Gooman

Just on that point ... we had assumed the same as well, but after a very thorough asbestos survey (which I would recommend you get) we found there was very little that needed to be done. The only asbestos in the heating system was a brown asbestos panel on the back of the furnace door (which was dangerous, and was removed for specialist disposal), and asbestos cement around the flue. The recommendation on that (as with most white asbestos) was to leave it in situ, with a suitable warning tag in the area not to disturb it. If your property is, like ours, a chalet-style roof (see pic) one of the most awkward parts of upgrading the building fabric was in properly insulating all the eaves spaces. Some of them were accessible as eaves cupboards, but some were sealed up. Existing insulation was 10mm fibreboard between rafters and on internal walls and a little (very little) loft insulation between floor joists.

If anyone is interested, please use the Contact section on their website to let them know ... it might galvanize a response!

PS - If I was going to try and persuade them that the UK is a worthwhile market, does anyone have any stats on how many domestic warm air heating systems are in use in the UK?

Just saw this really interesting development in my news feed - it's an ASHP (air-to-air I think) with an indoor unit in a tower format, specifically designed to replace a forced-air furnace: https://www.electricair.io/ Notably, it also cools and dehumidifies and includes a HEPA filter for air purification. Not available until next winter - and likely in the US only, at least at first. But promising nevertheless!

Apparently, J&S are rumoured to be working on an air-to-water ASHP unit + heat exchanger that would fit on the same ducting footprint as our current J&S unit. As it's air-to-water it would be eligible for the government grant (air-to-air isn't). When it might see the light of day is anyone's guess though!

@severnside - really interesting results! In the pic below your thermostat of the ducting in the garage - is that the new ducted unit? What of the ducting shown there is original? Our current J&S unit is in a brick-built cupboard with ducting above (for 1st floor) and below (underfloor for ground floor). I doubt the internal unit could be fitted in the ceiling of that cupboard and connect to the existing 1st floor duct - there's just not enough room to work in. Do you know if the unit you have can be mounted flat on a wall, or does it have to be in the orientation shown?

@Jeff V The other option is to go with something temporary until the Heat Pump/Duct tech is better proved ... second-hand J&S units do crop up on eBay from time-to-time, and might be an option if you have a local J&S-certified engineer who can install it.

I've heard from other users and from AC companies themselves that there could be some gotchas here - but no-one has any hard data on real-world examples! There's been various musings about difference in cross-sectional area of ducting in a typical warm-air system being much, much less than in a typical AC installation and the effect this might have. It would be great to get some results from an actual installation - both subjective and specific (airflow rates and temperature curves). If that's not a problem, the other potential issue is if you run the AC in cooling mode - the original ducting wouldn't have been designed for that and might have problems with condensation. That's going to be much harder to identify on a test - other than maybe putting a humidity probe and/or camera down the ducting to see what happens.

It's true that current heat pumps don't produce a high enough temperature airflow. But that is likely to change - there are some higher temperature models that are available now and more due to hit UK shores sometime soon (it's just a shame your unit has decided to go t*ts up at this point!): https://www.homebuilding.co.uk/advice/high-temperature-heat-pumps https://www.theheatinghub.co.uk/articles/high-temperature-heat-pumps It should be possible to link these with a Johnson & Starley Aquair - but you might want to check with J&S yourself. I have had informal conversations with them that suggest this should be possible, but we didn't get into specifics.

The door to our boiler cupboard had a brown asbestos panel on the back. Broadly speaking asbestos comes in three categories - white/grey (which is rarely a problem if you don't cut, drill or break it), brown (same as white/grey but a little more hazardous) and blue (very hazardous). We had the whole door removed and safely disposed of. We also had soffits in white asbestos, white asbestos in Marley floor tiles (who'd have thought?) and white asbestos in the cement around the boiler flue in the loft. All left in place - again, if you don't mess with it, it's not a problem. Was yours just a general survey or a specialist asbestos survey? I'd thoroughly recommend a specialist survey with a lab test on samples - that's what we had and it gave us great peace of mind. They'll put any old crap in a general survey just to cover themselves from you suing them later - quite often the stuff they come up with is total bs. If you didn't have a specialist asbestos survey with lab tests there's no guarantee that the ducts do actually contain asbestos. Can't think why anyone would put it on the inside of the ducts - it would impede airflow as it's not smooth. If it was used anywhere it would be on the outside - but ours had neither. If it is present, you don't need to actually remove the ductwork. As above, asbestos is generally safe if you don't disturb it. Just decommission the warm air heating and leave the ducts. The best way to negotiate is to show them examples of what else you could get in the same area for the same price. If they see that and have had trouble selling they might realise that they're overpriced.

I can believe that. All of our heating vents are at floor level, so the warmth starts at floor level and rises (rather than hitting the ceiling and then falling as it cools).

Given that this house is 50% larger, and looking at our bills (before the recent price increases) I'm certain that the warm air system is no more expensive to run than a wet system. Might even be slightly cheaper. But you're right, it's a lot more comfortable.

The boiler in the old house was a combi, only heating what was used, so wouldn't use as much energy as a whole tank. The immersion heater in the new house by contrast is heating the whole 140l hot water tank.

One other topic I'd be interested in any comments on ... in our previous home with a traditional wet system, our combi boiler was rated at 35kW output. With our current - 50% larger - home and a warm air system, the boiler is rated at 14.5kW. OK, it doesn't heat water, but that's using a 3kW immersion heater. Why is the energy output requirement so much lower? Is it because it's heating the air directly rather than heating water to heat the air? That's still a huge difference. ?

Yes indeed, but that's not the way that a traditional gas boiler+radiators wet system behaves. This is what you get with a traditional on/off thermostat: Opentherm improves things somewhat, removing the overshoot: (both graphs from https://theevohomeshop.co.uk/content/21-What-is-OpenTherm-and-why-use-Honeywell-evohome-with-it) But you'll notice in both cases that the temperature increase is linear. By contrast the rising curve on the warm air system changes gradient as it gets close to the target temperature.

FYI, a classic temperature curve with warm-air heating ... you can clearly see how the system throttles back the heat demand as it warms up and even spot the cycling of the temperature to within 0.1 of a degree once it reaches temperature.

Yep. There really doesn't seem to be anyone seriously addressing the case of using existing forced-air ducting for air-to-air ASHP systems. I would guess that part of the issue is that air-to-air doesn't qualify for the RHI grant, whereas air-to-water does. So all the installers are focusing on air-to-water. As other posters have noted, air-to-air ASHP is common in other countries. I found this comprehensive Canadian guide and workbook to sizing air-to-air ASHP systems - which includes the case of using existing ducting: https://www.nrcan.gc.ca/sites/nrcan/files/canmetenergy/pdf/ASHP Sizing and Selection Guide (EN).pdf Unhelpfully, the duct sizing guidance suggests "measuring the cross-sectional area of the supply trunk(s) near the equipment, before any branch take-offs". In my case, that's underneath the (huge) Johnson & Starley boiler. So that's not anything I'm about to do myself. I might ask our heating engineer about the sizing when he comes to do the service.

@robin colesThat's interesting. It's at odds with what a local commercial and residential HVAC and ASHP specialist has told me. He said that sending the lower temperature of ASHP air (around 40 deg rather than the 80 degC output from our current boiler) down our current ducts just wouldn't work. He added that air-to-air ASHP systems are usually specified with much larger ductwork. As the ductwork from the boiler and all our ground floor ducts are buried in concrete, it's not easy to replace. I guess that's also why Johnson & Starley themselves have their Aquair heat exchanger, as the fan is specified appropriately to shift enough air. There are some important caveats on the Mittens Heat Pumps site you linked to [with my notes in square brackets]: Obsolete ductwork techniques can influence efficiency levels [I'm guessing this means mainly small ducts and right-angle corners - both of which we have] They should be used in combination with good insulation and sealing [the lack of under-floor insulation is my biggest concern here] They are also perfect for smaller apartments and homes, or buildings where cooling/heating is needed only in certain spaces [although we're a 3-bed, we have a large ground floor area and so large rooms to heat] I guess really it would be dependent on getting a survey to look at the specifics of each individual property. It's too early for us at the moment. We've got too many other things in our renovation project that need to take priority at the moment. I did a new Google search based on some of the terms on the Mittens site and found these guys who claim to have nationwide coverage: https://www.heritageheatingandcooling.co.uk/blog/the-benefits-of-ducted-air-systems/ - you might want to try them?

External air is only used for combustion, not for circulation ... it's primarily recirculating internal air. So it just has to raise it from the current internal temperature to the desired temperature.

According to the boiler manual, at peak the fan should be pushing out 0.27 m3 a second

Me neither. Although we have the heating turning off when we leave the house, it feels comfortable within less than 5 minutes of arriving home. My home automation temperature monitoring suggests that it's currently raising the lounge temperature from 17deg to 18deg in 15 mins first thing in the morning.

Backdraft shutter on the kitchen hood. No fan in the bathroom yet ... we just open the window. Bathroom reno is still on the to-do list!

No, not noticed any sound transmission issues. But then there is only the two of us. An air-to-air ASHP heat exchanger (ie the indoor unit) is the option that I'm told is a no-go as the existing ductwork is too small for the lower air temperature. The ground floor ducts are around 100mm square, whereas first floor is 100mm x 250mm. J&S advised that air-to-water ASHP on their Aquair unit (which is designed for the existing ductwork) is unlikely to provide enough heat. So the only way that ASHP becomes viable is if the technology improved to provide warmer output. The only other electric alternative is an electric boiler with the J&S Aquair (like this: https://www.electric-heatingcompany.co.uk/electric-boilers/warmair-heating-replacement/) - but that's likely to result in much higher bills on the current electricity vs gas tariffs. That may change if the government decides to incentivise electricity over gas by removing the levies on electricity and applying them to gas - which may happen under the current green initiatives - see https://news.sky.com/story/boris-johnson-looking-at-new-green-levies-on-gas-as-prices-reach-new-high-leaked-document-reveals-12423200 for details on possible changes to gas vs electricity pricing.

The Johnson & Starley warm air boilers modulate the heat output - they don't work on a traditional "on/off" thermostat, but rather one that provides a resistance that varies proportional to the room temperature. The boiler will start up in the morning on full, then start to back off the output temperature, then keep the fan running longer, and then just end up "blipping" every couple of minutes at low output to maintain the temperature. So it probably us in fully condensing mode by that time. I hadn't heard of skirt or wing insulation ... might have been worth doing before we had our patio renewed! No chance now though. No heat recovery. There's no backdraft shutter on the combustion air - you couldn't do that as it would affect the ability to pull enough air through (as confirmed by our heating engineer). But that's not an issue now the combustion air goes directly into the boiler cupboard. The heating mainly recirculates air - there's air-return vents in the lounge, hallway and landing.

Yes ... but that would only supply one room at a time and is quite ugly/industrial looking. We didn't go with the garage conversion in the end but extended the kitchen instead. We just used a 2kW thermostatically-controlled wall-mounted electric heater as a top-up for the extra space. It's only on for a couple of hours in the morning and evening, and the thermostat keeps the on-time low.