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Heat pumps don't work below -10C, so this really depends on the local climate. You can use geothermal heat pumps, but now you're no longer near par.

Depends. The ones sold in Poland (and I guess Germany as the climate is similar) are often rated to work from -25C up.

Heat pumps are also loud.

Just as fossil fuel boilers are.

That's why you install them in the basement. If you can.

They really aren't. The outdoor units are comparable to air conditioners (60 db). I can't hear mine through the walls.