Single wall metal exhaust pipe should never be used to vent a water heater or furnace when the pipe passes through a combustible material of any kind due to risk of fire. The flashing rain collar will melt or warp if exposed to these temperatures passing through the single wall pipe, and loose the water tight seal it is designed for. All water heaters and furnaces not listed as high efficient should be vented using rated B vent pipe. B vent pipe is doubled wall pipe which isolates most of the exhaust heat to the interior pipe. All Oatey flashings are safe to use with B rated vent piping.
