Internal vs External Make-up Air Hoods
Why internal make-up air exhaust hoods?
The biggest reason for internal make-up air hoods is to reduce the costs related to providing tempered make-up air to the exhaust hoods(in both first cost and operating costs) this is short sighted.
An internal make-up air hood normally does not have the air tempered either for heating the incoming air in the winter or air-conditioning it in the summer.
One of the problems encountered with the internal make up system during cold weather and hot weather (humid days) with-in an air-conditioned space is condensation on the adjacent metal surfaces, ceiling grids, etc.
Internal make-up hoods are primarily use in the Southern United States.
Why external make-up air exhaust hoods?
External make-up air can be introduced in a variety of methods to provide better air flow to the exhaust hood, keeping smoke and vapor contained within the capture zone.
By tempering the air and dropping it in front of the exhaust hood reduces the amount of condensation build up on equipment, employees are more comfortable. Tempering of the air can be accomplished by either a heating/conditioning unit mounted to the return air duct or by increasing the size of the room units to compensate the air being exhausted through the exhaust hood. The best method for this is by placing the make-up air away from the hood either in the ceiling or walls but it needs to be done without disrupting the ability of the hood to capture the air and without causing discomfort to the staff.
The general opinion is that ‘Short Cycle Hoods (Short Circuit Hoods)” 2 are not recommended. Even though leading hood manufactures still have short circuit hoods in their catalog they indorse the recommendation that these hoods not be used.
1. Don Fisher, P.E.; Vernon A. Smith, P.E. Commercial Kitchen Ventilation, Engineered Systems, May 1, 2007.
2. Design Guide “Improving Commercial Kitchen Ventilation System Performance” California Energy Commission B500-03-0345 Rev 5.5.03; http://www.manualzon.com
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