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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.
They are just AC units in reverse. The biggest effect humidity is going to have is on how much condensation is going to form on the exterior radiator. That'll form frost that'll have to be melted in a defrosting cycle. That'll decrease performance and efficacy. Low humidity should keep that to a minimum.