"TP" wrote in message
.. .
Thanks, the installer retired. I have checked the manual and the
pipe dia and length are fine. I took a shop vacuum to both
pipes, just to be sure. Now the problem has seen to gone away.
The only thing that I noticed was there was not a screen on
the intake pipe. The manual doesn't specific any size of screen,
but shows a picture of one. Could I use standard window screen?
Or something heavier gage?

TP
If your PVC pipes (inlet and exhaust) do not have any kind of screening at
the outside ends you are looking for trouble. Those open pipes are quite
attractive to various animals who would just love to make a home inside one
of them. After one tried, even chewed through some plastic ribs across one
of them, I covered the opening with some chicken wire to keep them out.
MLD