Crofter

I could do, but I'm on the other side of the world right now and trying to get ballpark figures.

I'm not completely opposed to nuclear, but it's had a long time to prove itself and frankly it hasn't managed to do that, in the UK at least. It seems that other countries (e.g. France, India) have been able to make it work. There's a lot of hype about new types of reactor and small modular reactors, but I remain pretty sceptical. Solar and wind have become almost ridiculously cheap, so it makes sense to maximise that first.

Ok thanks for the reminder about Jeremy's spreadsheet. Air tightness clearly has an enormous effect. Unfortunately I have very little idea of just how bad it is in this house. It's a 1970s l bungalow, twin block walls with cavities filled. New 2G/3G windows and doors throughout. Floor is currently pretty draughty (T&G pine boards, suspended). I'm going to improve that as much as I can (probably insulation with a breather membrane stapled underneath). Loft has several layers of glasswool everywhere, must be about 300mm. Any ideas for a number to pull out of the air for ACH for a house like this?

Democratic Republic of Congo

As longer term users of the forum will be aware, I'm very much inclined to do things myself. I designed and built a small house from the ground up with absolutely minimal outside help. Partly it's because I've struggled to get trades in the past, and partly it's because I'm a cheapskate. Anyway, next project is ASHP for my 70s bungalow. Almost certainly going to go A2A. I've been leaning towards a ducted system but may go with multi split. Obviously a heat loss calculation is required. We just had a new EPC done and I was a bit surprised how superficial it was. If I get a pro to do my HLC, how accurate is it really going to be? Can I attempt this myself? So far I've just run numbers through a 'radsizer' online tool from a radiator supplier, and checked the Heatgeeks' rules of thumb, both of which return a value of around 5.5kw. There are plenty of assumptions in there. In practice I'm going to be installing a 5, 6, or 7kw system, so does it really make all that much difference trying to nail this down perfectly?

Exactly the same argument applies to EVs- even if further entirely by gas power stations, they're more efficient than ICE.

I definitely wouldn't do a double 90⁰ bend. From memory you're only allowed something like 6" total horizontal run, and anyway it would become impossible to sweep the flue after that. Are you absolutely sure there's no rear flue option? Your stove looks like a Charnwood. Mine came with the option of top or rear, but you won't see it without removing the lining bricks inside. (If you do find a rear flue exit, I can sell you my genuine Charnwood rear flue adapter!) 😁

I found that when my stove was pushed back in to the alcove, the chimney breast itself would get very hot... and leach most of that heat out in to the loft. A lot probably went down in to the foundations too. The chimney breast itself was boarded out with battens and plasterboard- except for under the floor and in the loft of course. It was far more efficient once we pulled the stove out in to the room.

I'm not sure about the structure of that brick work so won't comment on how to safely remove it. But I will say, getting the stove out on to the open will make a huge difference. We fl initially fitted our stove in to the recess created by removing the fire bricks in the fireplace. Used a clay adapter to connect to the chimney. Ran out like that for many years. When we renovated, I pulled the stove out, fitted a proper stainless liner, and boarded over the opening to leave a totally flat wall. Extended the hearth out in to the room and the stove is now free standing. It made a huge difference. The room heats up much quicker.

Do you want responsive occasional heating, or 'always on' heating?

I heard that the non f-gas units had worse performance?

I've just learned about the existence of intelligent duct controls. I hadn't really considered this before. I had kind of assumed that I would just set the system up with adjustable vents and/or baffles to deliver the required amount of heat to each room and just leave it alone after that. But maybe that's a bit naive.

This is one reason why I'm hoping to use a ducted system. It just seems to make sense to have big ducts emitting through grilles at floor level. I'm looking at a maximum airspeed of less than 4mph which hopefully will be barely noticeable.

Most people seem to be singing their praises, it's interesting to hear a dissenting voice. Do you remember what specific unit it was, and was it running at full speed most of the time?

Was this the conventional split unit with a wall mounted emitter? I wonder if I can find out any information about flow rate/speed to compare with my ducted plan.

Good question, I'd imagine it would be similar to the other option which is running it in the cold loft.

Good summary, thanks for taking the time to share it. Like you, I'm watching the Octopus situation closely. It would be a game changer if the trial became an option for everybody.

One of the big advantages I see with A2A is the faster response time. For a house like ours, which is either used by a working family and empty through the day, or at other times used as a holiday let, the current storage heaters are about the worst possible type of heating it could have. I don't think UFH would be the best match either. Anyway, I've got a suspended timber floor and there are limits to the amount of work I'm willing to do.

I've been crunching some numbers. I don't think I need a 10kw unit. Heatgeek's rules of thumb suggest around 5.5kw, and an onlinev radiator sizing tool returns a range of 4.5-6.5kw, depending on how exactly I answer it. So I think a 7.1kw unit ought to be ample? At full chat this is delivering 780m³/hr, or 216l/s. Which seems proportionally far less airflow than the bigger unit, and much easier to handle. This flow rate tallies with the numbers from engineering toolbox. Splitting it through four 200mm diameter ducts results in a max flow speed of 1.7m/s (3.8mph). I think I messed up earlier when calculating cross sectional area of the ducting, so despite the much lower flow rate the 7.1kw still works out to have about the same airspeed.

An another thing... It seems like a good idea to blow the warm air in at floor level. But where's the best place to put the return vents? I'm wondering if a ceiling vent, as far as possible from the inlet, is ideal. Or, given the air flow involved, would that turn the room in to a wind tunnel?

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.