During the heating season in cold weather, frost forms starting at the thinner (liquid line?) service valve of the heat pump and continues into the unit. Why is there no frost forming behind the valve on the tubing leading into the house? The unit defrosts normally. Thanks for any insights!

Frosting of a heat pump is normal in winter. However premature frosting around the liquid line valve and coil can be a sign of low refrigerant charge or restriction at the metering device.