Crofter

There will be a way of doing it from the inside. A reciprocating saw is your friend.

Ok, I've had my cup of tea now. 2640m³/hr is 0.733m³/s With a total duct outlet area of 0.4m² I make that 1.8m/s or about 4mph. That's the 10kw unit running full tilt, possibly double the actual heating requirement. Although there will also be losses with friction and bends. That doesn't sound too scary. But it shows that you definitely need some big ducts and vents to make this work. I'm making this up as I go along so more than happy to be corrected!

It's not on the same scale as MVHR. Then again, the more conventional A2A units without ducting seem to get on ok, with people using a single unit in the hallway to heat adjacent rooms. Shadow gaps could be really elegant, but when you look at the total vent area required I think a couple of dedicated vents might make more sense.

I did find an online calculator on engineering toolbox, and was playing around with that. Worst case scenario of 10kw heat loss, 21⁰ room temp, and 40⁰ supply temp, works out at 0.44m³/sec or a whopping 1584m³/hr. Yikes. Best case with 5kw, 19⁰, and 55⁰ supply, drops that to 417m³/hr. Which is in line with your own figures. I have yet to do a proper heat loss calculation, but a radiator sizing tool suggested about 7kw, making some hefty assumptions. One unit I'm considering (a Mitsubishi 10kw) has a max flow rate of 2640m³/hr. It certainly is a lot of air! Maybe that's massively oversized after all. Four 200mm ducts is about 0.4m². I should be able to work out the flow rate needed... after a cup of tea...

The system does recirculate. You need an inlet and outlet for each space. It doesn't draw in external air- it's a heating system, not ventilation.

So This should all be a solvable problem. But I'm realising it's fairly involved. I've not heard great things about the system design for A2A by the 'experts' and I'd like to get my head around the principles. So I guess I need to do a rough heat loss analysis first, to size the unit. For example, using Heatgeek's rules of thumb I'm getting maybe 60w/M2, which for my 93m2 property is about 5.5kw. That's about half what I was expecting. What happens if I oversize a good bit, up to say 10kw? Will the system cycle too much and lose efficiency? Next, pick a suitable ASHP unit and start playing around with flow rates and temperatures. That should let me start getting a handle on duct and vent sizes, and layouts. I'm finding very little info on all of this online. I suppose some of the principles will be the same as MVHR, but I guess it's a much larger volume of air being moved, and I'm going to want different rooms to be at different temperatures.

This was my experience too) Highland Council thing, perhaps?) I could just squeak in to the requirement, but it relied upon vegetation being cut back on the verges for 90m either side of my access. This wasn't my land so the need to 'demonstrate control' couldn't be met. But they accepted it. My backup plan would have been to point out that it was their responsibility to trim back the verges. A couple of houses built after mine, on the same road, had nothing like as good visibility, and still got through. One other thing that came up for me was that my LA have two different access standards- one is for a single house, the other for a mini estate of up to four houses. The junior planning officer who first dealt with my application initially demanded that I use the larger access, which physically wouldn't fit on the plot, because it would be shared between the house and the field behind it. Despite the field having alternative access, and fields not needing the same standard of access as a house. Fortunately he was over ruled by his boss who saw sense.

That low level wall outlet is basically what I'm hoping to do. But fitting a 200mm duct in to a 100mm stud wall will be interesting. I guess I can split it in to four 100mm ducts...

Lifting part of the floor isn't a great option, it's continuous pine T&G boards with the internal partitions built over the top. I think it would be pretty destructive and the floor would never be the same again.I've also just laid laminate through about half the house and don't want to lift that again! I really don't mind working underneath- there's about 2-3ft clearance everywhere. I replumbed the entire house recently and shifted everything from the loft to underfloor. The ducting in looking at is the same idea as I used for MVHR in my other house- flexible, expandable, lightweight, so dead easy to work with. Will need support but nothing complicated. I see in the US people use floor gratings. It must work, otherwise they wouldn't be doing it. But it feels a bit wrong to me, for some reason. Can imagine it working in rooms with hard flooring, I suppose, but not so much with carpet. I think I may have to resign myself to building a box-out and just have it positioned where it's partially hidden, e.g. behind a shelving unit.

Exactly. I don't want to force air down from up near the ceiling, I want it to emerge at floor level where it's needed. I expect a ducted system will be quieter than a mini split. I'd also like to be able to duct warm air to every space, which gets expensive with splits. But ducting is cheap. I don't really mind working under the floor. My loft is so full of junk that it's probably easier to work down there. Condensation drain is easier. Connection to outside unit is easier. Everything seems to point to a floor level installation, when the primary aim is heating.

Standard appears to be 200mm diameter ducting. So not Bruce sized but not small either. Maybe this is why ceiling mount is much more common.

No MVHR, this is for my 70s bungalow, not the cottage. I'm pretty tempted to fit a conventional (non ducted) A2A in the cottage, but probably after I've fitted PV.

Ah but that's the Costa del Moray, where they have this strage glowing orb in the sky.

The floorboards aren't anything like as gappy as the air bricks!! Seriously, somebody else must have thought of how to do this. Scratching my head a bit, I can hide box-outs for the bedrooms and bathroom, thanks the built in cupboards. In the kitchen, vents could go in the plinth under the units. The living room is trickier, no easy answer there... What generally accepted as the best place to draw air in to the system?

I'm leaning towards an underfloor ducted A2A system. The systems I'm looking at all seem to be designed for ceiling mount, which makes sense in air con mode. And it's dead easy to stick a vent in the ceiling. But for heating, it makes sense to pump out heat as low as possible. Being in the north of Scotland I don't expect to need cooling often, if ever. What's the standard practise for low level vents? I don't really want a grating on the floor itself. I'm hoping that it might be possible to fit something within the depth of the partition walls, but that's only 100mm and getting up through the wall plate will be a PITA. The alternative would involve some sort of box-out affair, which doesn't seem very elegant. I don't want to reinvent the wheel here...

I expect it's to do with a presumption against air conditioning. Planning is just as much about how a property is used and affects the surroundings, as it is about aesthetics.

I did not know that! In terms of aesthetics, there's not much difference in the external unit for A2A and A2W, is there? Both just a big white flat box with a fan in it.

Fair point about whether f-gas is needed on most installations. I'm probably being swayed by my own plans to install A2A. The more I've researched it, the more I like it. Seems a bit crazy that it's not more popular.

Sounds interesting! Did you go straight in to a classroom and start the courses, or did you get your hands dirty first?

Can't find it now but it would have been on the first page of a Google search. Courses cost around £900+VAT, 5d duration, and they recommend that you are working towards SVQ level 2 in refrigeration/AC engineering- which means you need to working in the industry already. However they also say that you can sign up without these qualifications, if you can persuade them that you have relevant experience. Could be a bit of a chicken and egg situation...

My crystal ball suggests that there will be a lot of f-gas cert engineers needed in the near future. I'm toying with a career change but I'd be starting from scratch (I don't have any trade under my belt). Is this a silly idea? I'm in my early 40s. I've found places offering the course and they're a bit ambiguous about entry qualifications- it sounds like you could blag it if you had some relevant experience.

No issue that I'm aware of, I've just more often heard about ducted units going on the loft space.

That makes sense. But most of the units I've seen on the market area designed for high level mounting. Any suggestions for a ducted system which could live under the floor. Is that a good idea? Would make condensate drainage nice and simple.

I don't think there's any reason not to, other than the extra cost.

I think the idea is that you have a gravel hardstanding and sit the Puraflo on that. But you'd really have to start talking to SEPA.