Max Demian wrote:
There was a heat pump system advertised a couple of years ago that took
the heat from "exhaust gasses". All right if you live in a laundry or
restaurant kitchen, but most people keep their windows closed in the
winter, with minimal ventilation.

Those are designed for super-insulated new-build small flats/houses, where
you have heat generating sources (appliances, people, bathrooms) but need to
change the air. Ordinarily people would crack a window, but these new
builds are highly sealed and so they have active ventilation. The
warm/moist/smelly air goes out, fresh air comes in. In a normal MVHR system
you just have a heat exchanger to transfer heat from stale to fresh air.
In these exhaust air heat pumps (EAHPs) they heat exchange, but also
use a heat pump to pull more external air and raise the temperature of the
fresh air beyond that of the heat exchanger. Because the place is
well-insulated, it doesn't need a lot of heat input to keep equilibrium.

The problems are that the heat pump only works to a certain output. Beyond
that there's only electrical resistance heating, which costs a lot. Some
systems silenty fall back to resistance mode without it being obvious.

When can happen is if the place is too leaky (eg the householder opens
windows or the ventilation ducts aren't properly installed), or the heat
pump is undersized, it falls back to resistance mode and the bills are
high.

https://en.wikipedia.org/wiki/Exhaust_air_heat_pump

Theo