[raymie1]

I have a fairly new house 2years old , the intake -exhaust from the furnace runs through the attic , through a 3" pvc concentric , everything works fine , however there is a "tapping"noice that comes from the concetric as the pipe heats up , its fairly loud because it echoes in the exhaust pipe , there is spray-foam insulation around the concentric as well as the attic rafters , any ideas ? Thanks

[hvac1000]

None by me. That is something that needs to be tracked down at your end.

[tsa7man]

The noise may very well be the PVC pipe expanding in length, as it heats up and slides in and out (as it cools) across the foam insulation. Spray in foam should not be right up to the PVC pipe.