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Heat pumps can't take the cold? Nordics debunk the myth
(techxplore.com)
This is a most excellent place for technology news and articles.
If that's the case, you simply installed a heat pump with too little capacity...
Heatpumps come in all sizes... I just looked up one that outputs 50 kW worth of heat, and if that isn't enough you can integrate up to 16 of them to output a total of 800 kW of heat.
That being said, if your house is badly insulated and drafty, you should fix that first, it will immediately cut your heating bill, no matter which heat source you use.