Quote:
Originally Posted by joasis
I am curious....so aside from the code violation, what is the actual, definitive reason for not allowing it? Is it reasoned that the flue pipe would allow the exhaust to "fall" back?
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You never want to mix natural draft appliances with forced draft appliances in a combined or common flue. The reason for this can be that the forced draft will cause an unnatural draft in the non-power vented appliance and cause combustion issues or even allow a flame out and reignition situation. It can also push its flue gases into the natural draft unit causing a back-drafting situation and a fire.
Some model codes allow this, but require automatic dampers to open and close and ignition lock-outs to be used to prevent simultaneous firing and also to prevent funky situations from arising with having a forced, high speed stream of flue gas being pushed into a cold stack of air (in the vertical part of the flue above the natural draft unit), and being forced into the building instead of being pushed outside.
Think of it this way... forced draft appliances pressurize their flues. When this happens, flue gases try to go anyplace they can to escape, and its not always up and out as intended. Don't mix natural draft appliances with forced draft appliances... ever, unless you're damn sure you understand exactly what happens at all times.
One other note is condensing appliances. They should never be put on a flue with a non-condensing unit unless there are ignition lockouts in place. I haven't looked at the latest model gas codes to see if this is addressed, but last time I looked several years ago, it wasn't.
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