[thsksbd]

If the unit is sized correctly for the appropriate climate, then its broken. There's no reason the system cant deal with 0C, the COP should still be well above 1.

Get the refrigerant level checked by someone that didn't install the system.

For what its worth this is why I don't like to rely on heat pumps. Furnaces are really primitive, simple devices: little more than a heat exchanger and a blower. A heat pump has a hermetically sealed volatile liquid undergoing phase changes by a compressor with seals.

They're cool. They're efficient. They're great.., but they're also far more complicated and we're living in an era of cheaply built consumer goods