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I have an HVAC system (heat pump / compressor outside the house, air handler under the house). Recently something strange started happening. While system is in heating mode (my Nest thermostat says "heating" and is orange indicating it is running the heat), the blower motor in the air handler will start turning off and on at exact intervals: 10 seconds off, then 45 seconds on. I used the timer on my iphone to time it and each on/off period is exactly the same, over and over. Meanwhile the heat pump keeps chugging along even when the blower turns off for 10 seconds, and the thermostat continues to say "heating".

My blower has a capacitor that I've replaced several times before, but I'm doubtful that it is the problem since it seems like the on/off behavior is on a timer.

My air handler has a timer-delayed relay that I replaced about a year ago. I'm leaning towards this being the problem, as it definitely has some timer logic built in that could have gone haywire.

Could be the thermostat, but I'd need help diagnosing this as the problem.

Other notes: - I ran the blower after it had been off for hours, and it immediately started the on/off behavior, so I doubt that the problem is the motor overheating... there's no way it overheated that fast. - The motor is only about a year old

Any suggestions or ideas as to what I should do to diagnose the issue?

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    You're going to have to get down there, and start poking around. Check to see if the motor is hot (maybe tripping on overheat). Check to see if the motor is getting voltage when it's not running. – Tester101 Jan 11 '18 at 19:26
  • @Tester101 I don't think the motor is overheating - When I turned it on after hours of not running, it immediately started the on/off cycle behavior... there's no way it would have been overheated at that point. – Rafe Jan 11 '18 at 19:30
  • Only one way to find out. – Tester101 Jan 11 '18 at 19:38
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    Does the furnace manual tell you to oil the blower bearings regularly? – Wayfaring Stranger Jan 11 '18 at 19:59
  • @WayfaringStranger No manual for this bad boy... it's a pretty old air handler. – Rafe Jan 11 '18 at 22:16

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