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exhaust inlet requirement for garages 1

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EnergyProfessional

Mechanical
Jan 20, 2010
1,279
typically I see designs that require garage exhaust to be taken not higher than 18" above floor. I believe the reasoning is CO is lighter and with such low intake of the exhaust air we capture CO better. I wanted to verify the exact requirement, but can't find a code that details the 18"? I only found it referenced for fuel dispensing facilities in IMC 502.1.2, which is not my case. Many people tell me about the 18", but no one can show me the code.

some questions I have are:
- do diesel garages have the same requirement (they produce more soot and NOx than CO)
- assuming the 18" requirement is real, how are angled intakes treated? To increase the screened area, we typically have 45° sloped intakes. teh back (at wall being lower, the front being higher. now which do we measure to satisfy the 18"requirement?

 
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How come Co is lighter (I guess you mean lighter than air) and we capture it better with lower intake.?
 
The reason for low exhaust pick-up location in garages is to collect fumes from gasoline spills before they get to some source of ignition.
 
OK, I think we agree low intake is good. question: how low exactly and is there an official code or something that describes it?
 
we didn't agree yet.
you are talking about CO not about C or O itself, and CO does not dissolve to C and O in the garage, also the garage has one or two exhaust outlets not an outlet near every engine.
also when Co spread in the garage it take all the garage volume as Dalton law, in a garage we are talking about concentration not volume, some codes ask to install CO or No sensor at certain heights from 9 feet and above, not at 18"
are you sure about 18" or it is 180"?
 
Carbon Monoxide (CO) is practically neutrally buoyant in air.
Molecular weights:
CO = (1) Carbon @ 12 + (1) Oxygen @ 16 = 28
N2 = (2) Nitrogen @ 14 = 28
Mixed air = 28.966 This means CO will basically mix evenly throughout the space.

I've always understood that the low level exhaust was because tailpipes are low. Codes I've had to follow before required 50% low level and 50% high level.
 
317069: you'r right.... but I think sensor location depends more on the manufacturer instructions. I often see CO at 4'height and Nox sensors at the ceiling.
I think the intake height is more related to typical vehicle exhaust location and to take care of possible oil/fule spills?

Compositepro: thanks for the hint, i didn't know they are free. i got mine from the fire department. anyway, 88A (Garage) and 88B (Repair Garage) don't seem to mention anything. I know NEC requires 12"in Div-1 locations, but this is a garage only. any idea which ones would explain/require the intake height?

My design is to exhaust from low above floor and supply makeup air up high to flush out evenly. but some design I see and people i talked to (and no, no one could tell me what code exactly!) indicate it has to be taken from floor level and from above. so I'm still looking for some code reference.

I'm in WI, which references and amends IMC. not all NFPA apply, only the ones referenced in IFC.
 
So I talked to Buldinginspection and they state IMC 502.14 used to have an 18" provision, butt that seems to have been removed in newer versions. The ICC website doesn't seem to show the old codes in detail, so I can't tell when it was removed. Oldest version I have around is 2009 our JHA version)

I also re-read NFPA 88A and 6.3 only seems to refer to cfm/ft² required (per our fire department NFPA 88A is not required, we follow IFC).
IFC doesn't seem to have specific vehicle garage requirements.

am I making this just too complicated if I don't have specific liquids or gases? Is it really (per code) just up t my judgment? I guess that is fine, just didn't want to walk into a trap....
 
Exhaust is to exhaust air from the garage not to remove oil or fuel spillage.
Vehicle exhaust usually connected to a pipe the end up outside without fan or any thing, this pipe is not considered as an exhaust system of the garage
 
Just a note to say that lukaiENG is correct. CO mixes rather thoroughly with air. I used to be a product manager for CO sensors, and we proved this experimentally. In fact, it will initially rise due to elevated temperature coming out of a vehicle exhaust, but it cools rapidly and disperses throughout the space.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies
 
317069: I know it is for exhaust. however, I like to improve IAQ with a given exhaust rate as much as possible. Most code requirements that prescribe an intake height, are concerned with VOC, liquids etc.

DRWeig: good data, thanks. Do you have any such data on NOx or diesel soot? Assuming it is warm as well, it probably will do the same thing.
 
NOx will sink to the ground quickly after it cools. Look for low spaces to sense it, unless you can get a fast-acting sensor up high.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies
 
I typically see CO sensors on the wall and NOx sensors on the ceiling in center. I was told this is based on manufacturer recommendation.

It probably has more to do with the fact, that installing sensors low and int he center of a garage in a clear span construction is a challenge. May be better if you have multiple columns, but in this cas there are none.

Either way, all seems to indicate taking all the dirt in at floor level is the best strategy. Especially with the makeup air unit discharging in high height. Taking air from high would mean all the clean OA just clears out the area in high height, where humans are not.

Thanks for all the comments.
 
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