10/3/2 rule

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Hi.
My question is in regards to existing chimneys. Can you install a woodstove chimney within 10 feet of another chimney. I'm installing a 6 inch class A chimney within 10 feet of my existing (4 inch I think) furnace chimney. Is it allowed and if so won't they compete for the "2 feet above any structure within 10 feet" up and out to outer space?
Thanks.
 
If the Class A cap is within 10 feet of the furnace venting,
then it HAS to be two feet higher than that furnace vent
Simple as that.
 
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Never knew that. Is it new? I guess the concern is a down draft from the output of one pulling in the exhaust of the other one.

What about all those fireplace with two fireplace and maybe an oil burner. All separate chimneys, but all top off within inches of each other. We had that for ever. Are they mandated to be removed?