Chicago Building Code - Ventilation - Exhaust/Supply

[pc1985]

We are designing for a retail store in Chicago.

Things I know so far: The code calls for 1 CFM/sq ft supply and exhaust. The exhaust is required to go to the outside atmosphere. Also required is that 1/3 of supply air be from outside, while the other 2/3 can be recirculated.

My question is wouldn't the space be under a constant negative pressure? Also it wouldn't seem feasible to heat/cool a significant amount of air just for exhaust makeup.

Anyone have thoughts or prior experience with this? Thanks!

 

[ChrisConley]

Heat recovery will help reduce the cost for make-up air. Ventilation is a requirement in all buildings.

The 1/3 - 2/3 seems odd, as the fresh air should be related to the exhaust, and the recirculated related to cooling load.

 

[pc1985]

My first thought was that "exhaust" in the code perhaps meant return air, but I spoke to the building department and they gave the 1/3 - 2/3 explanation.

I can't think of a reason to require exhaust to the outside in addition to ventilation air. It doesn't make sense to me from a ventilation standpoint or an energy conservation one.

I agree that heat recovery would help, but I'm not sure how practical it would be considering we have existing RTU's and ductwork and are looking to modify as little as possible.

We also took a look at the existing drawings and they were not providing any exhaust makeup, which makes me think the system would not work as it is.

 

[KiwiMace]

In a retail shop with a door to the outside, infiltration through the door would provide the makeup. All buildings are sufficiently leaky to provide for a small amount of exhaust.
However the 1/3 OA being a rule seems like a crock to me. I know Chicago has its own code, but what energy code would allow this as a blanket? Are you sure the city personnel were not providing a case example? If the required OA happened to be 1/3 SA then fine... I would just follow the code.

 

[pc1985]

Thanks for the replies.

Kiwi - The 1/3 OA is directly from the code. I was getting hung up on being told by these guys that the exhaust rate was 1 CFM/sq ft at all times.

For example, what they were saying is a 600 sq ft room would require 600 CFM of supply air. 1/3 of this, 200 CFM (min.) needs to be OA, and 2/3, 400 CFM could be recirculated. Then they also require 600 CFM of exhaust to go directly outdoors. --> My argument was that you then need to provide 400 CFM of Makeup, for essentially no reason other than to exhaust it.

After calling the BD again this morning, I got a strong sense that they did not understand the fundamentals of what they were requiring. I was told that our RTU could have a power exhaust, which would run all the time to provide the exhaust. The RTU would also bring in the 1/3 OA requirement, AND recirculate the remaining 2/3. I basically said that doesn't really make sense to me, and that was pretty much the end of the convo.

In any case I (hopefully) resolved the problem citing the code, apologies for the long-winded post

 

[chicopee]

Based on the description in your original post and using your 600 sq.ft. as an example, then,you would have the following:
600 CFM of fresh air supply to air handling unit
1200 CFM of reciculated air to air handling unit
1800 CFM of mixed air from air handling unit to room
600 CFM of exhaust air thru openings
With those numbers your proportions and suggested 1CFM/Sq.ft. are maintained.

 

[pc1985]

Agree with you chicopee, but in my opinion that would still be oversizing the system just to accommodate the exhaust requirement, essentially 133 SF/ton or 15 Air changes for a retail space.

I think the intent of the code was to not pressurize the building or, in the instance that a unit might not be running, that the space will still get some OA via infiltration due to the exhaust running.

Elsewhere in the code they state that the exhaust should be 90-100% of the OA supplied. I cited this part of the code and exhausted the portion of OA (aka the 1/3), to keep the space under neutral pressure. I didn't feel right designing the system the way they had suggested when I thought it was incorrect and couldn't get a reasonable explanation, other than basically "just do it this way"

 

[chicopee]

You may want to double check the specification of 1CFM/sq.ft. which may include reciculated air in addition to outside fresh air. That would change all my values by 1/3rd.

 

[DuctHunter]

I live in Chicago and have designed many buildings here. I'm not sure if this answers your question or not, but what the code says is that 1/3 of the CODE REQUIRED air be from outdoors. So for an office space which by code requires 0.6 cfm per sq. ft., 0.2 cfm/sq ft. must be OA. Some people confuse this to mean that 1/3 of the total supply air needs to be OA.

So if you are supplying 600 cfm to a space that may only require a min. of 300 cfm by code. The OA portion only needs to be 100 cfm.

Does this answer your question ?

 

[DuctHunter]

Unless the Ventilation tables specifically say that all air be exhausted to the outdoors, you can recirculate the remaining 2/3 of the supply air. I'm quite sure you don't need to exhaust the retail space. When the code says exhaust it typically means that you must remove air from the space, not necessarily to the outdoors.

Other tips for you: Be sure in your final design documents you include the "City of Chicago" Ventilation tables, and the "City of Chicago" standard design notes on the cover sheet, Pressurization tables, and heat charts. Your drawings will not even be looked at without this required information.

If you need help with any of these items please let me know.