our radiators. They say “for heating the whole house are really tricky with UK climate and relative poor insulation” – but is this just an outdated view? Or reflective of the fact that most people wouldn’t want to run their units continuously to provide the same kind of heat a radiator at high flow temps would achieve?

A2A are better at heating than cooling, if the system is designed to offset your solar gain it will be way over the required capacity to heat your home unless there is gaping holes in the building.

Posted by: @donciccio

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I understand that a heat pump installer would be able to provide a system that ignores the heat requirements upstairs if these are already supplied by a sustainable system and I’d still be eligible for grant? So we’d only need a 5 or 7kw unit for three downstairs rooms and a cylinder which would be a fair bit cheaper than a 10 or 12kW unit.

No Grant if you already have low carbon heating which A2A is. Also the installer would have to declare that the system is a hybrid and the property benefits from some areas being heated by other sources and in this case it will not be eligible for the grant.

Posted by: @donciccio

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I only feel reluctant because it seems like this isn’t a common option and even the installer seems not to recommend it.

In most cases A2A can end up more expensive and very hard to hide within the fabric of the building, pipe are much larger where they inc insulation, units are much bigger outside or multiple / multi units to do a whole house and capacities are limited, so where you can chose 100w increments for rads on A2W you only get 2kW, 2.5kW, 4kW, 6kW, 7kW, 8kW 10kW and so on jumps in capacities, if a small room only needs 320w heat-loss and as an internal room only 200w heat-gain a 2000w unit is overkill. Then you also have capacity limit complications when using the Multi units, you cannot go to far over the index so selecting can be a challenge if doing 5 or 6 rooms.

Posted by: @donciccio

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Is there any reason I’m not considering that would make A2A less effective than boiler or A2W heating? (I would be getting a large battery with enough capacity to power my heating through a full day on all but the very coldest days, charged at cheap overnight EV rates).

So be carful here, lets say you end up with 20kW (due to cooling requirements) of A2A systems, at peak they will need to use around 6kW of power so you need to make sure your inverter on both batteries and converter are rated above this or you will pull from the grid at the same time, you also really need to allow for the home draw as well which can be a steady 2kW. I am not sure if they do a 8-10kW inverter on batteries and solar inverter back to AC as I am not a solar specialist.

Posted by: @donciccio

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• Planning permission – for two external units in my end-terrace/semi, I’d need planning permission. This looks like it would cost c. £300 in my local authority.

Does anyone have any experience of whole home air-to-air heating and cooling, particularly in older, less insulated properties that could persuade me one way or another?

This choice needs to be yours, A2A is a bit more intrusive and can end up costing a bit more for a whole house but heat is heat, they heat every room the same as a A2W would. Being forced draft convection there are better suited to draughty less insulated homes as they are much faster reacting, using radiation / convection via A2W is low and slow, A2A can blast 50deg C within 15 mins and sustain that per room without any efficiency issues at a SCOP of AV 4.

We have only ever fully installed A2A in 6 properties in 20 years out of 1000's and we install around 800 systems per year for that 20 years. These properties are always furbished from the skin so services can be hidden and can cost around £2,000 - £3,000 per room so are often high end homes.

Although I do not have many examples of whole house heating and cooling by A2A, we have installed 100's of whole building A2A commercially using the same systems that go in homes and effectively a building is a building heat loss is heat loss its just math. It is also worth noting that very often commercial buildings are not always insulated the best and A2A still works. 

So your challenges are as follows:

1) finding a good company to design and install for you that truly understand A2A in the home

2) finding a way to install the systems so they are less intrusive if you like the minimalistic look

3) getting planning for 2 systems outside

4) passing the sound assessment test as the larger units on A2A are louder than the larger units on A2W 

I hope what I have added is helpful and I wish you luck.

Thank you for that very helpful and thorough response!

Posted by: @ashp-bobba

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No Grant if you already have low carbon heating which A2A is. Also the installer would have to declare that the system is a hybrid and the property benefits from some areas being heated by other sources and in this case it will not be eligible for the grant.

Hmm this bit is interesting, potentially a spanner in some of my thinking. Looking at some of the Ofgem guidance, it says: "In some cases, a BUS‑funded heat pump may be installed alongside other low‑carbon heating systems, where this is permitted under the scheme rules. Your installer will confirm whether this applies to your installation."

Would existing air-con/air-to-air not fall under that?

So if I wanted a mixed-system of some air-to-air and some air-to-water, and be eligible for the BUS grant, it would have to be done as a single installation? (Can't do A2A first as then wouldn't be eligible for grant. Can't do A2W first as it would have to be sized for the whole house to pass MCS/BUS sign-off)

Posted by: @donciccio

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Thank you for that very helpful and thorough response!

Posted by: @ashp-bobba

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No Grant if you already have low carbon heating which A2A is. Also the installer would have to declare that the system is a hybrid and the property benefits from some areas being heated by other sources and in this case it will not be eligible for the grant.

Hmm this bit is interesting, potentially a spanner in some of my thinking. Looking at some of the Ofgem guidance, it says: "In some cases, a BUS‑funded heat pump may be installed alongside other low‑carbon heating systems, where this is permitted under the scheme rules. Your installer will confirm whether this applies to your installation."

Would existing air-con/air-to-air not fall under that?

So if I wanted a mixed-system of some air-to-air and some air-to-water, and be eligible for the BUS grant, it would have to be done as a single installation? (Can't do A2A first as then wouldn't be eligible for grant. Can't do A2W first as it would have to be sized for the whole house to pass MCS/BUS sign-off)

Fundamentally I have the same interpretation as @ashp-bobba.  The actual legislation is here should you want chapter and verse on what is and is not allowed.  As you may want to do something odd or staged, I recommend you study this carefully.  Parts 2 and 3 are the key parts for this purpose.  Order of doing things may matter!

@jamespa thank you, very interesting to see the legislation.

Looking at it, I can't quite see that it would disallow the grant if you already had air-to-air heating in some parts of the house?

(2) A heat pump meets the suitability criteria in relation to the eligible property ... for which it is installed where—

[...]

(b) it is capable of meeting the full space heating and hot water heating demands of that property, whether alone or in combination with supplementary appliances that are not biomass boilers,

(ba) it has sufficient kilowatt thermal capacity to meet the full space heating demand for the eligible property alone, whether or not it is installed alongside a supplementary appliance, and

(c) it replaces the heat generating components of the original heating system installed in that property (where applicable), other than any supplementary appliance.

Where "supplementary appliance" is defined as

a heating appliance installed in an eligible property that is not fuelled by fossil fuel

para 2ba there seems key - the installed heat pump needs to be sized to heat the whole house, irrespective of any supplementary appliances.

That would suggest that, potentially perversely, if you had some A2A already, you'd be eligible for the BUS grant if you got an A2W heat pump to meet your whole house requirements, but not for one that would be sized to only heat the parts of your house not supplied by A2A.

But I'm sure that you repliers have much more experience in the field than me (I'm just a guy trying to google things), so is there something else in there that would make one ineligible for the bus grant if you already had partial A2A heating?

@donciccio I think you are correct and perhaps I was a bit binary with my response. So if the ASHP A2W covers the whole heating and hot water I think you can get the grant, if it does not you have to justify why and that the rest is covered by low carbon renewable tech. So in essence you are talking about duplication rather than substitution.

Ofgem have a very good help line for home owners and can answer your question directly, I would say thats the best option here to find out more? 

@ashp-bobba ah that's good to know, I'll get in contact with them. Appreciate the detailed answers on this forum!