How Critical is Ductwork?

Newly developed fans are quieter and more efficient, mainly due to the larger ductwork they employ. The increase in duct size is a critical factor in reducing sound. Increasing the duct size means that at a given flow rate (CFM), air can move more slowly. A six-inch diameter duct is more than twice as large as a four-inch duct and will pass the same volume of air at less than half the velocity, resulting in quieter operation.

Courtesy of: Broan-NuTone

Another consideration: long duct runs with multiple turns build up static pressure, robbing air flow and increasing noise. That is a concern of the mechanical contractor, but it is a factor architects should be aware of, because the location of the outside discharge relative to the fan will have a bearing on their performance. If duct runs must be lengthy, the duct size should be increased. Rigid ductwork is recommended over flexible ducts, and it is important that high quality, low static pressure wall and roof caps be used.

Remember, all ductwork needs to be vented outside the home.

Ventilating the Kitchen

The primary purpose for kitchen ventilation, as it is in the bathroom, is moisture control. The kitchen is complicated, however, by elements not present in the bathroom.

Cooking grease is vaporized, becomes mobile and can remain in the air for several days.

Smoke is another potential by-product. Both grease and smoke are easily distributed throughout the home if not immediately and completely removed at their source.

A common problem in the kitchen is that the cook top fan is not ducted to the outside. Range hoods and over-the-range (OTR) microwaves that are installed non-ducted are not ventilation devices. Their fans recirculate smoke, heat, grease and moisture from the cooking surface into the kitchen and throughout the home.Kitchen ventilation must be ducted to the outdoors.

Another common problem is that many range hoods are only used as light sources for cooking, nightlights and ambient lighting, largely due to undesirable noise levels.

It is for this reason that leaders in kitchen ventilation continue to reduce sound levels with the introduction of revolutionary new range hood technology-some earning HVI's certified sound rating of one SONE. It can also be achieved by over-sizing the blower and then operating the range hood at a lower performance level.

In addition, external and in-line blower systems provide alternate means to remove sound from the kitchen while providing appropriate performance levels-up to 1500 CFM.

For kitchen ventilation to be effective, designers will also want to adhere to the following general rules:

• To determine the appropriate kitchen CFM required for the cook surface, consideration should be given to increasing the CFM if:
• • More aggressive cooking style is practiced;
• The hood is installed higher than manufacturer recommends;
• The capture area is compromised by width or depth; or
• The duct run is unusually long or complex.
• Install range hoods at the lowest acceptable height to maximize capture while complying with manufacturers' recommendation for minimum height requirements- typically 18-24 inches.
• Use a hood that is at least as wide as the cook top.
• The hood should be deep enough to completely cover rear burners and half the front burners.
• If the hood is installed over an island or peninsula cook top, the hood should extend 3" on each side and back, and at least to the middle of the front burners.
• Increase the hood size and depth to compensate for higher mounting heights or when there are greater cooking demands than usual.
• As a minimal guide, maintain the same square inch area of duct as originated at the hood. Airflow can be maximized by increasing duct size from the hood.

For peninsulas or islands with cook tops, where no option exists for a canopy hood:

• Use a downdraft system.
• Design the counter so there is a raised section behind the cook top to reduce the effects of cross drafts.
• Choose the downdraft to address the height for the typical cookware.
• Make sure that the downdraft system is appropriate for the BTU output.
• Shelter the cooking area to prevent drafts-cross drafts from nearby doors and windows will reduce the effectiveness of the fan.

The Home Ventilating Institute (HVI) is a non-profit association representing manufacturers of home ventilating products from the United States, Canada, Asia and Europe. HVI offers a variety of services including test standards and certification programs that provide a voluntary means for residential ventilation manufacturers to report product performance information based upon uniformly applied testing standards and procedures performed by independent laboratories.

The Certified Rating Programs of HVI were created to provide a fair and credible method of comparing ventilation performance of similar products, and architects and specifiers can rely on HVI certified products to perform as represented. HVI represents a wide range of home ventilating products including bathroom fans, kitchen range hoods, downdraft kitchen fans, inline fans, heat/energy recovery ventilators, single and multi-port exhaust fans, exterior mounted fans, balanced ventilators, whole house cooling fans, powered attic ventilators, passive fresh air inlets and static ventilation devices for attics and crawl spaces.