I have a 3.5 ton XP14 Lennox heat pump that has not been behaving normally this heating season. I say this because I have been keeping track of its "heating" profile for the past 2 years. Basically I have a temperature sensor in the output plenum right after the coil which sends my PC temperature information during the heating cycle.

Last year at 32F outside the sensor showed 94F during heating, at 26F outside the sensor showed 92F at the coil.

This year at 32F outside the sensor reads 89.5F and at 26F the sensor shows 88F, at 40F outside the sensor sits at 91.8F.

No auxiliary heating was on during all these measurements.

This year the heat pump regularly goes into defrost, I noticed that sometimes it would do so even without frost visible. At 41F it was still going into defrost every so often. The XP14 has a demand defrost system.

I suspected a low charge may be causing these output temp variations, however since the pump behaved normally otherwise and gave me enough heating I hesitated to call for service.

Last night the temperature dropped to the lowest this year to 23F, and I found the pump this morning all frosted up and running continuosly, the sensor showed only 83F at the coil, when outside it was 29F. It was obvious that the demand defrost stopped working at some point during the night. I shut off the system for 10 minutes and on restart the pump immediately went into defrost, melted all the accumulated ice and starting working within the coil temperature profiles recorded this year ( which are on average 4F lower than last year's)

My question: are these symptoms indicative of a low charge?

I'll be calling for service next week and I'd like get a forecast of what could be wrong with the pump.

Thanks in advance