Rheem AP13945-2 - Manual (Page 10)

Venting Through Closed Spaces
If the vent piping passes through a closed space, a minimum
clearance of 8” (20 cm) when installed horizontally, 6” (15 cm)
when installed vertically should be maintained between the vent
pipe and combustibles and noncombustibles. Be sure to follow
local codes and vent manufacturers installation instructions.
For maintenance and inspection purposes, the following access
panels are required.
●
Two (2) inspection access panels large enough to allow
access for venting inspection. One (1) of these access
panels should be close to where the vent pipe enters the
ceiling. The other access panel should be near the vent
termination.
●
A ventilation access panel with a 16 in 2 (103 cm 2 ) opening
should be provided every 10 ft (3 m).