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I have a Goodman furnace, 92% efficient, GKS91155DXAF.

The exhaust vent flue was previously 2 inch PVC with one 45 degree elbow about 6 feet long overall, and it was routed directly into a piece of 6" round metal chimney vent that also provided exhaust for a gas water heater. The fumes from both went out the roof of my home. It was a terrible design, probably not to code and the moisture dripped out of the chimney vent through a drilled hole into a bucket instead of draining back through the furnace flue as designed. The combustion air intake was also 2 inch PVC about 4 feet long with a 90° elbow and a small screen to prevent blockage.

Based on instructions in the owners manual, I converted the exhaust flue to a 20' vertical/horizontal run of 2 inch PVC that exits through the side of my home. The PVC exhaust flu rises about 4' above the furnace and uses two 45 degree elbows to transition to a horizontal run of about 16 feet with correct slope backwards into the furnace to keep the moisture from collecting in the horizontal pipe.

After changing to this set up, the furnace would not light and the failure indicator was three flashes, which suggested a problem with pressure sensor air intake, exhaust, inducer blower motor, etc. It's a fairly new unit and all of the above parts and systems appear correct or functional plus the moisture is draining correctly thru/past the inducer blower.

It seemed as if I changed the pressure inside the unit so if I disconnect the exhaust vent from the top of the furnace, the furnace lights immediately. If I remove the 4' of combustion intake vent PVC from the top of the furnace, it lights as well. The owners manual suggest that 20 feet of 2 inch PVC with two 45° elbows is absolutely within the parameters of correct exhaust flue.

My thoughts are:

  • do I need a different pressure switch?
  • Do I need a higher flow inducer blower?
  • Is there a possibility the apparently correctly functioning inducer blower is simply not producing enough air for the pressure switch?
  • What other things should I consider in order to keep the new 20' 2" PVC exhaust system that I've installed and ensure the correct pressure in the unit at start up and when running?

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  • Where is your combustion air intake coming from? Is it the little elbow at the left corner of the furnace? – FreeMan Mar 16 at 15:13
  • Yes, originally it was a 4 foot vertical piece of 2 inch PVC with a 90° elbow and a small plastic screen, so the combustion air intake comes from the room in which the furnace is located, and the room is very large. However, when the furnace wouldn't light, I removed the PVC pipe and simply installed the 90° elbow. Then the unit will light. The same thing occurs if I disconnect the exhaust flue PVC; the unit will light. But if I connect the exhaust flue as you see in the picture and I reconnect 4 foot of PVC and... – Ray C Mar 17 at 1:26
  • the 90° elbow onto the combustion air intake, the unit won't light, and I get three flashes on the LED which suggests an issue with the intake or exhaust or induction blower, or pressure switch etc. – Ray C Mar 17 at 1:26
  • The moisture in the exhaust flue is draining correctly, there is no moisture in the inducer motor blower, the pressure switch appears to be working correctly (mouth suction test only) but I have not checked it with technical equipment, and everything inside the unit is clean and dry and it is a fairly new furnace, maybe two years old? The only thing I've done is change the PVC exhaust from joining the water heater pipe that you see in the photo to a normal 4' vertical/16' horizontal run with two 45 degree elbows. – Ray C Mar 17 at 1:35
  • Can an inducer blower be bad or so inefficient as not to produce the necessary vacuum even if it appears to be new and fully functional, no noises etc.? Also, can a pressure switch appear to be functioning normally But be slightly off in the pressure level triggers? If so I will change both of them. – Ray C Mar 18 at 16:55

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