Nigel, you got me thinking about a cold-weather add-on..
Assuming that I had a small geothermal 60F (15.5C) heat source..
I could dip into that geo-source during extremely cold weather.
(When there was very little heat in the air to be harvested).
Here's the first revision.
The Geo-source: Perhaps a few hundred gallons of solar boosted water in storage.?.
Or maybe a pair of small wells in the back yard?