Sizing a vent hood?

Can anyone lead me toward a direction on how to size a vent hood and make up air system?

Contact George Mills that is his cup of tea.

George will be helpful, however, best to check with local building inspector for the standards that apply in your jurisdiction…

Hi snow man:

Replying to sizing a vent hood:

Sizing the hood is the simplest part of the ventilation equation.

The hood must extend 6 inches over all sides of the equipment under it.

Exception: If against a wall only a 3 in air brake is required between the hood and the wall, no extention over the equipment. Most competent hood manufactures automatically build in a 3 inch air brake on hoods that are against a wall.

In addition curtains are allowed on the ends of a hood if they do not interfere with the operation of the equipment. The size of the hood can be reduced to where the curtains are adjacent to the equipment.

The above is only the start to designing a hood - ventilation system. The amount of air to be extracted is determined by the various units of equipment under the hood. That can very from 300 CFM per foot hood over ovens to 525 CFM per foot of hood over a broiler.

What ever the total CFM of air extracted, an equal amount of air, no less then 10 degrees cooler then acceptable room temperature, must be returned to the building. That is known as make up air.

Considering that the cost of warming make up air averages about $1.50 per CFM per year that can become a significant cost.

Make up air units are not, as of now, required by code to cool the warm weather air they bring into a building. The business operator has the option of, adding a rather expensive cooling system to his make up air unit, suffering the summer heat the unit brings in or installing a lot of additional A/C .

It should be apparent to all that for considerations of operating cost and work area comfort a properly designed ventilation system can cut both operating costs and improve work area comfort.

Example a pizzeria hood over a high volume conveyor oven that is used in most high volume shops. Assuming an 111 in long by 60 in deep oven it requires a 123 in long by 66 in deep hood.

Assuming an un certified hood, ( a certified hood has been tested by UL or ETL and the lowest CFM at which they can operated has been certified) uncertified hoods require more air to be exhausted, That hood will require 4700 CFM of exhaust and 4700 CFM of make up air.

At about $1.50 per CFM of make up air per year that is about a $6000.00 cost, per year, for gas and electricity. That figure can be substantially more or less depending on the geographical location. Additional costs are incurred for additional A/C to combat hot summer air brought into the building if the make up air unit does not include A/C

Note also that the above calculations apply to ventilation systems recently installed or to be installed now under current codes.

Without going further into details let me state that we are now installing ventilation systems,

over conveyor ovens, such as described above, that only extract 800 CFM of air from a building.

George Mills


Thank you for the information. What is an air brake? How do you determine the amount of cf of air that’s needed?


Thank you for the information. What is an air brake? How do you determine the amount of cf of air that’s needed?

A 3 in air brake for a hood indicates the hood has stops on it so it cannot be installed closer than three inches to the wall.

If you are going to use a certified hood, that is a hood that has been tested by UL or LTL the hood manufacturer will indicate the CFM required to be exhausted.

If you are going to use an uncertified hood (bad idea) you will need to determine the cfm required by code for each item under the hood. The highest rated item under the hood determines the CFM of exhaust per lineal foot of the hoods perimeter that is not against a wall.

If you have only have only a pizza oven under the hood the exhaust rate for a non certified hood is 300 CFM per foot of perimeter of the hood.

If you use a non certified hood the exhaust rate will be very high and you will need an expensive to buy and an even more expensive to run make up air unit.

For instance a hood 9 Ft X 6 ft for a conveyor oven will have 21 ft of open perimeter if against a wall. An uncertified hood will require 21 X 300 CFM and will require 6300 cfm of exhaust air and a make up air unit to bring in 6300 cfm of heated air.

That is compared to certified hoods that will only take 800 cfm of air from your building. Will not need a make up air unit, if coupled with a properly sized A/C, (no larger usually than what the shop itself would require). Such a ventilation system usually costs no more that a hood, make up air system and costs thousand per year less to run.

George Mills

I should note that most certified hoods have a relatively low air supply into the hood it self.That air does not require heating as it never enters the room it is drawn out of the hood by the exhaust fan. XLT has a hood that does not have an air supply and has exhaust rates as low as 800 CFM depending on the oven used.

George Mills

Mr Mills:

Do you have any experience with the XLT AVI hood? I have purchased the triple stack oven and was thinking of adding there hood. It is very pricey and pretty new. It does appear in theory that it would work. Just trying to get an outside opinion.

Jeff H

Hi Jeff: We have not placed any XLT-AVI type hoods as yet so I have had no feed back from any clients.

Check with your local jurisdiction before ordering. It is an entirely new consept and you would want to have it cleared by your Building, Health, and Fire department before ordering. We have had certified hoods rejected by authorities in several instances. Your Local authorities have the final say no matter what certifications a ventilation system carries.

George Mills