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Matt Fellman
02-11-2014, 10:31 AM
Is this a problem or just a bad idea?

Wood stove insert and 90+ furnace vents about 6" apart at the top of a chimney. The pipe is only about 3 years old and is covered with soot but not melted or otherwise damaged.
John Kogel
02-11-2014, 10:41 AM
I can't see why the furnace vent should be taller than the chimney. If it was shorter, it wouldn't catch the soot.
Jim Robinson
02-11-2014, 11:53 AM
The intake side may be an issue. Rheem's instructions for a 90+ call for 3 feet of clearance to any other exhaust. Not sure if they consider the flue an exhaust or not, or if there is 3 feet present there.
Jimmy Roberts
02-11-2014, 08:11 PM
Is this a problem or just a bad idea?

Wood stove insert and 90+ furnace vents about 6" apart at the top of a chimney. The pipe is only about 3 years old and is covered with soot but not melted or otherwise damaged.


A 90+ furnace uses a forced air combustion chamber, like all other high efficiency units. The pipe in question appears to be exhaust, and therefore, soot contamination from the flu stack, into the furnace, is not a problem. Considering the presence of soot on the pipe, it is possible that there could be sufficient heat from the flu, to deform and possibly shrink that pipe. This pipe should be shortened and elbowed away from the flu. Also, with the exhaust positioned vertical, it is possible for sufficient rain water to enter the pipe, and corrode the heat exchanger.
Raymond Wand
02-12-2014, 05:02 AM
1. Why is the exhaust reduced at top? Is it okay as per manufactures instructions?
2. PVC Schedule 636 is rated for 65 deg. Celsius. (149 deg F.)
Jim Robinson
02-12-2014, 09:51 AM
A 90+ furnace uses a forced air combustion chamber, like all other high efficiency units. The pipe in question appears to be exhaust, and therefore, soot contamination from the flu stack, into the furnace, is not a problem. Considering the presence of soot on the pipe, it is possible that there could be sufficient heat from the flu, to deform and possibly shrink that pipe. This pipe should be shortened and elbowed away from the flu. Also, with the exhaust positioned vertical, it is possible for sufficient rain water to enter the pipe, and corrode the heat exchanger.


The installation instructions will call for the pipe to be vertical. The system is designed for moisture to come down the PVC exhaust and be handled by the condensate drain at the bottom.