What would make this even more useful (if possible) would be including 1) the UK equivalent of an ASHRE Manual J and 2) an ACH rating for the dwelling.

Heat pumps are great, but suffer from piss-poor installations and shitty salespeople. When I renovated my DC rowhouse, I talked to 6 different companies about the HVAC install. Only one, ONE, would do a Manual J. I had already done a Manual J myself (it's not hard at all) so I could compare their calculations to my own. They were slightly different (they calculated a greater load than I, but at the time, the insulation systems I used in the house were uncommon, so most of this difference was due to their lack of familiarity).

In the end, my rowhouse needed such a small unit (1.5T IIRC) that I couldn't get the SEER I wanted because no one makes high end units that small... I ended up slightly oversized at 2T, but that was necessary to get a unit from a good manufacturer (Lennox), rather than a pile of garbage flipper grade unit from someone like Goodrich.

What the heat pump industry is going to suffer from is the utter and blatant disregard to right-sizing units. The other 5 companies I talked to? They just walked around the house and then said shit like "This gonna need a 5T unit" and left.

Also, flex duct is bane of any central non-high velocity system. If an installer mentions flex duct for anything other than a short run to a register, run away from them. Flex duct is the sign of a lazy installer who is going to cut corners everywhere else they can, and especially where you can't see it.

In the same vein, if the installer doesn't have their own sheet metal shop, make damn sure they're buying your ductwork from a sheet metal shop and not from HomeDepot. One company I interviewed refused to do a Manual D (duct sizing) and said that a standard (i.e. we're going to get it at HomeDepot) 9x13 duct will be "plenty" for the return. The return (per the Manual D) ended up barely fitting in the chase alloted for it which is something like 30"x 24".

So be educated consumers:

- Do a Manual J and Manual D yourself, they're not hard

- Right size your unit. It should be running 90% of the time for max efficiency, so yeah, a smaller unit might take longer to cool or heat, but you shouldn't be turning the unit on or off but two or three times a year. A super high efficiency unit that is oversized is going to be terrible and inefficient. Don't fall for their shitty sales tactic of "but it's not going to be able to cool your house down as quickly as this 5T unit!".

- Demand good work. Like a plumber, never ever ever let an HVAC installer in your house unmonitored. I had to fight with my installers because they wanted to move a chase to the middle of the room "because it'll be easier for us". No, you have the plans, you quoted on the plans, so I don't give a shit that it's hard.

- Insulate and seal your duct work. You can't go back and do this, so make sure it's in your contract, and make sure they actually do it.

- Make sure you service your unit annually, it's a couple hundred bucks of peace of mind.