Sorry for the long post. Lots of detail, though! I have a 25 year old (I know... External Serial No. 3094E24778 so true 1994 model) Carrier (External Model 38AY024300) Heat pump (I am not sure of the model numbers/serial numbers of the crawlspace portion, just yet). This is the downstairs unit in a 2400 square foot house in North Carolina. It is original to the house. I've replaced motor capacitors once or twice in this outdoor unit. I'm no stranger to electrical work and have had to deal with a few electrical hijinks in this house since we bought it in 2003, mostly related to kitchen and bathroom remodels. I offer this info as evidence that I have some skill in electrical work... When I left for work this morning, the heat pump was functioning normally. It was about 38 degrees this morning and drizzling rain, temp hovered around 40-45 degrees all day. The wife calls me later this morning indicating the downstairs heat is not working. To be more specific, no compressor noise, no outside fan noise, and no inside blower noise (I've trained her well on the details collection). Now, once or twice in winters past, usually during a rainy or icy period, this has happened, but self-corrects after a day or two, when I'm able to get into deeper troubleshooting; I've always noticed that there is no 24vac present at the thermostat during these failure events. In one instance last year I was able to apply 24vac myself at the appropriate contacts at the thermostat and "command" the heat on successfully. I get home today and started at the thermostat and discovered the same thing - no 24vac at the thermostat. I went to the outside unit, checked breakers which were not tripped and seemed to have snappy throw (I did once replace them due to lightning which caused one phase to not work). I switched them to Off and back to On, just to make sure. I switched them off again and pulled the side panel on the outdoor heat exchanger unit, switched the breakers back on, and was able to measure 240VAC at the contactor contacts, as well as 120 VAC from each one to ground, so I know the breakers must be functional. My initial thought way back when was that the 24vac control transformer might have failed, or maybe a control board. I bought a replacement transformer in preparation to have it when climbing under the house to start troubleshooting back then, but before I could get under there the system started working again. I have done some research on the low-voltage control circuitry of these systems in general, and have learned there are a number of safety cutouts, limit switches, etc for the 24vac control circuit. A last note or two. The wife reported that after noting the temp drop to about 60 in the house, she thought to try emergency heat, and it kicked on. It brought the house up to temp (we keep it at about 64) and turned off, and did not turn back on. When I came home the thermostat was commanding the heat pump on, but nothing was running, and no 24vac at the thermostat terminals. Second last note. In troubleshooting starting at the outside unit, I used one of those laser thermometers to see if I could find anything "warm" or "hot" - Everything measured 42 degrees or so (expected), except the evaporator/condenser fan motor which read about 51 degrees F. That tells me that there's probably a bad motor capacitor for that fan, and it got very hot much earlier in the day. Third last note: The thermostats are LUX PT2110, which are self-powered units (run off 2xAA batteries instead of 24vac control source). So, at this point, I'm thinking to ask some experts their thoughts or resources - a schematic diagram would be lovely, or some basic description of the circuit theory about how the various components of the 24vac system (outside, inside air handler, inside thermostat) interconnect along with cutout switches and whatnot, and that kind of thing. Otherwise, I guess I'll need to start drawing one myself and tracing out the circuit, one component at a time. I know it's an old unit, but it's been functioning well; and I hope to squeeze another couple of years out of it so I don't have to pick which kid can't go to college in a few years. Any help at all would be very appreciated.

We moved recently to a home in Arizona with a heat pump. It’s been cold here and the heat has been working fine, but this morning we noticed that for about 15 minutes the hot air was replaced with cold air. And the interior temp actually went down one degree. But now it’s blowing hot again and seems to be working. Any ideas on this one?