Sciencemadness Discussion Board

Using a chimney as an exhaust route for fumehood

bogmonkey - 9-1-2010 at 09:33

I live in an old house and my bedroom has a little fireplace in it. Obviously the thing hasn't been used in years but I've been wanting to build a fume hood for a while now and recently the thought came to mind that since I already have a chimney in my room half the job has been done for me.

Basically the corner of my room where I want to install this fumehood is right beside the fireplace. There was a small grate above the fireplace which connects to the chimney I assume that was installed later (after the invention of electrical heaters) on for ventilation. I ripped the grate off and behind it is a little square opening (3.5 by 3 inches) and although the wall around fireplace seems to be drywall the area behind the grate is concrete. Since I already have this gap in the wall which leads into the chimney I'm thinking why not kill 2 birds with one stone and ventilate my room at the same time. Although I could connect the fumehood to this hole I would have a blatantly obvious ventilation duct leading to it. Instead I can make a hole in the wall where the fume hood is.

I don't have a camera at the moment sorry I can't take any pictures but I'll draw a diagram.

So what I want to do is have two inlets to the chimney, one will be ventilation for my room (that square hole in the wall) and the other will be from the fumehood which I will connect to the chimney through one of those aluminum ventilation ducts. The hard part obviously will be getting the air and gases all the way up the chimney so it can be expelled. If I was to install a decent exhaust fan on the top of the chimney that would definitely ventilate my room but I dunno if it would be good enough for the fume hood. Also I'd have to get up on the roof which will be fairly dodgy since its a sloped roof. If I was to install the fan a good bit of the way up (I can probably get at it in the attic) would that be enough to expel fumes out the chimney or is installing the fan at the top of the chimney the only way to go about it? Also a problem with having two inlets will be reduced suction force so I will make it so I can close one inlet when the other is in use. What I really wanna know is about the fans. I know nothing about HVAC so I don't even know what kind of fan I should get. On top of that I don't know where I should put the fan. I hear that its best to put the fan at the end of the airway where the air is being expelled. In my case thats the top of the chimney so I'll have to get up on the roof (its sloped) for that and I dunno how the hell I'll manage to get up there let alone carry all the stuff I need to install the fan up there.

Since the chimney outlet is so far away from the fume hood should I put in two fans 1 on top of the fumehood and one in the chimney?
Any advice?

BTW: I forgot to label what those two big lines on both sides of the fireplace mean. The wall sticks out by about 4 inches in that area. That will probably be useful for the fumehood because I can probably make a hole in that 4 inch side of the protrusion.

[Edited on 9-1-2010 by bogmonkey]

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[Edited on 9-1-2010 by bogmonkey]

Mr. Wizard - 9-1-2010 at 13:49

Maybe I'm not seeing the obvious here, but I always wondered why everybody wants to put a fan or electric motor in the path of the very corrosive, flammable, or toxic substance they don't want in the room with them and their equipment.

An easy solution would be to put a pipe up the chimney to act as an eductor, or jet pump. The theory is a fast moving jet of air, made by a nozzle on a PVC pipe, powered by a vacuum cleaner or leaf blower, will pull along or entrain a much larger volume of air. There are no moving parts to install in the chimney or roof. A fast and noisy blower may be overkill, but you could experiment with slower fans to see if you were moving enough air. A simple liquid manometer, using fluorescent green anti-freeze, with the inside end at a slight angle from the horizontal would give you an idea of the pressure differential obtained. Be sure to allow for air to come into the room for 'make up air', otherwise you will draw a lower pressure inside, and the flow will stop.

hissingnoise - 9-1-2010 at 14:11

I don't think the room you sleep in is the right one for chemisty. . .reagents *or* experiments.


Magpie - 9-1-2010 at 15:41

http://www.sciencemadness.org/talk/viewthread.php?tid=11145

anotheronebitesthedust - 10-1-2010 at 15:46

Could a fume hood be made out of a bathroom exhaust fan or dryer exhaust fan. It would be neat to see some schematics on that.

watson.fawkes - 10-1-2010 at 20:39

Quote: Originally posted by bogmonkey  
Since the chimney outlet is so far away from the fume hood should I put in two fans 1 on top of the fumehood and one in the chimney?
Any advice?
I can't recommend that anyone use a chimney as an exhaust flue, not without taking special care. The most important reason is that it's not typical for a chimney to be a sealed, single pipe with only two ends. Old chimneys that were used for functional heat often were a plenum with multiple openings, relying on natural draft to avoid smoke, which nevertheless was unavoidable at least some time. If you introduce hazardous fumes into such a plenum, it's pretty difficult to assure that the fume only leave by the desired exhaust opening. Many old houses had other openings below the fireplace for ash removal, either outside at ground level or in the basement. Two story chimneys were not uncommon. For example, the grate above the fireplace was likely used previously as the exhaust for a wood-burning, iron stove; I've seen such things in kitchen for exhaust of gas ovens. This isn't the only way sealing can fail. Old mortar frequently has air leaks; not necessarily hazardous for fire exhaust, but not so good for fumes from a chemical hood.

As for locating a fan on the top of the chimney, that's pretty much the opposite of inconspicuous. The top of the chimney, though, is the only place to put a fan that ensure negative (inward) pressure difference across all the openings to the plenum of the chimney cavity. Getting adequate mechanical stability for a fan on top of a masonry structure that's not designed for it is sketchy. Then you've got outside electrical wiring to deal with. And none of it's within the building code. Did I mention it was conspicuous?

I'll second the recommendation not to put a fume hood in a bedroom. Bad idea.

bogmonkey - 11-1-2010 at 08:20

Quote: Originally posted by Mr. Wizard  
Maybe I'm not seeing the obvious here, but I always wondered why everybody wants to put a fan or electric motor in the path of the very corrosive, flammable, or toxic substance they don't want in the room with them and their equipment.

An easy solution would be to put a pipe up the chimney to act as an eductor, or jet pump. The theory is a fast moving jet of air, made by a nozzle on a PVC pipe, powered by a vacuum cleaner or leaf blower, will pull along or entrain a much larger volume of air. There are no moving parts to install in the chimney or roof. A fast and noisy blower may be overkill, but you could experiment with slower fans to see if you were moving enough air. A simple liquid manometer, using fluorescent green anti-freeze, with the inside end at a slight angle from the horizontal would give you an idea of the pressure differential obtained. Be sure to allow for air to come into the room for 'make up air', otherwise you will draw a lower pressure inside, and the flow will stop.


Thanks a lot! Thats the approach I'm gonna take. Thanks for informing me about that manometer trick too I'm new to all this.
Quote: Originally posted by hissingnoise  
I don't think the room you sleep in is the right one for chemisty. . .reagents *or* experiments.

Don't have any alternative unfortunately.

[Edited on 11-1-2010 by bogmonkey]

bogmonkey - 11-1-2010 at 08:27

Quote: Originally posted by watson.fawkes  
Quote: Originally posted by bogmonkey  
Since the chimney outlet is so far away from the fume hood should I put in two fans 1 on top of the fumehood and one in the chimney?
Any advice?
I can't recommend that anyone use a chimney as an exhaust flue, not without taking special care....


Good point. The eductor approach Mr_Wizard was talking about would solve that problem though. In the area I'm in a fan at the top of the chimney wouldn't be very conspicuous. Visibly as least, people would definitely hear it though.

As for putting the fumehood in a bedroom I know its not the best idea but unfortunately I don't have a shed or garage to practice chemistry. I could setup a tarp and do it out the back garden I suppose that would defeat the problem of fumes saving me the hassle of building a fume hood. There are even electrical outlets out there. Besides the equipment and materials being exposed to the elements are there many inherent problems associated with setting up an outdoor lab?

[Edited on 11-1-2010 by bogmonkey]

watson.fawkes - 11-1-2010 at 14:21

Quote: Originally posted by bogmonkey  
The eductor approach Mr_Wizard was talking about would solve that problem though.
It might. I would still worry about alternative flow paths, though. Should you pursue this, do a manometer check at all the other orifices to check for adequate pressure drop there as well.

Putting aside the question of the wisdom of using a bedroom, is there a reason that it's infeasible to put a separate chimney up through the ceiling and roof?
Quote:
As for putting the fumehood in a bedroom I know its not the best idea but unfortunately I don't have a shed or garage to practice chemistry.
Given the total cost of practicing chemistry, the materials for building a shed roof are pretty modest. Just by thinking about a fume hood, you are considering construction. Perhaps another kind of construction might be more useful.