I'm surprised at your experience with Aira's sales people,  I was very happy with the way they conducted themselves (as with all the other I contacted except when the contact at Octopus changed). I think Aira operate on a sales person giving you the basic picture and fixed quote for free followed by a detailed and paid for survey. Others do the full survey, they may ask for payment. Worth noting for me that Aira's full survey recommended more radiators that the sales person had estimated, but there was no price change.
Also, just because you have three year old radiators doesn't mean they are the right size for a heat pump. It will be interesting to hear what your other surveyors tell you.
Dropping the 15 year guarantee is an interesting one. Maybe Aira think that commercially they can get a bigger market share by dropping the outlay in by doing so. I used a rule of thumb of £200 p.a. service cost, so 10 years gives them £2k to play with.
As for the gas boiler cut off date target, I speculate that this gets the government off a hook to hang itself by when it isn't being reached fast enough at the next election, particularly give the cock up on the winter fuel payment announcement. I suspect that softer nudges will come into play such as decoupling gas and electricity prices. Also I read that putting gas heating in new builds is likely to be uneconomic under the FHS - Future Homes Standard.

Aira completed the technical survey for our property today. The assessor was very thorough and probably spent more than 3.5 hours going through all the minute details. Really impressed with his knowledge and attention to detail. If the installation team is as good as him, we are onto a winner. Overall, no curve balls in our case. I should get the detailed technical design soon and they can go forward with the installation as planned in a week.

A bit more progress on our side. Aira sent the detailed technical design on Friday. They are going for 46 degrees flow temperature and 38 degrees return temperature. After carefully going through the design, I had a few queries. After a bit of to and fro, they are planning six radiator changes, and reuse some of the downstairs radiators for the bedroom floor. I wanted to reduce the carbon footprint as much as possible, so rather than go for all new K22 radiators upstairs they are going to repurpose the larger K11 radiators from downstairs. The technical team has been very knowledgeable and responsive. The good experience continues and hopefully they can start the installation on Monday.

With a temperature difference on flow and return of 8C is the Aira able to operate comfortably with that? or are they using hydraulic  separation I.e. 4 port buffer aka low loss header. There are many arguments against that.
Totally agree with reuse of perfectly good radiators, we did with one but only one was the right physical dimensions.

All Aira systems are designed with a 4 port buffer. They say the heat pump has been designed from the ground up optimised to use a buffer. They have an additional temperature sensor on the buffer that the system reads. I don't think most other systems have this. My Aira system performed poorly to start with because this buffer sensor had not been installed correctly. It was reading ambient temperature not the buffer temperature and the heat pump was working hard to make the buffer heat up when it was actually already at target temp. Now the system works much better. I've put my own temperature sensors on the buffers flow and return because i'm curious and it is generally 8 degrees difference between flow and return. No problems with heating the house at -5'c a few nights ago and I've not seen Flow temp higher than 43'c. I can't tell how regular it was defrosting though. the Data in the app isn't detailed enough to tell and I'm not standing out in the cold to watch it 🤪  

@mikeh Spoilsport!😉  Actually, I have only been outside at the appropriate time to see our pump defrost once in nearly two years - but it is quite spectacular to see! Toodles.

@judith Our design includes a 40l buffer tank, and if I remember correctly, I think they mentioned a buffer is included by default. They have mentioned a SCOP of 4.1 in the design spec, so would be interesting to see what we achieve in real use.

Yeah we have a 40l buffer too. Problem is the App doesn't tell us the COP or SCOP. So we can't really tell. Hope they add this soon. They keep adding new features so finger crossed.  

So the team turned up just before 08:00 on Monday January 6. They had a quick tour around the house and the job. Then they set to, one was mainly in the place where the existing 23 year old gas combi boiler was and where the Aira Indoor unit, Buffer tank, connection to our existing hot water supply system and the existing central heating. As I had said before, the plan was to replace 6 out of 8 radiators. Of the six, three would be vertical, as the wall space was insufficient. We had a more stylish vertical in the bathroom but that arrived the next day, One guy was mainly working around taking out the old radiators, there were two others: the electrician who started on the electrical set up , which was crowded as we have Solar PV with FiT (and meter), Moixa battery (4.8kWh), and a EVSE (Ohme ePod). Thankfully we had had our DNO supply sorted back in 2018 for the EVSE and a 100A main supply fuse. But the electrical cupboard was very crowded. So in Day 1, we had no heating, no hot water. The gas boiler had been removed and the supply capped off. They did provide a couple of fan heaters which Aira had said in comms the week preceding and whether we had any special needs for anyone at risk etc. The outdoor unit was in place. 
Day 2 was radiator fixing, getting the indoor unit, buffer tank, etc. set up. The electrician set up all of the wiring between the new unit in the cupboard to the various Aira units. The final adding into the electrical main supply required the use of a certified person, which was a (busy) subcontractor with protective visor, insulated gloves etc. That was the only time the power in the house was off. That was about 15 minutes.
Day 3 was pretty much starting things up, and there was a leak, but in my existing pipework. They quickly fixed it. And by 15:00 of Day 3 we were getting heat and hot water. 
In conclusion about the installation, we were very pleased, with the work done, the attitude and behaviour of the Aira staff. It was the coldest week in the last 12 months. We survived.

 I will comment again when I have a week or so of experience. Of note in the photos, is the FuseBox. Bizarrely, the indicator light flashes green and amber when things are working well. If it is a steady colour then something is wrong. Seems counter-intuitive to me. The Indoor unit is 100l, and the Outdoor unit is the 6kW one.

Here's my follow-up report after living with the new heating and hot water system. All is good, we are warm, in fact a bit too warm, so I have taken the thermostat down. The thermostat is located in the downstairs hallway. It is now set to 18degC. After about a week of usage, the app got Nighttime cooldown option so, I set that to take the target temp down by 3degC (the maximum amount) between 21:00-06:00. Cannot say that we have suffered coldness in the house. As far as hot water is concerned, we have not run out of hot water. There are two adults, only a shower (no bath), so the 100l indoor unit has sufficient capacity for our needs. 

So what would I say in terms of conclusions: 
We are happy with the performance. Not sure yet about the cost of this comfort, but from commissioning the Aira system (late 8-January), the app reports that the system has used 320kWh in January. With a massive peak usage on Thursday 9 January of 55.10kWh. This is getting the very cold house and hot water in what was the very cold snap (external temps around -6degC). 

There are improvements I want to see in the Aira app, but I will say more later.

@vsmith1 Thanks for the updates, very helpful.

Our installation started today, and they have designed both downstairs living areas and upstairs bedrooms at 21 degrees. I did raise with their design team saying we like the bedrooms slightly cooler, but apparently it's the MCS standard. Maybe, like what you have done, I will turn down the thermostat and use the nighttime cooldown option too.

On another note, I discovered that the guys who fitted our bifolding doors in the big open plan room installed the trickle vents below the holes on the door, instead of covering the gap!! So heat was just leaking through the gap which is conveniently not so obvious given its location. Just because the gas boiler was pumping loads of heat into the room, I didn't notice until I was checking the gas consumption to make sure that we would be ok when the heat pump is putting in roughly how much ever is the heat loss for each room. Covered up the holes and the boiler stopped the excessive cycling and uses approximately what's been calculated as the heat loss for the room. Now, we have to move the trickle vents to cover the holes so that it can work as it should when we open the vents.

@chandykris I don’t believe that the MCS have a ‘Standard’ for room temperatures though some installers will attempt to hide behind such ‘standards’; at most the MCS may make recommendations or suggest guidelines but these are subject to the user’s requirements - which may for various reason (such as medical needs) be at variance. The installer should heed such requests and respect them! We made our requirements clear to our installer at the design stage for these very reasons.

When all is said and done - you will still have the lockshield valve adjustments to fall back on! Regards, Toodles.