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Im setting up shop in a 22x20 garage. I just had the underside of the roof sprayed with closed cell foam, and the walls filled with cellulose. It has several glass block windows but no windows that open. It has one pass through door, and a 16ft door that is relatively well sealed. I plan on gluing an inch of foam board to the big door to insulate it better. The garage is pretty tight and well sealed, the only real leakage being around the 16ft door. I bought a hanging natural gas forced air heater from Menards, a Beacon Morris brand. It has the option of using outside air for combustion. Im not really sure if the extra effort and expense of running the combustion air from outside would be of any benefit.

Ventilation is going to be a problem in there, with the doors closed just a few minutes of welding is creating a cloud that just hangs there forever. Im thinking that using the combustion air inside the garage would at least help move more air through the garage. I know I am going to need a better solution to my welding fumes problem and am looking into older threads for more options.

What do you guys think......and why?
 

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Check you local code, I think you need some form of outside combustion air, and probably a vent system for the welding fumes. I'm in Arizona so you might use a different mechanical code but i'll try to find my code book and tell you what it says.
 

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You are definitely going to want to use outside air. The biggest reason for this is that if you have any type of combustable fumes, ie. gas, brake cleaner, starting fluid, etc..., then those fumes will be sucked through the furnace. Most likely, these fumes will never create any type of an explosion hazard, but they will burn, creating CO gas. Enough of that in a space that doesnt vent well, and you know how that ends.
 

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Did the furnace come with any formulas to calculate the size of outside air vents? Many do. When I put the gas log in my cabin, it had a fairly detailed series of formulas to determine the size of air vents. IIRC, there may also have been some guidance on positioning of the vents to best scrub the crud out of the room and to the fire/discharge.

If it's as tight as you say it is, you'll need to have a sufficiently sized vent to replenish the air that you're burning to keep the fire lit and you alive.

Don't forget that your welding activities consume available oxygen as well and replace it with at least some less than healthy by-products.
 

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this has very little to do with the furnace but i'm gonna ask ya anyway.. you do have soffit vents and or some sort of ridgevent or gable vent correct? a building like anything else needs to breath. if they sprayed the foam over the opening for the ridgevent then you've sealed the building off. this is bad for a few reasons but one being CO. with any welding, burning, etc you need ventilation for hazardous gases.. CO and others.. like fumes put off my welding stainless for example. judging by your glass block windows i'm assumeing the building is CMU construction? a CMU building that doesn't breath is a a mold factory. not to mention in the summer that place will be a sweatbox with no way for the hot air near the ceiling to vent out.


personally i'd do the furnace setup with outside air combustion.
 

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In reference to the two issues you mentioned, I recommend addressing each of them separately. That will be more energy efficient.

The furnace needs outside air for combustion, one way or another. Either its ducted directly to the outside, or you should have a permanent opening into the space. If the unit has the option of being ducted, that is a superior option. Sometimes its called direct venting or sealed combustion.

A permanent opening allows for infiltration of cold air when its not required. With the ducted option, that is not an issue. Also, if you put in an exhaust fan for welding and other fumes, it will have no effect on the burners performance if the combustion air is ducted. In an unducted scenario, the exhaust fan would not only pull air through whatever wall opening you have, but may also pull down the furnace venting creating a dangerous backdrafting situation.

For the second item, put in an exhaust fan, either on a manual switch or a timer.
 

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You might also want to consider that a heater without a vent supplying fresh air will take its air supply from the heated space, mix it with fuel, burn it, and exhaust it through the roof vent. This situation will create a (albeit small) negative air pressure that will draw outside air in through leaks in the structure (i.e. garage door). This air will need to be heated up to room temperature, requiring the heater to do more work than it would otherwise.

I would suggest doing the work to supply outside air to the furnace and then use a vent as needed for the welding.
 
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