Do you want to know how much CFM you need for your range hood?
Choosing the right CFM is important for an effective range hood --- not undersized or oversized.
In this guide, I will go over...
- How to choose the correct CFM for your home
- Different factors that will affect the CFM or fan speed
- Some negative issues with a too powerful CFM
Let's get started with this guide!
What Is CFM?
CFM stands for cubic feet per minute and is the standard rating for range hoods and bathroom fans.
It basically says how powerful the fan moves air.
The most common CFMs for range hoods are between 150 to 600 CFM but can be as high as 1,000+ for certain home chef range hoods.
What Is The Best CFM For My Range Hood?
Ideally, you want a kitchen exhaust fan that is powerful enough to adequately remove smoke, odors, and indoor pollutants. One of the biggest hazards of indoor cooking is actually nitrogen dioxide which is a respiratory irritant.
Home Ventilating Institute
Probably the most authoritative source for range hood CFM recommendations is from HVI. HVI is the Home Ventilating Institute and is a non-profit organization founded by major venting appliance companies to rate and test appliances.
Kitchen hoods (and bathroom exhaust fans) that are certified by HVI tend to be more trustworthy than other products --- because they can't cheat on their specs.
According to HVI, these are the bare bone minimums and the ideal CFMs for a kitchen range hood.
30" Range Hood
Here is the probably the most common range hood size in the industry --- the standard 30" wide range vent hood. For a 30" range hood, you want a minimum of 100 CFM.
The recommended ideal CFM for a 30" range hood is 250 CFM.
36" Range Hood
For a 36" range hood, you want a minimum of 120 CFM. The recommended ideal CFM for a 36" range hood is 300 CFM.
48" Range Hood
For a 48" range hood, you want a minimum of 160 CFM. The recommended ideal CFM for a 48" range hood is 400 CFM.
Choosing CFM For Kitchen Hood Based On BTU
In addition to the recommendations by the Home Ventilating Institute, another common approach is to choose CFM based on the total burner output BTU of your range.
BTU stands for british thermal unit and it is a measurement of heat. Basically, it is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. In this scientific world, the joule is used instead of BTU since heat and energy are technically the same thing.
A general rule is to choose 1 CFM per 100 BTU of total burner output. The manual or online specs of the oven will list the BTU output of each burner.
Here is an example of typical burner BTUs...
#1. Burner 17,000 BTU
#2. Burner 15,000 BTU
#3. Burner 9,500 BTU
#4. Burner 9,500 BTU
#5. Burner 5,000 BTU
Total Burner BTU = 56,000
56,000 BTU / 100 = 560 CFM
If your gas stove has a 56,000 BTU total output for the burners, then you would want a vent hood with at least 560 CFMs.
Read Also: How Do I Vent A Bathroom Fan Into An Attic?
Negative Air Pressure Hazards
It's important to be aware of potential problems with negative air pressure in a home.
It is strongly recommended by HVI that you if install a range hood with 600 CFM or more, you should provide makeup air. Makeup air is brought in from the outside to balance the loss of air pushed out by the vent hood.
In newer homes, we have made them so energy efficient (tight) that it can be difficult to bring in air from the outside to balance the loss of air from the kitchen hood. In older and leaky homes this isn't likely to be a problem.
Makeup Air Systems With Motorized Dampers
Homeowners should be aware that installing a makeup air system can be an elaborate and expensive endeavor. It is basically a motorized damper that is connected to the range hood. When the range hood turns on, the motorized damper opens, and allows in outside air.
If the negative air pressure problem isn't resolved, it can cause backdrafting of other gas appliances such as the water heater or furnace. Backdrafting is when the exhaust gases get pulled back into the home rather being pushed outside.
This can cause carbon monoxide poisoning in a home.
Things That Affect CFM of a Kitchen Exhaust Fan
Turns are one of the top ways that CFM will be reduced with a range vent hood. You want to minimize or eliminate turns altogether --- but sometimes that just isn't possible.
If you have two sharp turns in your duct run, add at least 5' in between each turn. Make sure that the turn uses a reticulated metal elbow so that the airflow smoothly changes direction.
Quality of Ducting
It is highly recommended that you only use smooth metal ducting for your range hood.
If you use foil or corrugated metal ducting, it will reduce performance and it will increase the noise of your stove hood. Grease and debris will catch on corrugated ducting, and may become an eventual fire hazard.
The best ducting is galvanized steel metal ducting because it is the strongest type of duct and it will be the quietest.
It is best to always go with the built-in duct size of your stove hood. If you use a vent reducer to adapt to a smaller duct, it will reduce performance (the rated CFM) and it will increase the noise.
The roof or wall vent cap will also have an affect on the range hood CFM. If the vent cap has small holed cover, or if it abruptly stops air flow, it will reduce the range hood CFM.
The best range hood caps have gravity closed dampers and doesn't reduce air flow suddenly.
Length of Vent Pipe
The longer it takes for the vented air to get to the outside, the more performance degradation the range hood will experience. The best vent runs are very short such as less than a few feet.
Of course, many times it isn't feasible to have a short vent run such as if you have to go through an attic. If you have the choice, choose the shortest route possible to the outside with the fewest number of turns.
What's The Bottom Line?
When choosing the right CFM for your range hood, it general, you want to follow the recommendations of the Home Ventilating Institute. You also want to take into account the quality of your duct, the number of turns, and other factors. You may also want to look at the total BTU output of your range burners.