If your heat pump capacity was calculated using MCS heat loss calculations which it has to be for permitted development use, it is likely to be oversized, it would probably be quicker to 'adjust' your heat loss calculation and come up with a slightly small capacity heat pump that is within the permitted development size.

Yes, a very bad one indeed.  However it will be entirely dependent on the attitude taken by your local planning authority.

The problem is that its totally binary, the development is either PD or it isn't.  If it isn't then the decision is made by your local planning authority against the criteria in the local plan and they don't need to take the PD rules into account at all.  They may consider every aspect of the development and may, for example, impose much more severe constraints on some aspects that are imposed by the PD rules.  You might think that all they would only consider the aspects in which your proposal departs from PD rules, but that it not the case and there is nothing in (English) law which requires it to be the case

So for example my LPA wants me to achieve a noise level of the most affected assessment point of 25dB(A), as opposed to the 37dB(A) required if I were to install under PD.  This is both unachievable and objectively nuts, because 25dB(A) outside translates to 10dB(A) inside which is 20dB below the levels deemed desirable by BS 8233 (the British standard that sets out noise design criteria for residential buildings).   I have been battling this now for nearly two years.  I have been messed around bigtime along the way and most recently have had to remind them very bluntly that they are obliged by law to make a determination within a certain time (they were trying every trick to avoid doing so).  It will end up being refused and will go to appeal, so the whole process will be 2.5years end to end and there is still no guarantee of success.  My LPA is run by the Green party BTW (!) and their executive member in charge of planning has wittered on about how she wouldn't want her gardening disturbed by the neighbours ASHP.

That said, others have reported much less obstructive or even positively helpful LPAs, which is why I say it will depend entirely on your LPA.  You could seek some pre-planning advice or look through some decisions they have made to see if you can work out their approach.

If you cant establish that your LPA is cooperative I tend to agree with @gary that, if you can find a unit which fits within the PD rules, then its probably best to do so.  Unfortunately all the Samsung Gen7 units are the same size and 12kW tends to push you to double fan units not all of which will fit.  However this in turn depends on whether you really need 12kW.  I was told I need nearly 16kW when in fact the measured loss is 7.5kW.

Finally, Government is currently consulting on relaxing the 0.6cum rule, I am guessing this will happen, so it might also work simply to wait!

There are plenty of 11kw units around Mitsi, Valliant and Dakin all have single fan 11kW units and as James said you may not even need a unit that large anyway.

Posted by: @gary

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There are plenty of 11kw units around Mitsi, Valliant and Dakin all have single fan 11kW units and as James said you may not even need a unit that large anyway.

Vaillant Arotherm plus 10kW is over the PD limit.   Vaillant tend to underrate btw, so the sticker capacity is very much worst case and you may find that, at your actual operating conditions, the output is greater.  If I ever get planning consent I hope to fit the Vaillant 7kW machine, which is actually good for 8.5kW at my design operating conditions.

Daikin sticker ratings are iffy, you need to check the actual output at your operating conditions, it can be lower than the sticker rating and vary quite a lot with conditions.  They seem, so far as I can tell from the specs, to have a lot fewer actual hardware models than they have sticker models, and play games with the firmware to peak the output differently.  They also make it very difficult to find the information (their stock response is - ask your installer).  

The Mitsi 11.2kW unit fits within the PD rules and delivers 11.2kW pretty consistently according to their tables.  

LG units are quite thin by comparison to many others so this might help with the PD rules.  

There are of course many other makes, so you will be able to find something that fits if you can flex a bit on 12kW (and probably even if you can't)

Don't forget that minimum output is nearly as important as maximum.  You ideally don't want to be short cycling when your house demand is half the maximum (which will be the majority of the time).  My 7.5kW house spends almost all of the heating season sitting at around 4kW, so my design criteria include a requirement that the unit modulates down to 4kW at 12C and ~37C flow temp (applying weather compensation).  Heat pumps that are output capped in firmware often don't modulate down as far (because essentially you are dealing with a higher capacity unit the output of which has been firmware limited).

Where does the cubic capacity size come from...it does not seem to appear in Scottish planning rules on PD? I haven't checked Wales and NI yet.

Edit just checked...1m/3 in Wales and no mention of cubic capacity in NI.

They are the permitted development requirements for England.

• Development is permitted only if the air source heat pump installation complies with the Microgeneration Certification Scheme Planning Standards (MCS 020) or equivalent standards.
• The volume of the air source heat pump’s outdoor compressor unit (including housing) must not exceed 0.6 cubic metres
• Only the first installation of an air source heat pump would be permitted development, and only if there is no existing wind turbine on a building or within the curtilage of that property. Additional wind turbines or air source heat pumps at the same property requires an application for planning permission
• All parts of the air source heat pump must be at least one metre from the property boundary
• Installations on pitched roofs are not permitted development. If installed on a flat roof all parts of the air source heat pump must be at least one metre from the external edge of that roof
• Permitted development rights do not apply for installations within the curtilage of a Listed Building or within a site designated as a Scheduled Monument
• On land within a Conservation Area or World Heritage Site the air source heat pump must not be installed on a wall or roof which fronts a highway or be nearer to any highway which bounds the property than any part of the building
• On land that is not within a Conservation Area or World Heritage Site, the air source heat pump must not be installed on any part of a wall above the level of the ground floor storey if that wall fronts a highway.

You are right there are smaller ones available but the only R290 units that fit under the PD limit are 8kw ones (too small according the the survey/estimations) and the larger ones that fit are R32 ones.

Not too qualified to comment on whether the R290 brings any substantive benefits and whether newer is better. It’s a refrigerant minefield 🙂

Posted by: @jamespa

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If I ever get planning consent I hope to fit the Vaillant 7kW machine, which is actually good for 8.5kW at my design operating conditions.

Were you able to find (comprehensive) performance numbers for the Arotherm Plus heat pumps? The best I could find was the technical data from the installation manuals (page 133 & 134). What interests me is that the performance curves are not intuitive for R290. For instance the VWL 75/6 A 230V S2 has the following data 

• A7/W35 4.60 kW output nominal
• A7/W45 4.20 kW output
• A7/W55 5.00 kW output
• A7/W65 6.30 kW output
• A2/W35 3.10 kW output
• A-7/W35 7.00 kW output

Note that only the A7/W35 lists output as nominal (along with a min/max power output range).

The Mitsubishi data book is far more comprehensive in this regard, giving minimum nominal and maximum output at a wider range of temperatures.

@chickenbig

The attached has lots of figures starting page 90.  Not sure where I found it.

@imbimp please keep us updated on what you decide.

Will do - currently navigating the planning process, so we’ll see 🙂