The question of how much overhang should I design into the hood is one based on the equipment below the hood. All the codes require a minimum of 6” front overhang. But is that enough?

ASHRAE testing under laboratory specific conditions always points to more overhang is better. A 12” overhang is considerably more effective at giving additional volume within in the hood area to allow the exhaust plume time to be taken up through the filters and improve the CKV performance factor.

Most of you designers know however, that not all equipment is created equal. Some items like ovens for instance can be much deeper, front to back than an adjacent range or fryer. The overhang should really be based on the deepest or largest piece of equipment. In the case of convection ovens, there are far too many hoods out there that have less than 6” of hood overhang. Consequently, when convection doors are opened, a huge amount of steam flows out and escapes the hood reservoir.

When sizing a hood to accommodate a convection hood, that automatically enhances the ability of the hood to function better since the overhang on ranges and fryers has been increased considerably, perhaps even as much as 18”. That is a good thing.

But why is this a good thing you ask? Very simply. When a Type I is specified as a listed hood by a testing agency such as UL, ULC or ETL, the air volume required for satisfactory operation is usually lower than that of a non-listed hood. See ASHRAE Chapter 31 for specifics. Air volume is calculated across the face or length of the hood based on light, medium, heavy and extra-heavy duty categories of cooking equipment. The depth of the hood based on overhang does not increase the amount of air required to be exhausted to properly perform.

In fact many items come into play such as side overhang, side panels, make-up air, ceiling registers and cross drafts from doorways, pass-thru openings and the like. For a very good explanation, see the newly issued FCSI White Paper recently authored by Don Fisher and Vern Smith. Contact FCSI at www.fcsi.org and click on the Commercial Kitchen Ventilation White Paper on the home page.

Another aspect is the hood type chosen. Overhangs are greatly affected by the hood design such as a wall hood which is probably the more common design used. Single and double island canopy hoods generally require more air due to the fact they are most affected by cross drafts and currents. The proximity or back shelf type hood sit lower over the equipment and are generally the most effective at effluent capture and do not overhang the front of the cooking equipment.

Use of a non-listed hood requires a different amount of exhaust air, usually higher. In the City of Chicago who recently overhauled all of their codes, they retain the old system of sizing the exhaust air volume by the amount of square feet of the hood. This does result in significantly more air being exhausted than a comparably sized listed hood. Officials in the city, plan to fix that intentional oversight, but they told us it was not a priority and would take about seven years to get around to once again recognizing listed hoods. Apparently those of you who design for Chicago still have about six years to wait.

We hope that this short article will cause designers to consider the issue of sufficient overhang when designing their next hood. It will benefit the kitchen staff by keeping them more comfortable and will benefit the owner when specifying a listed hood in reduced energy costs for the life of the kitchen. Another option to consider when considering energy use, is a variable speed fan controlled by equipment installed in the hood as manufactured by the Melink Company.