That is a good question, and there's so much confusion about this. I'll reach out to my installer and heat pump expert network for guidance. 

Posted by: @andrewj

↑

I've seen a number of images in posts of ASHPs that use R290 installed under windows.  I thought these had to have minimum clearances from door and window openings, drains and so on in case of leaks.  Is this not the case and it's perfectly ok to place in such a position?

I firmly believe (but am open to correction) that the answer is yes (I also hope so, mine is!). 

The operative word (I believe) is under, the protection zone extends either side of the unit and in front of the unit but not above.  I seem to recall that Samsung diagram is clearest on this point because it shows a pseudo 3d view, but I may be wrong.  I haven't yet seen any diagram which forces an interpretation that the protection zone extends above.  If anyone has please say.  Propane sinks, which is relevant.

I may be wrong.

@andrewj with the urgency of the need to reduce GWP internationally I would presume there would be a phasing out of new R32 installs. After all the environmental benefits are significant 

“R290 has a GWP rating of 3, whereas R32 has a rating of 675.

Presumably there would be a 10 year support for spares after it being discontinued but that’s purely a guess.

Regarding gas hazard I think I saw a Heat Geek video where it was stated the safety legislation of window clearance is a hangover from gas boiler legislation where the gas supply might be piped town gas of enormous volumes, where a leak could be catastrophic. Whereas the volume of propane gas in a R290 heat pump is usually outside the property and of finite volume. (Not sure if the exact volume.)

Another advantage of R290 HPs would appear that the models I’ve seen generally have lower minimum operating outputs which means they do not cycle quite as much as R32 equivalents currently available. At least, that’s what I’ve seen, so far as Mitsubishi models are concerned.

@editor Thanks, that would be very useful.  My property has windows across all sides so locating a R290 would be problematic depending on rules.  An R32 would be ok hence my follow on question in respect to future proofing.

@jamespa The Vaillant diagrams do have doors and windows below the top level of the heat pump with relevant clearances but nothing in respect to anything above.

@sunandair It seems a bit strange that I could have a canister of propane attached to a bbq under my kitchen window but not a heat pump!!  Hopefully someone has more info.

Posted by: @andrewj

↑

@jamespa The Vaillant diagrams do have doors and windows below the top level of the heat pump with relevant clearances but nothing in respect to anything above.

Which I personally take to mean that there is no restriction on what is above.  Otherwise they would have to show a clearance and a protection zone

This 3d diagram form Samsung possibly makes that it clearer 

@jamespa I see.  Taking that and re-looking at the Vaillant images, the protective zone doesn't rise very far above the unit - no measurement given so presumably just a minimal clearance.

@andrewj will you be installing it yourself?

Posted by: @andrewj

↑

@sunandair It seems a bit strange that I could have a canister of propane attached to a bbq under my kitchen window but not a heat pump!!  Hopefully someone has more info.

I have an 11kg propane cylinder 1.5m below our kitchen window feeding our cooker. 

@editor No.  I'm getting some quotes later this month so I'm just trying to work out what and where things might go and whether any choice I make in my head is moot and the reality is it has to be a Daiken because of location.  I have Octopus and Heat Geek coming around so the choice is going to be between Cosy, Daiken or Vaillant.  I know I'll get the low down at the time but I like to be informed.

The other thought I have is whether I should get in touch with Aira: it's not been clearly laid out why they want a buffer.  In your video about this the message I picked up is that manufacturers want a buffer to provide separation to limit possibilities of blame if there is an issue.  Although they seem to be really responsive from the postings so far so I could be doing them a disservice but in any case, given that the vast majority of installations don't need a buffer, why are they so insistent??