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We have a Carrier 58STA/STX Furnace that is only short cycling in Heat and not A/C. When it cuts off it is throwing Code 33 which is pointing to the following issues:

  • Dirty Filter or Restricted Duct System - Replaced Filters & Using a MERV 2, still trips
  • Loose Blower Wheel - Removed today and was not loose/excessive play
  • Defective Switch or Connections
  • Defective Blower Motor or Capacitor - Capacitor was at 50% of nominal, replaced and still occurs, have not done anything with blower motor.
  • Inadequate Combustion Air Supply (Flame Rollout Switch Open) - Flame Rollout Switch stays closed
  • Restricted Vent Sizing - Spec'd to House
  • Excessive Wind - Happens on Windy or Calm Days

If I either remove the filter or remove the blower access cover it does not throw the code.

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    Thank you for informing us of your situation. Is there a question you want to attach to your post. – Alaska Man Jun 29 '20 at 21:46
  • Have you had the static pressure of your duct system measured? Has it always done this, or was there some event that prompted it to start doing this? – ThreePhaseEel Jun 30 '20 at 0:34
  • Was your system designed with a merv2 filter ? Your blower gains may be full of dust it is really wild that that slight curve can get a thick enough layer of dust / dirt to drop your flow / pressure. – Ed Beal Jul 1 '20 at 21:01
  • We purchased the home in September of last year so this year was the first Winter with it. I don't recall it happening immediately but I also may have not noticed. I had used high MERV 4 - 7 and started dropping it as I noticed more issues. – Pat Jul 9 '20 at 19:21
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I was able to figure out it was the Draft Safeguard that was tripping. After digging around online some more there were a couple recommendations:

Right when we purchased the home we redid the flooring immediately from linoleum that had a threshold at the closet to engineered hardwood and we did not put a threshold at the closet (effectively increasing the gap at the bottom). I both verified the intake was clear and also placed towels at the door gap and let the furnace run in which it finally did not short cycle.

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  • Glad you were able to lick the issue! – ThreePhaseEel Jul 15 '20 at 15:52

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