Kirk said:
Greetings from snowy New York City:
I am particularly interested in understanding chimney physics and
:question:
(1) why fireplaces need separate flues, and
The problem with shared flues is that there will inevitably be differences in how the appliances connected will draft. It is possible, especially if you are having a marginal draft situation, for the smoke from one appliance to "backdraft" and exit through the air intake of the other appliance. Since smoke often contains Carbon Monoxide, this can easily be a deadly situation. Since there is no real way to tell under what circumstances this will happen, code these days almost totally prohibits shared flues*. Safety aside, a shared flue setup will usually mean that one of the appliances will get shortchanged in the draft area and perform poorly as a result. (Like lots of other things in physics, nature always takes the shortest path...) There may be some older structures with shared flues that some folks will argue are "grandfathered", however those really are still not safe.
* The exception on shared flues up until fairly recently was that a furnace and hot water heater could share if they were located next to each other on the same floor, and both used the same fuel - either gas or oil. I'm not sure if that is still allowed or not. It is becoming somewhat of a moot point in any case now that we are seeing more and more high-efficiency furnaces and h/w heaters that vent out the wall and don't USE a flue...
(2) why chimneys from a lower building need to be raised above the roof of a taller, adjacent building.
This was addressed in an earlier post, and is normally addressed by the 10-3-2 rule (a chimney must exit at least 3' above the high side of the roof where it exits and 2' above anything within 10' of it. The basic idea is to ensure that the chimney exits far enough above the structure so that the smoke will be carried away, and you will have good draft. When you have a taller nearby building (or a hill, cliff, big trees, etc.) this can disrupt the airflow around your chimney and cause it to not draft well. The usual cure is to increase the height of the chimney, though it usually isn't needed to exceed the height of the obstruction.
We own a 5 story Brownstone next to a 12 story apartment building built later without extending our chimneys--and now complaining that our woodburning fireplace smoke gets in their windows...
That can be nasty, you may want to talk to a GOOD lawyer (I know - contradiction in terms, but... %-P )
Did you own the Brownstone before the other building went up?
Have you been actively burning wood the entire time?
Did you object to the permitting of the new building on grounds of messing up your draft?
What sort of local rules apply?
IANAL, but my understanding is that if you owned the building first, and have been burning wood all along, you are a "pre-existing condition" and the new building's owners need to either compensate you for fixing it, or deal. OTOH, If you just started burning, then you are disturbing their peaceful use of their property by dumping your smoke on them, and can be made to stop burning or fix the problem some other way (if possible)
This is based on some of the "objectionable neigboor" cases I've heard about over the years. The base rule that I've always heard is that the first person doing the activity rules - If I have a pig farm and you build a subdivision next to it, you can't complain. But I can't start a NEW pig farm next to your existing subdivision, or even change from raising vegetables to raising pigs on my existing farm....
However given that you are in NY, a city notorious for denying basic human rights to people, anything can go...
Good luck,
Gooserider
(edited to fix quoting)
