# Negative air pressure in commercial kitchen



## Sifu (Apr 21, 2020)

I have a commercial kitchen with what seems like an unusually large amount of negative air pressure.  The RTU is providing 6000cfm, and returning 4300cfm for a positive pressure of 1700cfm.  The two exhaust fans pull 3900cfm out.  This leaves a -2200cfm air pressure being replaced by the dining spaces.  This seems excessive, is there a standard?  I usually look for a slight negative pressure and have seen recommendations for 15% differential from balanced.
Code says the facility must be balanced if it contains a commercial kitchen.  In this case the facility has a total positive pressure of 750cfm (about 5% positive pressure)


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## classicT (Apr 21, 2020)

I'd say that it is permissibly balanced.


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## steveray (Apr 21, 2020)

We usually get closer than that, but not sure what a "best practices" would really be...


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## cda (Apr 21, 2020)

Require balance test results??


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## TheCommish (Apr 21, 2020)

2015 IMC


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## e hilton (Apr 22, 2020)

Sifu ... having a problem following your numbers.  In the first para you have a negative 2200 cfm, but in the second para you have positive 750 ... ?


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## north star (Apr 22, 2020)

*$ ~ $ ~ $*

Those who regularly and competently design these air
circulating systems [ should ] have the owner's budget
in mind and what the intended type of exhausts are to
be removed.

Heavily grease \ oil laden particulates will require a
higher amount of cfm's to exhaust........That said, the
designer should be experienced enough to balance the
required amount of cfm's, with the "right sized" equipment,
with the owner's budget, and not just design in equipment
with higher cfm capabilities  [ i.e. - a balance  ].

*$ ~ $ ~ $*


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## Sifu (Apr 22, 2020)

The kitchen has a negative 2200 cfm, that is 3900cfm being exhausted by the fans, 6000 brought in by the RTU with 4300 returned.  That leaves 1700cfm of makeup air for the fans to use-equaling -2200.  (All makeup air is provided by the RTU's in this facility)  The kitchen is relying on the dining room RTU's to provide the rest of the makeup air.  

The entire building has a plus 750 cfm, as the dining RTU's do make up the difference and then some.  The issue is that the code says the "facility" must be balanced.  In this case, the kitchen is unbalanced, pulling more than 50% of the required makeup air from the dining area, and the building is unbalanced, oversupplying the building by 750cfm.  I am not too concerned with the oversupply of the building as it is a pretty small percentage based on the totals (4%), but I am having trouble reconciling the more than 50% differential in the kitchen.  The kitchen is separated from the dining areas except for two, two-way swing doors, which it seems will be standing open all the time with the negative pressure.


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## north star (Apr 22, 2020)

*@ ~ @ ~ @*

Compose your comments, along with the submitted numericals,
and send to the RDP, ...the owner or both.........Request a
clarification, as you believe the system is unbalanced for the
Kitchen Area......See what develops.

Please let us know how this is resolved, or other developments.

*@ ~ @ ~ @*


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## Sifu (Apr 22, 2020)

north star said:


> *@ ~ @ ~ @*
> 
> Compose your comments, along with the submitted numericals,
> and send to the RDP, ...the owner or both.........Request a
> ...



We think alike.  I already asked him to clarify the imbalance and reconcile it with the code section.  I am sure he will show me the error of my ways if I am out of line.  I think this was a value engineering judgement, which is fine, but I want to make sure they meet the minimum requirements.


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## Paul Sweet (Apr 22, 2020)

A pass-thru or transfer grille(s) will get the surplus air from the dining room into the kitchen.  Commercial kitchens often have double-swing doors without latches which the waiters are pushing open every minute or so to bring food out.  If there's too much negative pressure it will pull them open enough to relieve it.

The problem is when (not if) they fire up the hood exhaust to start preparing food before customers arrive and they turn the dining room A/C on.  Once I saw air being sucked in thru the water heater flue when this happened!  The RTU fans need to be interlocked with the hood exhaust to keep this from happening.


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## classicT (Apr 22, 2020)

Paul Sweet said:


> A pass-thru or transfer grille(s) will get the surplus air from the dining room into the kitchen.  Commercial kitchens often have double-swing doors without latches which the waiters are pushing open every minute or so to bring food out.  If there's too much negative pressure it will pull them open enough to relieve it.
> 
> The problem is when (not if) they fire up the hood exhaust to start preparing food before customers arrive and they turn the dining room A/C on.  Once I saw air being sucked in thru the water heater flue when this happened!  The RTU fans need to be interlocked with the hood exhaust to keep this from happening.


Paul has got it. I do not see an issue with the building being slightly pressurized (750cfm per OP). I do agree with Paul that the exhaust and supply must be interlocked. This will be atypical, because most supply systems are on-demand type. The exhaust hood will run nearly continuous.


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## Sifu (Apr 23, 2020)

OK, I had to take my PJ's off and go into the office to get it, but I I got my Commercial Kitchen Hood Application Guide book and found this to validate my concern.

"The amount of makeup air is usually _slightly less_ than the air exhausted.  This difference in air volume maintains a _slightly negative pressure in the kitchen,_" & "The difference between the exhaust and makeup air is usually supplied by transfer air from other adjoining spaces, such as the dining area".  

It even includes a nice little drawing showing a 10% difference.  

All the RTU's are interlcoked to the exhaust fans so they have that covered.


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## klarenbeek (Apr 23, 2020)

IMC 508.1 states "The amount of makeup air supplied to the *building * from all sources shall be approximately equal to the amount of exhaust air for all systems in the building."  As long as all RTUs that provide makeup air are interlocked with the hoods, this system meets minimum code requirements. There are so many different scenarios for commercial kitchens that as long as the makeup air has a permanently open path to get to the hood I consider makeup air requirements met. In places like large grocery stores or schools where fresh air required by chapter 4 is provided through the air handlers well in excess of what the hoods are exhausting, a dedicated makeup air just for the kitchen is overkill. 
Unless it's actually causing problems, there's no reason the makeup air can't come from the dining area through a permanent fixed opening.  Your killing two birds with one stone because the dining area requires a certain amount of outside air per chapter 4 anyway.  The same air can be used for both purposes. 403.2.2 actually specifically allows it.


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## Sifu (Apr 23, 2020)

I understand what you are saying, its just not consistent with the commentary and other documents that all say "slight" negative pressure or some such wording.  Would you say that none of the replacement air needs to be introduced directly into the kitchen?  Or how much iis enough?  Should I know what the hood MFR recommends?  Maybe a comment for them to verify the recommended negative pressure not exceed MFR specifications?

This from the Commercial Kitchen Hood Application Guide:
_"Most hood manufacturers recommend that the negative pressure not exceed .02 inch of water column"  _


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## klarenbeek (Apr 23, 2020)

But if you have a permanently open pass through window or open walkway from the dining room to the kitchen, the makeup air will still be introduced to the kitchen based on the positive pressure from the dining area. It shouldn't matter that it comes through a hole in the wall instead of a diffuser in the ceiling.


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## north star (Apr 23, 2020)

*@ ~ @ ~ @*

One difference in the amount of cfm's being supplied
to the Kitchen is the cost of the equipment producing
those cfm's.........If the cfm's could be lowered to
produce a slight negative pressure, and still be compliant,
that would help the owner's "bottom line" in the
operations of the restaurant.......If those cfm's could
be pulled through openings from the dining area, so be
it, but costs should [  IMO  ] be a primary concern;
as well, having a safe, compliant and functional exhaust
system.

Why throw money away on excessive, un-needed cfm's ?

*& = & = &*


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## Sifu (Apr 24, 2020)

What I see is that there is no "code" that I can find that dictates a specific volume of make-up air from a specific source.  I do see dozens of contributory recommendations that say "slightly negative" pressure in the "commercial cooking areas" are allowed and or recommended.  These sources are the IMC commentaries, the application guides, and various papers written on the subject.  In the end I can't really see the negative impact of having 60% of your make-up air coming from the dining room as long as all the sources add up to the required volume except that the swinging doors may not want to close, which is their problem I guess.  I may leave the comment just to see if the designing engineer can enlighten me since I have other comments anyway.  I am going to try to find a copy of ASHRAE 154 to see if it has any additional guidance.  (if anyone has a copy or knows of anything in it that addresses this that they can share I would appreciate it)

I do have one additional question:
The hoods provide the required exhaust for cooking, but when no cooking is being done they are not on.  Should there be a completely separate, dedicated exhaust fan to provide the required .7cfm exhaust from IMC t403.3.1.1?


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## klarenbeek (Apr 24, 2020)

I typically don't require the general exhaust in kitchens with a hood exhausting to the outside. The hoods are usually in operation when the kitchen is occupied, especially when typical air contaminants that the general space exhaust is meant to take care of are being produced.

I do have a hard time accepting full height swinging doors in the only opening from the dining area to the kitchen though. it is a restriction to airflow, plus to easy to prop something against it either accidentally or on purpose, particularly when the kitchen staff is prepping before the restaurant opens to the public. Saloon style swinging doors I have allowed.


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## jj1289 (May 6, 2020)

Another way to look at it is egress.  Unbalanced systems can affect the egress doors.  Had a small restaurant constructed and they contractor choose not to install the correct make-up air fan resulting in an unbalanced system.  Had them fire up the kitchen equipment as if it was being used.  They could not open the egress door without a lot of force.  After they got the door open, it slammed shut.


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## klarenbeek (May 7, 2020)

Insufficient or no makeup air can cause a lot more problems than that.  The hood system will pull from wherever it can get it, including out of the sewer system right through the traps. We had one restaurant built several years back where decades ago there was an old landfill. The makeup air unit was 3 phase and didn't get wired right so the blower spun backwards not supplying any makeup air. When they first started the hoods, the exhaust pulled methane right out of the ground. Fortunately it was discovered very quickly .


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## Bryant (Oct 14, 2020)

I am not an expert by any stretch and it is difficult at best to apply all of the systems within systems concerning the totality of mechanical systems without a basic practical engineering point of view IMHO. That being said, I think the mechanical code (chapter 4 & 5) really is about checking of minimum  numbers as they apply to ventilation/exhaust air or perhaps better stated, ventilation & exhaust systems functionality, nothing to do with human comfort. 
Human comfort in a kitchen really gets blown away by all the exchange rates that intermingle with the kitchen exhaust system(s) IMO.
From a conservation approach (IECC), the intent IMO, is to abandon the energy sucking circuit venting of commercial hoods which cause all kinds of imbalances to the positive or negative pressure differences by having equipment running 24/7 without direct control intervention. Remember positive or negative pressure is atmospheric pressure. In the case of hoods being slightly negative, the idea is to keep the air in the room, hence not smelling your steak cooking a 100 feet away from the kitchen, is an improper design. The opposite, positive pressure, think hospital room keeping unwanted air out.
Coming back to the logic of providing the make up air to the kitchen hood from other spaces (RTU's. HVAC, MAU's, etc.) is not a bad idea, slightly negative at the hood(s). The energy savings are enormous versa all of that human comfort air being sucked out by the exhaust with excessive negative pressure. 
But it is always a challenge capturing the exact CFM's required (the total rate of ventilation air) when unknowns exist without confirmed compliance with the air barrier requirements for exfiltration/infiltration accounted for, hence the requirements to meet the leakage rates.
The reason for opining, is from a project I am reviewing with under floor supply plenums. OMAU's, RTU's & kitchen hoods and not accounting for types of exhaust fans. i.e. bathroom, IT closets & etc..
Speaking off numbers game. Does anyone  know of a site that provides some free training on mechanical systems, that kind of explains the madness. The code and commentary only go so far in knowledge....
Thanks
Bryant


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## cda (Oct 14, 2020)

Bryant said:


> I am not an expert by any stretch and it is difficult at best to apply all of the systems within systems concerning the totality of mechanical systems without a basic practical engineering point of view IMHO. That being said, I think the mechanical code (chapter 4 & 5) really is about checking of minimum  numbers as they apply to ventilation/exhaust air or perhaps better stated, ventilation & exhaust systems functionality, nothing to do with human comfort.
> Human comfort in a kitchen really gets blown away by all the exchange rates that intermingle with the kitchen exhaust system(s) IMO.
> From a conservation approach (IECC), the intent IMO, is to abandon the energy sucking circuit venting of commercial hoods which cause all kinds of imbalances to the positive or negative pressure differences by having equipment running 24/7 without direct control intervention. Remember positive or negative pressure is atmospheric pressure. In the case of hoods being slightly negative, the idea is to keep the air in the room, hence not smelling your steak cooking a 100 feet away from the kitchen, is an improper design. The opposite, positive pressure, think hospital room keeping unwanted air out.
> Coming back to the logic of providing the make up air to the kitchen hood from other spaces (RTU's. HVAC, MAU's, etc.) is not a bad idea, slightly negative at the hood(s). The energy savings are enormous versa all of that human comfort air being sucked out by the exhaust with excessive negative pressure.
> ...





you might look thru this site and see if you find a class, book or help







						Hazards in a Hood and the Importance of Inspections  |   Phil Ackland
					






					www.philackland.com


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## Sifu (Oct 14, 2020)

Check with Captiveaire.  While they may have a proprietary agenda, the do offer classes, usually in association with local code chapters or jurisdictions.  And if you can get past the self-promotion, they are very good and informative classes.


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