What is causing this?

I have no idea what is causing the rust around this fresh air pipe? Can anyone help me out with this one and the inlet on the outside actually have 90 degree elbow pointing down so it is not rain getting.

I assume its a High efficiency unit since it has PVC. That rust looks old , possibly a prior issue that has been resolved. Maybe old air intake pipe didnt have a goose neck or facing downward outside and allowed water infiltration prior.

Was it all rusted when inside when you removed the panel? Could it be from a previous leak above?

It was pretty clean inside behind the door and appeared to only be on the top. It was really weird because the other pipe had a big chunk out on the PVC pipe and actually created a lip for the water to pool on but that was not the one rusting. I had to look at it twice to confirm it was not that one that had the rust.

That’s the exhaust side not the intake side. The corrosion is from moisture condensing out of the exhaust gasses. They probably didn’t seal the PVC to the adapter and the condensate with corrosive combustion products is leaking at the connection. Most that I know of call for sealing the PVC to the adapter with RTV.

Based on the disconnected fitting on the intake side and no visible primer or glue, it doesn’t look like they glued the PVC connections. A similar separation on the exhaust side would be dangerous.

That makes sense thanks for the response as I am calling this unit out for other reasons as well but wanted to have an idea for myself why that was there and what might have caused it.

Thank you for your replies!

Looks like the exhaust. A 90% furnace makes a lot of water. There may be some leaking on the furnace there at the connection.