Bill Zoller, a US Inspect Home Inspector out of Loveland, Colorado, shares this picture and explains what could happen. If you really do not know what you are doing, hire someone who does, or ask someone who knows...
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Answer: Dangerous Configuration
As is often the case, this installer knew just enough to be dangerous. Very dangerous.
What you are looking at is two smaller bath exhaust fan vent ducts configured into the larger furnace exhaust flue. Of course, the bath exhaust fans are designed to pull conditioned (moist) interior air and vent to the exterior, hopefully. The larger pipe is intended to vent furnace gases full of carbon monoxide, absolutely.
But what happens when a negative atmospheric pressure is created, such as wind buffeting the roof exit cap, or interior ceiling fans pulling air in a reverse fashion back into the home via the bath exhaust vents? At this point, those same poisonous gases you thought you were venting to the atmosphere are suddenly back in the home. Not a safe situation. There should be no potential for carbon monoxide gases to return to the interior of the home once they leave the heating system.
The fix? The bath exhaust fans need to be totally separate from the HVAC exhaust system. Vent those baths exhaust fans separately and directly to the roof exterior.
(Posted at U.S. Inspect blog)
Chris Smith CSSBB
Chay Realty Inc., Brokerage