UKH

This question is specifically about the air to air type of system. Can anyone who has this, or maybe even someone who installs it, give me an opinion on what they think of this type of heating system, and what installation cost is roughly (I appreciate this varies by output but if you specify that I can work from there)? Obviously I've Googled it and read loads of stuff from various sources (although most of it refers to the air-water systems) to come to an opinion that it might be what I need, so I have an idea of the obvious pros and cons, but an up-to-date UK-based opinion would be handy.

Well I'm not UK based but I've buddy with air to air here in Bavaria. And installed plenty of ducted hest systems in supetyachts. Unless the house is purpose-built for it it's a no-go because even if you can get the feed ducting in you need the outgoing heat recovery system. You can't blow in if there isn't the equivalent exhaust so you need a heat-recovery system somewhere up in the house which is relatively gigantic. And the perceived temparatures mean the plants have to operate less efficiently than a wet system (the temperature has to be higher).

You mean a system that just blows warm air from the unit inside, like an aircon unit but warm. 

I can reply more later, but yes we have one, or rather two. 1 up, 1 down. It works fine but once air temps are -20 or so, then it's becoming less efficient, although they'll heat with air temps down -30. Insulation is key, proper insulation, not uk standards. Fairly open plan is desirable too, to get the air around. 

I got a quote for one about 6 months ago. my house is on the limit of practicality as its quite large and is detached. the main drawback is that there needs to be a additional device to bump up the heat to that of a gas boiler if it is to be used for radiator heating. The pump was the largest they produce and the quote was about £15k. Much of that was in labour and a surprising amount was to administrate the grant application but it has become apparently that the government grant awarder was a US company just buggering about sucking up UK taxpayer money. The grant has been withdrawn for the time being so this isn't the best time to be installing one. Wait until the green homes grant comes back which is probably not too far off due to COP26. But expect it to cost about £5-8K after the grant unless you are of low income and then you can get it free.

I have an ASHP, but that is air to water. They are great.

The type you describe are really only suited for purpose built homes where all waste hot air can be used as heat source. You need very good insulation and well planned ventilation. There seems to be many complaints about these type installed in social housing and costing the tenants lots in electricity.

Retro fit I doubt you'd recover enough of the waste heat and probably won't be well enough insulated if just cavity wall+roof+double glazing.

Yes, the ones I looked at online have an external compressor then aircon type units are fed from that. You can run 5 internal units off the external compressor. That does limit things a bit (but UK houses are small anyway), and I was going to use standard electric heaters in a couple of the smaller rooms that aren't used much. The insulation isn't great, but I can upgrade it to an extent, although not anything like you'd expect on a new build. During this winter I've kept the old electric hot air system off and used electric convector heaters with thermostats in the main areas to see exactly what heat output I need in each room.  So, based on this and the heat output I've seen in the technical documents I looked at it should work (although I'm not a heating engineer so maybe I've overlooked something). Winters here (SW UK low down near coast) are mild, generally no worse than -4 deg C a couple of nights a year, so that seems well within the operating range of the systems I looked at. I was going to keep the hot water supply on the old immersion heater system that runs off cheap-rate overnight electricity.  Originally I was going with gas before covid struck. I even had the supply put in and had all the internal pipe runs planned after lifting the floors, but the signs here from government are that gas is on the way out, although slowly, and as someone in this thread has indicated there maybe grants available for this type of thing.

I'd read something along those lines as well a while back and so discounted the idea. It was only recently I started looking at this again and I was getting a slightly more positive message from what I was reading.  So, what you've said is adding to my caution. I suppose ideally it would be good to hear from someone who'd retrofitted one of these currently-available systems to their property.

Have you looked at small VRF systems?

Commonly used in small commercial developments with really high efficiencies and easy installation.

Drawbacks are going to be the size of the indoor units and the potential for problem noise due to them containing fans to blow air through them.

No idea how much they cost in a domestic setting, but have designed many small offices with them.

You could use them without a mechanical ventilation system.

Ours are retro fitted in an old house, upstairs we have just one internal unit covering 60-70m2. The same down, each have their own external unit. We were advised they work best if operating independently. The rooms they can't reach with normal open plan air flow, bathroom, halls, some bedrooms have separate heating. 

Insulation; double glazed, walls are around 100mm although some have more, some have internal and external insulation to avoid thermal bridging. Loft has 400mm. Avoiding unnecessary heat loss is key, we have veranda/ porches on both doors in, if you count doors into halls, you have 3 doors to go through to go from living space to outside, so in winter you aren't going to have that sudden rush of cold air in. Costs for both pumps £5k ish. 

That's interesting. Similar areas, and I assume you're in a colder climate. All my entrances used during winter have porches with doors plus halls in addition between there and main living spaces. My insulation upstairs is the real problem I think. But your reply indicates this might work.

That looks similar to the systems I was referring to. I'll check it out in more detail.

I should point out when I say wall insulation, just mean actual insulation, adding to the overall thickness of it, not navity filling, which I don't believe personally are any good long term, cavities in brick walls have a job to do. 

What temperature range do you experience during winter?

Probably the most common way of heating houses in NZ. We currently have two, and another three will be fitted soon as part of our renovations. 

NZ homes typically single glazed and unlikely to have insulation, even new build regs have similar thermal specs as the UK mandated in the 80s. So when we got some decent loft insulation and thermally insulated curtains even the council’s eco-advisor said “just add heat pumps until warm enough - it will be cheaper than double glazing”! 
 

We have Daikin units and low interior wall fan units. The new system has a single large exterior unit and three internal units. You might be better off looking on some Australian/NZ fora for ideas. 
 

good luck 

Rarely below -20c. But will have a 2-4mth period every winter permanently below 0c.

Thanks, can I ask you the same as above, what sort of winter temperature range do you get there in NZ?

Considerably warmer and more UK like. Maybe 20 frosty mornings a year here for n Dunedin. Overnight temps usually single digits in winter. Plus if it gets really hot midsummer we can run the AC on cool - happens a couple of days every couple of years. 
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