The heat pump when first installed at the end of December would heat the house by its-self without much if any help from the furnace with an outside temp of 15 to 24 F . Now it has to have help from the furnace most of the time at a 31 F outside temp. When I noticed this I went outside to look at the coils and found what looks like 6 " to 8" of ice built up inside the unit coils and all. Could a low charge cause this ? 4 ton unit with a 5 ton coil , both from Lennox. Heated charge not getting to lower coils because of a low charge? When installed it was under charged. A few weeks after installation it had more refrigerant put in. Charge was never checked with "super heat/cool," the charge was only done by weighing refrigerant. Low charge or should the defrost setting be moved up from 50F. The rest of the coil is free of ice it is just the very bottom 6" that is solid ice. Lennox furnace model #SLP98UH110XV60C Lennox cased coil model # CX34-62C Lennox heat pump model # XP21-048-230 Lennox comfort wi-fi touch. Once the electric meter is installed I will use a low setpoint of about 33 - 35F.