Air-to-air ASHP replacing warm air heating

[Archer]

On 01/12/2024 at 15:43, gravelld said:

Really interesting thread. We're looking at a renovation project at the moment with a late 70s forced air heating system. First priority would be upgrading the building fabric (and likely removing a lot of asbestos).

 

But after that, it would be interesting to use the existing ducting.

 

If we achieve good levels of airtightness, would it be possible to use an exhaust air heat pump to also combine ventilation?

Good luck with it all however you decide to progress. Our house is also 70's and we ended up keeping the ground floor steel ducts and adding a second zone in with new ducting and vents from the ceiling in the first floor. 

 

Obviously with heat pumps the supply temperature is much lower than gas warm air so you need to run the system for longer essentially to achieve the same input of heat. Most of the time this is beneficial because you can get the same level of comfort at a lower set air temp when the system is running. Ours feels a tiny bit underpowered when it's really cold. It still gets up to temperature but it takes hours to do so. Only seem to notice this at freezing and below, I think because of the heat pump defrost cycles and generally greater heat loss ( our house isn't that well insulated) 

 

I'm still not entirely sure how we got it wrong. The A2A heat pump itself is oversized so should be fine. My suspicion is that the 2 indoor air handlers are too small because the upstairs duct work is custom made so unlikely to be the problem. Either way, it's a bit annoying but otherwise the retrofit has worked fine. Much better than the 70's original gas system we replaced.