Range hoods are a key player in the kitchen mechanical system. If they are not tied to a makeup air system, they make their own.
Many people do not consider the kitchen range hood to be an important part of a house’s mechanical ventilation system, but they are. Monster range hoods that suck large volumes of air (500 CFM - 1,000 CFM) out of a house are (by definition) sucking large volumes of air into the house at the same time.
Exhaust = supply in the ventilation equation.
Like it or not, the kitchen exhaust has to play nice with the rest of the house's mechanical system.
This is especially important in tight houses and energy efficient retrofits.
Today, we're consolidating that research into a concise hit list for you to execute in the field.
Standards, codes, and high performance ratings
ASHRAE 62.2 is a standard that defines optimal ventilation rates for homes. Building codes and Energy Star use 62.2 to define mandatory ventilation rates for homes. Kitchen fans are part of that equation.
Range hoods typically turn on when a person turns them on and they turn off when a person turns them off. This is called 'intermittent use' or 'occupant operated' by people who like to make sentences harder to read than they need to be.
Fans can also run continuously.
Continuous exhaust fans must be quiet, or people will unhook them—less than 3 sones is suggested by HVI.
Energy Star goes a little further for continuous-flow kitchen fans: they should have low power and have multiple operating speeds to bump up for spot ventilation.
The range hood duct should have a smooth interior finish (easy to clean), be airtight, and be equipped with a backdraft damper.
5 Items for you checklist
Kitchen exhaust fans are often installed by many different trades: remodelers, electricians, range hood vendors ... whoever does it should follow these steps:
1/ Choose the right size exhaust fan for the application.
- Kitchen fans operated intermittently should move 100 CFM of air or more. Energy Star recommends selecting a fan with a rating of 150 to 200 CFM to pull at least 100 CFM when measured.
- Continuous fans should move 25 CFM of air. Energy Star recommends fans that will get you at least 5 air changes per hour (ACH) based on the kitchen's volume.
2/ Install the fan above the stove: either in the range hood, cabinet, or in the wall.
- Cut a hole for the fan or duct slightly larger than needed and air seal the opening with caulk or low-expansion foam after installing fan.
- Don’t use duct tape, use mechanical fasteners, foil tape, mastic, or all three. Duct tape is for wallets.
- Exhaust hoods installed over broiler units should have at least 24 inches clearance above the cook top and the hood should extend the full width and depth of the cooktop.
3/ Vent the fan to the outside: not into the attic.
- Or the crawlspace. Even if it is vented. Period.
- Straight duct is better than flex duct, fewer bends are better.
- Don’t downsize the ducts: small vents can trap grease and cause a fire.
- Seal the seams in the ductwork with the aforementioned metal tape, mastic, or both.
- Don’t dent the ducts. Misshapen ducts really change the way the air flows through the pipe, and not for the better.
4/ Place the outlet away from soffit vents (or any other intake vent for that matter).
- The outlet cover should be protected from chipmunks, et.al. with some sort of covering: louvers, screen, or grill.
- Exhaust air should not be directed onto walkways.
5/ Turn it on and walk outside.
- Make sure that the fan exhausts outside, duh.
- Confirm that openings cut in the ceiling and walls are air sealed.
- Confirm that the exhaust duct is sealed to the fan.
- Check the sone rating: under 3 sones for switched fans, under 1.0 sone for continuous fans.
- Test the flow rate.
It is important to test the actual flow rate for a couple of reasons. Code is one of them. Another is this:
Duct work, termination choices, and installation may decrease the measured cubic feet per minute below the factory-rated value.
- M1503: RANGE HOODS
- M1504: INSTALLATION OF MICROWAVE OVENS
- M1505: OVERHEAD EXHAUST HOODS
- M1601 DUCT CONSTRUCTION
- Building America Kitchen Exhaust Guide
- ASHRAE Standard 62.2, Ventilation for Acceptable Indoor Air Quality in Low-Rise Residential Buildings
- ENERGY STAR Certified Homes, Version 3 (Rev. 07) Inspection Checklist for National Program Requirements
- DOE Zero Energy Ready Home National Program Requirements