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Hello, everyone.

I have a friend who is an executive chef seeking my help about his kitchen which is discomforting to him and with his staff.

The last HVAC contractor who made the rehab of his kitchen supplied them with a motor capacity that is half the capacity from the originally installed 15HP to only 7.5HP (bummer).

Based from the total kitchen area of 1821 sq.ft. and a ceiling height of 8 ft, the overall volume of air inside the kitchen is 14339 cu.ft. and their island type exhaust hood size is 18ft x 6 ft x 2 ft.

I counted 27 various cooking (open tops ranges, stock pot stoves, grill, griddle, convection and microwave ovens, wok, fryer, upright refrigerators and freezers, rice cookers, counter table equipment like bar blender, panini grill, food processor) . They also have a hood dishwasher that produces steam.

Our actual readings using anemometer results a total air coming into the kitchen from both fresh air and conditioned air is 32,016 cfm while readings from the exhaust hood is only 12,396 cfm.

Will a 10HP motor centrifugal blower should be enough in lieu of the 15HP that was originally installed based from the above kitchen size? Definitely a 7.5HP isn't sufficient. What is the air change rate to be used?

I'm getting a total of 37,142 cfm air to ventilate using ACR of 53, is this correct?

The ACR of 53 was computed based on per unit of kitchen equipment CFM per sq-ft area of hood divided the total air volume of the kitchen.

Appreciate any help.

Chefs Mate


HP is meaningless since fan efficiency and the associated ductwork matter. install a 6"duct and you need a 1000 hp fan for the same airflow.

What you need is exhaust flowrate and that by code mostly is based on the size of stoves, dishwashers etc.

but this is for IAQ. For comfort, it matters how the make-up air is treated. cooled, heated and so on. With a given airflow, air-conditioned air may make it comfortable, non-conditioned air not. Obviously depends on climate. So there is more needed than you provided.


I assume ACR means Air Change Rate?

53 means just less than one per minute, which, from your data, is what the ventilation system is currently doing. Most data seem to have 60 air changes per hour to be the max required amount for a

Your total inlet air volume (32,000cfm) seems to be much more than outlet air flow (12,000 cfm) which doesn't make sense. Is some of that recirculated air going through an air conditioning package?

What exactly is the issue?
Temperature or stuffiness or fumes or ?
What temperature is the incoming air?
Is the incoming air spread out or does it short circuit and go direct from inlet into the extraction hood?

Are the open tops gas burners or electric?

There is a LOT more to it than just upping the size of the fan extracting the air.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.


What is the fresh outside air flow rate. It should at least be equal to cooking hood exhaust flow and other exhaust flows such as dishwasher hood. Where does fresh air come from? Any recirculated air through HVAC unit does not count. What was 15 hp motor changed out to 7.5 hp? Was this the HVAC unit? If so then the HVAC unit was downsized by contractor? Seems like HVAC unit should be for cooling both internal loads and fresh air makeup which would make it a rather large unit.


Dear All;

Thank you very much for taking the time to respond to my above message.

@LittleInch - Yes, ACR is the Air Change Rate. ACR 60 seems right from what I've read. Total air inlet flow rate of 32,016 is a combined ambient air @77°F (26,562) and air-conditioned air @68°F (5,454) and their ceiling vents were spread out. I'm just concerned about the 8 vents that were located near the exhaust hood. I have attached a drawing for you to see.

But then again, the exhaust hood is extracting air very weak as per actual readings shows. I am sending you actual readings and list of gas cooking equipment also momentarily.

The main issue here is that the kitchen area temperature is too hot to cause discomfort.

Thank you.


If it's a temperature issue, then I think you need to look at the makeup air equipment; that seems to be a rather large amount of CFM for a kitchen of that size. Does the makeup air unit have a cooling coil? If not, there's nothing you can do about the temperature if it's bringing in unconditioned outside air.

If it has a heater on it, what is the turndown? If it's a single-stage burner or even a two-stage, than it's going to be very difficult to control the temperature. I would typically specify at least a 4:1 and up to a modulating gas burner for this application.


Looking at the layout shown in the sketch and the information provided on the Excel spreadsheet, I have the following comments:

From the looks of the layout it appears that makeup air is only supplied through the 8 supply air grilles at the hoods and the other supply air grilles shown have been disenabled (NA) for some reason.

The makeup air is much more than the air exhausted through the hood therefore either the makeup air quantity shown is incorrect or I assume that there is an additional relief vent somewhere letting the air out, or the excess air is going out the doors and into the hotel area. However typically for kitchens where smoke and fumes are present makeup air is less than ehaust air so that a negative pressure is maintained in the room. This forces extra air required for makeup to come from adjacent rooms so flow is from adjacent rooms to kitchen and fumes don't escape kitchen.

The temperature of the makup air is shown as 77F. I assume that it is either due to being in a mild northern climate or it is precooled by a Make Up Air Unit with cooling coil. If not and maximum temperature of makeup air is really higher then this will in itself cause heat input to room depending on how hot ambient temperature is.

It appears that the only HVAC unit serving the large area is aircon 3 since other two units are in separated rooms. Aircon 3 appears to be about 5 tons based on supply air flowrate. Aircon 2 at 3335 seems very large capacity for such a small room that it is in. It is possible that the 5 ton unit is satifactory to cool the entire larger area if the makup air is really 77F maximum.

It appears that the hood over the cooking equipment may not be design correctly. I believe that typically hoods are designed for extending a minimum 6" beyond cooking equipment (or 1/3 hight of hood above cooking surface) and no higher than 4 feet above surface of cooking equipment. A lot of heat that would normally be passed out through the hood with the exhaust may be entering the kitchen in addition to the radiat heat given off by the flames. See following link.



Check cooking hood design for proper coverage, height above surface and flowrate. Insure that hood has proper coverage, height above cooking surfaces and exhaust flowrate.

Balance makup air flow so that it is slightly less than hood exhaust rate plus other exhausts fans/hoods that may exists.

Reuse the makeup air grilles currently not flowing (NA) so that the cool makeup are is spread more evenly in the space (assuming the makeup air is really 77F maximum and therefore makeup air also serves to cool room). With only grilles operating near the cooking hood all air is short circuited directly to hood without passing through room to remove heat in room.

Insure that the aircon 3 is operating at rated capacity. The sensible heat capacity output of the unit can be determined by equation Capacity BTU/H = 1.08 CFM (dT) Where dT is difference in return air temperature amd supply air temperture of the unit, and CFM is the flowrate of the unit in cubic feet per minute. The actual capacity will be a little above the sensible capacity due to heat of condensation of the water vapor as the flow goes through the cooling coil.


Thanks everyone and apologies for the late responding as I was away.

Just to let you know, we are near the equator (southeast asia - the Philippines).

We will work on the exhaust system since its easier to modify on the fresh air based from the list of equipment computed CFM required to move air at recommended ACR for commercial kitchen.

Again, thank you very much for your assistance and clarification, Snickster, LittleInch, HVAC-Novice and nuuvox000.

Really appreciate it.


Hi, AdrienneGC.

Thanks for your feedback. These are great references. Appreciate your help on these.


If you really want to see if your supply air is being short circuited directly into the exaust and air distribution patterns inside the kitchen, you can perform a smoke test.

Assure that kitchen is negative. As said above, it is highly unusual that a kitchen is kept positive in relation to the restaurant/other areas. Food smell is usually good, kitchen smell can be really annoying and a real headache, especially if the CEO doesn't particularly like that smell. Trust me, I know...
-Get a small smoke machine in Ali Baba, Tao Bao, Amazon or any other shop. These are dirt cheap. The one that I bought couple years ago cost like 20USD.
-Isolate all fire service detectors
-Inject the smoke either on the supply air duct (preferably) or at the kitchen itself.
-Don't forget to warn your duty team and the stake holders that you are doing this test


Is the exhaust hood fan connected directly by shaft to the 7.5 hp motor or is there a set of belts with pulleys to the 7.5 hp motor? What I am driving at is to increase the RPM of the exhaust fan which can be relatively easy to change with a belt and pulley system as long as the maintenance staff recognize a "possible" change of the circuit breakers may be needed. There is also a matter of difference between volumetric air air flow coming in the kitchen and drawn out thru the exhaust hood;how do you reconcile for that large difference?. A push pull exhaust system over the cooking appliances would have been a better solution to the kitchen staff discomfort.


@MedicineEng - Thank you for the advise. We already did this and due to weak suction of exhaust hood, we can barely see make up air doubling back through exhaust hood. But, it appears make up air 5 and 6 does.

@chicopee - it is belt-driven. This is my initial findings actually in lieu of replacing the motor capacity, we could adjust on the blower size and pulleys (making emphasis on the motor ampere ratings).

Thank you very much everyone who provided insights and suggestions. I really appreciate it. As soon as the owner approves the retrofit, I will make sure everyone of you will be informed on how it went and acknowledge all the people that provided me all the ideas and information that helped resolve my friend's dilemma.thumbsup2

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