Whats more relevant is that a badly installed air/air system has a COP of near 4. Given the kind of people who do a lot of installs this is a feature as is the fact the average American can self install one. Efficiency of heatpumps in a lot of properties will be governed by the quality of install which is one reason I think air/air ought to be used for newbuilds because they people doing the air/water install generally haven't got a clue.

@majordennisbloodnok @jamespa 

I stand corrected, A2A need defrost cycles just like A2W.

But on the relative efficiency, given that there are no water circuit losses, A2A tends to be slightly more efficient. Not a reason for us to change our mind about our ASHP!

Couple of things: A2A do defrost, they sometimes do stall or very gently (almost unnoticeable) move the air of the inside coil if it has to dissipate some heat to the indoor space. I can assure you at -6 they defrost every 30 to 60 mins if the heating load in the home is high.

They tend to be more efficient as they are DX systems, only using the refrigerant to move or collect heat from outside via a metal collector (the condenser coil "or evaporator if described in heating mode" so the outside coil. This coil directly influences the refrigerant and then through its stages of evaporation, compression and expansion makes its way still using the refrigerant as the medium to effect the opposite coil inside, this is called direct expansion. 

An A2W does all the above inside the monoblock ASHP and then moves the heat to inside the home via water medium, although water is an amazing heat moving medium it is not as good as refrigerants. The DX systems also can modulates just ever so slightly better allowing a bit more optimisation but this is all designed in by the manufacturer not the field staff or installers.

DX systems, splits or AC packages, what ever you want to call them are also matched by the manufacturer, so you get the correct external units that match the indoor units and these have been optimised to work with each other. A2W is field designed, companies that are good at designing can get A2W to hit the 400% and above, others cannot, some homes will naturally be a 400+% home and some others will not as the saving of £80 per year is not worth £2000 worth of upgrade, so some people live with 360% efficient systems and get their installation for 5k rather than 7k.

DX coil - DX coil = refrigerant directly absorbing and delivering the energy (more efficient) 

Other systems - DX coil - DX coil - Water - Emitter 

Half the stages, no water pumps or additional valves which all add more power consumption, an A2W unit effectively has the same components as a whole AC system with a plate DX to water in place of a (DX HE - Indoor coil) and then has additional bits added on top to move the heat to the home. Also worth noting that naturally the more changes you have to move the heat the more energy losses as heat transfers - this is some additional entropy in action.

Where A2W shines is most homes already have a reasonable water circuit thats usable if in part and to place a DX system in every room from a boiler would be more expensive to cover the whole house.

I hope I did not go to technical above...  

Posted by: @batpred

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Not a reason for us to change our mind about our ASHP!

I quite agree.

I'm a bit cautious now about commenting since I'm not trying to be awkward. However, @batpred, you happen to be the person who's posted some comments that lead into my questions, so please don't take offence; nothing personal intended. It's just that I want to challenge some comments I've heard several people throwing around not because I think they're wrong but because I just don't know (and my instinct is to not accept assumptions out of hand).

Up to this point, I've generally heard that A2W systems typically provide a SCOP of about 3 - 4 whereas A2A systems typically provide 2.5 - 3.5. Looking in a little further, I'm not sure this tells the whole story since the reasoning appears to be that A2A systems normally run hotter to heat the space directly, and that sounds far more like running an A2A heat pump like a boiler. However, it's also often suggested - at least where I've been reading information on them - that A2A systems are often set up to deliver heat where people are, so reducing efficiency but also reducing wasted energy in heating empty rooms. The net summary I've been able to glean so far is that A2W is normally more cost effective in better insulated homes whereas A2A wins out where more localised heating is required, especially when insulation isn't so good.

Now I've outlined what my understanding is from reading around the subject, I would REALLY appreciate someone like @ashp-bobba who knows what they're talking about to fill in what I already know are big gaps to take the theory into the real world:

• Is there a significant difference in efficiency between A2W systems and A2A?
• Given efficiency and effectiveness aren't the same thing, what are the key factors affecting real world running costs for each system (aside from factors that affect both similarly)?
• Looking at the variation of home sizes and of insulation at dreadful, poor, good and Passivhaus standard, which is the typically better choice for each scenario? A2W or A2A?

Obviously, I'm interested in ignoring for the moment the fact A2A can do really well in the summer for cooling too, and that A2W can do DHW as well as space heating; those are inevitably distinctions that will sway many people's choices, but I just want to compare on the overlap in functionality. As a result, @batpred, you may be right about A2A being slightly more efficient but equally it may be possible A2A is not as efficient but is perhaps still cheaper to run. I just don't know and am keen to find out.

For passivehaus btw you also need to consider just using an electric heater (or even a large dog) as at that level of efficiency it can be that you simply don't get sane payback versus just using an electric heater, gaming pc etc.

...and as if by magic...

Here was me asking for a bit more in-depth and @ashp-bobba dives in before I hit submit to do exactly that. Thank you.