It definitly could leak, but so can most of the pipe joints and sectional elbows. If your chimney has a steady draft, then the pipe will be under a slight vacuum, and pull air from the room into the pipe.
The only time that the pipe may leak would be if it had positive pressure. Such as when the combustion blower first comes on and the chimney isn't pulling that strong yet, or if you had really gusty/windy weather and you had some flow reversals in the chimney.
Just to be safe I'd seal it, you can use good hi-Temp RTV gasket sealant or furnace cement. But don't neglect the other joints or the fitting going into the chimney if it is masonry. All can leak some under pressure.
You did use three or more screws in each slip joint for security right??
Last edited by LsFarm
on Tue Oct 02, 2007 6:50 pm, edited 1 time in total.