Asked on Jan 19, 2015

Outdoor fan on my house heat pump A/C system works but won't turn on

by APG81
When I set the T-stat of my house 3 ton split heat pump (A/C) system for either cooling or heating, the compressor turns on but the outdoor fan doesn't. However if I go outside and manually activate the contactor by depressing the relay arm, both the compressor and fan turn on. This tells me the compressor, fan and run capacitor are probably ok. So the question is, how come the T-stat calling for cooling or heating only turns on the compressor and not the fan? My system has a defrost control board, however if that were bad, I would think the fan wouldn't turn on even when manually operating the contactor relay.
  3 answers
  • Shira Shira on Jan 20, 2015
    @Woodbridge Environmental do you have any advice for @APG81?

  • Heat pumps are strange devices, although simple in concept manufactures tend to install all sorts of fancy controls in order to boost efficiency of their system. Seeing that when you manually operate the fan and compressor relay and both operate, your correct in saying that the fan is ok, But when the inside control asks for heat or AC and only the compressor runs and not the fan is sounds as though the relay is at fault. There are a few things that prevent the fan from running, Being stuck in defrost mode, being shut down because of low suction pressures. I would suggest that although it appears that your not afraid to get into the system and get dirty, that you have a local pro diagnose the system for you. If you want, provide the manufacture of the system with model number and I can review the wiring diagram to see how exactly everything is controlled. Without that its simply a guess.

  • APG81 APG81 on Jan 20, 2015
    Thanks Woodbridge Env, turns out one of your guesses was correct, I eventually figured out that the defrost temp sensor was closed and this is what was preventing fan operation. Apparently this sensor needs to reach 80F before it opens again and I guess I prevented that from happening since I turned the system off when I noticed the fan wasn't working O_O Live and learn! Thanks.