Two chimneys next to each other

[kcbenson]

I'm planning to replace a factory-built chimney. Right now there are three chimneys that all exit a chase on the roof at the same height (see attached photo--yes, the chase needs some work too). I keep reading that this can cause problems, and that adjacent chimneys should be at least 3 ft different height. Is this true?

Ken Benson

 

[begreen]

Yes, under the right conditions smoke can be sucked via downdraft into an adjacent flue. I would at least make the center one taller.

 

[webbie]

A difference in height of about one foot should do the job fairly well.

 

[kcbenson]

At the moment, only one chimney is active (the left one). I'm going to remove the right one, replace the middle one, and resize the chase. So when I'm done there will be only two chimneys, both serving boilers in the basement (oil burner and wood boiler).

What conditions cause downdraft? I'm not getting any downdraft now. The middle chimney is abandoned and terminates with a huge open pipe right next to the headboard in the master bedroom.

Ken Benson

 

[webbie]

The usual condition is the "stack effect" within a house.
Picture it this way - air inside your house generally is warm and rises up. This causes a slight vacuum (negative pressure) on lower floors or close the floor. When a chimney is not in operation, there is not reason for air to flow up it - so instead the chimney pulls outside air into the home to satisfy the pull of that negative pressure.

 

[kcbenson]

Probably then it would make sense to plan the less used chimney (the oil burner) to be shorter than the more used chimney (the wood boiler). And maybe I'll keep a few sections from chimney #3 (which I'm removing) in storage just in case I change my mind and want to add some height to chimney #1. The existing chimneys are from a manufacturer (Thor, in Syracuse NY) that no longer seems to exist.

Right?

Ken Benson

 

[webbie]

that sounds right - thor is long gone.......
and it would be best for the wood chimney to be higher.

 

[Fsappo]

I agree, make the wood chimney higher. I'm not sure if the NFPA211 mentions it specifically, but I always went by the one foot rule.