I'm in Raleigh, NC and it's unusually cold (23 degrees right now). My house is 4 months old, very well insulated (energy star cert). The upstairs is about 1700 sq ft. I have a lennox XP-16 - 3 ton unit. Plain old lennox air handler. Right now t-stat set at 72, the return air temp is 72, and the supply is only 79. Should I be seeing more than a 7 degree increase? Seems low to me. Also, last night it got to about 15 outside and even with the 5kw resistance heat, the temp dropped to 68 inside. Is 15F cold enough that the HP and aux heat shouldn't have been able to keep up?