Humidity issue in HE furnace box – GreenBuildingAdvisor

I have a 2 year old 98% efficient direct vent High Efficiency Trane Furnace. The unit uses a 2 pipe (PVC) intake and exhaust vent up to my attic where it combines into a concentric vent through the roof. The vent has a rain cap and it is in place. It is installed in my basement in which I have a dehumidifier that keeps the humidity around 50% in the summer and shoulder seasons. The basement is not yet finished and tends to sit around 67 in the summer and 65 in the winter. I’m in zone 4A mixed humid. I just remodeled and hit 1.5 ACH50. (1950’s ranch)

I started to have a no heat/ no flame code and lockout a few times in the last 2 weeks. My HVAC guy (the installer) and I went to dig into it and found a non insignificant amount of surface rust on some sheet metal parts and fasteners in the cabinet enclosure. We pulled the burners a there was rust blocking the end ribbons so flame would not spread across the entire burner assembly leading to the ‘no flame’ code and lockout. He wire brushed it clear and we were able to fire it back up with no issues.

He and I are concerned with the surface rust. He says it is not normal and he has seen it on 2 other similar units (but not the same model). For example the black iron gas line looks new outside the cabinet but after it goes through the gasket into the side it shows significant surface rust. He thinks humidity is getting too high in the cabinet in the shoulder seasons when it is very humid outside but heat is still called overnight. We think the intake is pulling in the outside humidity then it is condensing on cold surfaces (hence the cold gas supply line shows the most rust). Important to note the cabinet is sealed from the basement so it is not humidity from the interior.

We are reaching out to the Trane rep.

Has anyone else seen this?

Any solutions?

For a bandaid solution, we are thinking of removing a gasket in an unused 3″ penetration in the side of the cabinet to get air communication with lower humidity air in the basement. (The dehu intake is about 24″ inches away.) The downside if this is it will then use some basement air for combustion air.