Time To Replace Some Heat Pumps-Or At Least One

Ddaddy said

If they are icing up, they are likely low on Freon. It’s usually the first sign the system is failing...and probably leaking.

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The outside coils will ice up whenever the outside temp is low enough and the humidity outside is high enough. Happens with a perfectly running system.... which is why they have reversing valves to thaw themselves out. If very dry air is outside, they can run down to the 10F, but again, get pretty inefficient under those situations.

And some sort of backup to cover the situations when they just won't run efficiently (or at all with really cold, dry conditions). Electric resistance heat is a common backup, but gas is a good one for colder climates. As with DDdaddy's system, our home system uses LP gas as a backup, and switches to that below 35F. The bonus is when it goes to the gas backup, we get nice warm air, much more comfy.