Approximately air makeup

[Mr. Inspector]

2018 IMC 508.1 requires commercial kitchen makeup air shall be approximately equal to the amount of exhaust air. What does it mean to be "approximately"? How close should it be?
Looking at a kitchen hood that will exhaust 3000 CFU but air make up is 2600.
Is a 14% difference as in this hood is considered 'approximately"?

 

[steveray]

They usually want the kitchen to be slightly negative to keep the smoke. smells in there....

 

[Sifu]

I typically see up to 20%. But then I see positive pressurization in adjacent spaces like dining rooms that tend to make up the deficit. In that way, there is additional makeup air provided, it's just not being supplied by the MAU supplying the kitchen. I reviewed one yesterday with two type I hoods and a type II hood (dishwash) in different kitchens, serving a large adjacent dining area. One type I hood exhausted 23% more air than the MUA, the other 15%, and the type II was all exhaust with no dedicated MAU, but the excess transfer air from the dining area made up all but 133cfm. That resulted in a fraction off of perfectly balanced. At least I think...these things are difficult to understand for me, and I end up running the numbers until they make enough sense to make me a believer.

 

[Mr. Inspector]

I typically see up to 20%. But then I see positive pressurization in adjacent spaces like dining rooms that tend to make up the deficit. In that way, there is additional makeup air provided, it's just not being supplied by the MAU supplying the kitchen. I reviewed one yesterday with two type I hoods and a type II hood (dishwash) in different kitchens, serving a large adjacent dining area. One type I hood exhausted 23% more air than the MUA, the other 15%, and the type II was all exhaust with no dedicated MAU, but the excess transfer air from the dining area made up all but 133cfm. That resulted in a fraction off of perfectly balanced. At least I think...these things are difficult to understand for me, and I end up running the numbers until they make enough sense to make me a believer.

So how do I do that if all I got was the kitchen exhaust system plans for this existing almost 100 year old restaurant. Do they need an architect to make complete plans of the building including the HVAC system to prove the makeup air is adequate?

 

[Sifu]

Request an air balance schedule. It should tell the whole story. Ask the question..."where is your makeup air coming from?" and explain the need for a balanced system. You may get a blank stare. The makeup air could be provided by a transfer grill directly from the outside (depending on where your are that can be complicated as it must be within 10° of the ambient air or the HVAC system must compensate for it). More likely there is a makeup air system already, and maybe it has the capacity to account for an increase. Code sources use terms like "approximately" and "slight". .02"w/c. is often used to represent "slight". The terms are subjective, and I personally have no idea what .02"w/c looks like, so I just do the best I can and ask them to show me a "slight" negative pressure or "approximately" balanced system.

 

[Sifu]

2018 IMC

508.1.2 Air balance. Design plans for a facility with a
commercial kitchen ventilation system shall include a
schedule or diagram indicating the design outdoor air balance.
The design outdoor air balance shall indicate all
exhaust and replacement air for the facility, plus the net
exfiltration if applicable. The total replacement air airflow
rate shall equal the total exhaust airflow rate plus the net
exfiltration.