According to Pete C. :

I'd revise that to "a geothermal heat pump with a correctly sized ground
loop" since in stable 55 or so degree soil a properly sized loop will
always be able to extract sufficient heat.

We went through the process of costing out a geothermal. They
usually size it such that there will be a couple of days per year
where the geothermal _alone_ won't quite cut it, and put
supplementary heat in for those days. Also, it keeps you from
freezing up if something goes wrong with the HP.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.