Yeah, Norwegian cities are good examples of places with sub-10º annual mean temperatures that can still use heat pumps very effectively. Because their normal lows aren't all that low (Bergen's January average low is above 0º), heat pumps have absolutely no problem working efficiently.
The problem with heat pumps (particularly mini splits) in North America is that they're overpriced and HVAC companies way overcharge for installing them, making them less viable. I installed 2 at my house with an HVAC tech friend's help and with him getting me the units at cost. Had I paid the company he works for it would have been a $13-15K project; it ended up costing me about $6K, with me paying him $100/hr for his 4 hours work to install them. So basically the company would have pocketed about $7-9K on a half-day installation just with their markups on the units and labor. And it's not like they're holding inventory; he just drove to the supplier, bought them for half what his company would charge customers, and brought them to my house.
There's a reason why you see mini splits all over the side of apartment buildings in lower income countries, and that's because for most of the world they're cheap heating/cooling sources. In a lot of countries, you can just buy mini splits off the shelf from Walmart or Costco (or the equivalent) for under $1K CAD.
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