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?