I have a Carrier 3 ton unit on a horizontal 3 ton field. Installed in June with desuperheater for DHW. Cooling season was as expected. Heating season stays mostly on the auxilary heat. The contractor diagnosed the system and indicated it was working fine even though the auxiliary heat was coming on when the temperature was below 50 F. This was corrected by removing the compressor lock out on the thermostat. Now it is colder outside and it is rare to hear the compressor working. Here are some of my amature observations: 1. The hot water is hotter than when the system is off because of mild weather. This tells me that the compressor must be working when I don't hear it. 2. The system is working hard to keep the house at 67 F with outside temperatures at 30 F, meaning it is on most of the time. 3. When I lock out the auxiliary heat through the thermostat, the house gets cold overnight. It will go into the 50s in 10 hours when it is 30 ouside. I can hear the compressor run occasionally. 4. When I turn the system off at the thermostat, wait, and turn it back on, the compressor with run for a minute and stop and revert to auxiliary heat. 5. This is an older house. The energy audit indicated the house was drafty with a coeficient about 3 times normal for a modern house this size (3500 sqft). Some of this is fixed, but not all of it. Is it possible that every thing is working and the loop is so cold the unit is protecting the compressor? If so, how can I confirm this? Great reading the forum, but I didn't see this, yet.