Uncap 7 inch furnace exhaust pipe in basement

stevelarson70 · February 21

Before I bought my home it was converted to natural gas. The oil burning furnace was removed. I want to uncap the pipe so I can use the chimney to exhaust air. There are two pipes capped in my basement, one on top of the other. The lower pipe is capped flush to the wall, and the upper pipe sticks out of the wall, and is capped also. I want to know which one is the exhaust. This is a townhouse with a shared wall and chimney. The neighbor still uses the chimney because he has an old fuel oil burning furnace. Thank you for any help.

Jamie Hall · February 21

top one was the old exhaust. However, you MUST get a qualified chimney sweep in there to inspect that chimney and make absolutely sure that the flu from that connection is not shared with any other connection.

EdTheHeaterMan · February 21

It is unlikely that you and your neighbor use (or used) the same flue in that shared chimney. There are probably two terracotta flue liners in that chimney. One for each of you.

The top elbow is where the oil burner exhaust connector was installed. The bottom one is a clean-out. Nothing was connected there. Both of those openings are connected to the same flue. That is the way it should be... But you should double check that there is no cross connection between the oil burner in the other home and the one you want to use as an exhaust vent. Is should be pretty easy to do. Just take the elbow out of the wall when the oil burner is operating next door and see if you get fumes inside your basement. That is the best test. Actually looking at the roof for too flues, and looking up the flue from inside with a mirror and a flashlight to see if everything is clear.

Have some furnace cement ready to put that elbow and cap back if you find that there is a cross connection. That way you can patch it up right away if there is a problem.

realliveplumber · February 21

What do you mean by "exhaust air"?

stevelarson70 · February 22

@realliveplumber I want to exhaust air from a cannabis grow.

Jamie Hall · February 22

Oh. Well, I won't comment on the crop. However, your humidity is going to be off the charts in that exhaust air. That is going to cause a lot of condensation in that poor chimney. At the very least I would want to add a Continuus stainless steel flue liner or that old chimney will have a short and miserable life.

stevelarson70 · February 22

@Jamie Hall thank you for the very good information. The chimney is from 1920 so I will figure something else out. Thank you to everyone who replied. My wife and I are in our 70s and the Internet isn’t so easy to use. Thank you everyone!

mattmia2 · February 22

I would be surprised if a chimney from the 20's has even a terra cotta liner.

EdTheHeaterMan · February 22

...And the neighbors be enjoying the exhaust gasses. Maybe even get some lawn chairs, sit on the roof by the chimney and munch on some Fritos corn chips. TOTALLY DUDE!

Waher · February 22

A stainless steel flue liner isn’t too expensive. A local chimney sweep should be able to install without too much difficulty.

A question to ask is if your basement will need make up air to balance the desired level of ventilation up that flue. You don’t want to back draft other appliances or a fireplace in your home that could create a carbon monoxide hazard or other unpleasant effects.

ethicalpaul · February 22

Just room air would hurt a chimney? It must cause dramatically less condensation than hot combustion gas, no?

ChrisJ · February 22

With all of the appliances that have been connected directly to a flue like that with nothing more than a drafthood (wide open to the room 24/7) for the past 70 years I can't imagine why this one would be a big problem.

Jamie Hall · February 22

ethicalpaul said:
Just room air would hurt a chimney? It must cause dramatically less condensation than hot combustion gas, no?

Room air wouldn't. But the OP mentioned something about a cannabis grow. That's not a couple of amaryllis in a pot... like any greenhouse, that humidity is going to be way up there.

ChrisJ · February 22

Jamie Hall said:

ethicalpaul said:
Just room air would hurt a chimney? It must cause dramatically less condensation than hot combustion gas, no?
Room air wouldn't. But the OP mentioned something about a cannabis grow. That's not a couple of amaryllis in a pot... like any greenhouse, that humidity is going to be way up there.

I thought about that.
But what temperature and RH is the flue gas from a natural gas furnace with a drafthood after mixing with all of the cool room air?

ethicalpaul · February 22

Yes, what Chris said. Surely even a grow room exhaust wouldn't have nearly the condensate nor acidity of any given gas appliance I would think.

My house was built in 1913 and had an unlined chimney with a coal boiler, then an oil boiler and now a gas boiler and it wasn't lined until I bought it in 2017. It had a long life that didn't seem that unhappy.

GGross · February 22

I'm not sure how it compares to flue gas condensation in quantity, but a rather intelligent engineer that I trust told me to treat cannabis grow operations like you would a pool room, they can be that wet. All depends on the owner though, I think there would be some small considerations that could be made to deal with the humidity, and just being aware of the potential is probably good enough for a small operation

Larry Weingarten · February 22

Hi @stevelarson70 , It seems that some sort of inexpensive plastic liner could be used as well, maybe some thin-walled large drainage pipe. For the benefit of the wood-framed building, keeping the basement under negative pressure and getting the humidity out would be a good thing.

Yours, Larry

mattmia2 · February 22

You could use expandable aluminum duct too.

Uncap 7 inch furnace exhaust pipe in basement

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