backdraft

Hello, I installed a new Rheem 50 gallon natural gas water heater to replace my old Rheem 50 gallon. I noticed after it ran for probably a half-hour the plastic trim pieces around the water lines was melting. The vent is very simple just an elbow on about a 45 then straight out the block wall...

We have a new home and one bathroom fan in particular has horrible cold backdraft from outside. The damper with the fan is angled at the top and bottom. It's not stuck. I suspect it is leaking in air or worse blowing open from the other side if enough air is hitting the flap. That particular...