The other morning, when I got up I bumped the thermostat on the heat pump up a couple of degrees like I do every morning. When I did, the lamp that was on in the living room seemed to dim a little for a second, but I didn't think anything of it. After about an hour, I noticed that the fan was still running and the blue "Auxiliary Heat" light had come on. The unit was putting out cool air, it was about 34 degrees outside at the time. Went outside, the outside unit was not running. Discovered that the breaker that powers the outside unit had tripped. Flipped it back on, went inside and turned the system back on, seemed to be working fine, putting out heat, for about 30 minutes. Then I noticed again the the blower was still running, blue light on again, putting out cool air; the ouside unit not running again but breaker not tripped.
I shut-off the power, opened the panel to the inside unit, and one of the legs on the middle stack of the sequencer had burned off. Had a similar problem in the summer of 2010 that caused the AC not to work, so I ordered a new sequencer, two actually so I would have a spare.
I replaced the sequencer and a couple of wires that were burned, and.....still doing the same thing. The compressor in the outer unit will not run, the fan on the outer unit runs sometimes.
I've checked the continuity on all of the wires coming off the sequencer, all good. But, when measuring voltages, I get a good 27v at the secondary of the transformer, but only about 16v to the coil of the sequencer. I've also noticed the the red "Emergency Heat" light on the thermostat will flash for a second when I activate the unit. I'm assuming the 24v signal to the sequencer would be coming from the thermostat. Any ideas why it would be low? I used to wire industrial control panels for a living, though it's been a few years, so I have a pretty good understanding of electricity and industrial controls, but not much experience with HVAC. Any help would be appreciated. Thanks.
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Update...was mis-reading the digital meter, what I thought was 16v at the coil of the sequencer must have actually been 16mv, so basically zero. Have good voltage at thermostat, and on activation it's sending voltage to correct contacts, but no voltage at sequencer or compressor contactor. On "Emergency Heat", red light comes on but still no voltage to sequencer. When I manually engage compressor contactor, fan runs, compressor doesn't. Checked dual run/fan capacitor, rated at 6uF and 36 uF, reading 5uF and 34uF, so I guess it's ok. If problem with compressor, why won't emergency heat work? Aaaaahhhh!!!! Driving me batty.