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A duct is designed to transfer air between spaces in a building. The ducting inside your bath fan—like any ventilation product—is essential to its success. The Home Ventilation Institute(HVI) recommends that bath fans be ducted to the outside.

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Ducting Guidelines

In the case of a bathroom fan, the warm, stale, moist, dank air—which could otherwise spawn health problems, like mold—is safely and efficiently transferred from your living space to the great wide open. Duct-free fans are available, but these are not recommended for your bathroom. A duct-free bath fan will not remove air from the room, as it is not a ventilating device. 

So, how can you be sure you have the correct ducting and it performs to its maximum potential? Just as size is important for the performance of your bathroom fan, so is the length of its ducts. 

When you’re sizing an exhaust fan that does not open directly to the outside but is ducted, it’s important to ensure it has the capability to move stale air throughout the duct and ultimately to the outside. Here, we first need to understand static pressure and equivalent duct length. 

Static Pressure: Inside every duct, constant pressure is exerted at any point from all directions. Similarly, an exhaust fan must overcome resistance (also referred to as static pressure) when pushing air through the duct to the outside.  

Your bath fan must have the ability to overcome this static pressure to effectively and efficiently duct stale air out of your bathroom. This can be done by calculating the equivalent length of any duct. 

Calculating Equivalent Duct Length (EDL): Simply measuring the length of a duct is not enough to know how much static pressure an exhaust fan must overcome. Ducts may have one or more elbows, turns or wall caps which add to the static pressure in a duct. As a result, you must calculate the equivalent duct run and not the actual duct run to size an exhaust fan (like your bath fan) properly. 

The static pressure in any duct run differs according to the duct material, number of elbows and turns, exterior wall cap and wall jacks. Listed below are the standard values for different duct components: 

·         Smooth metal duct 

o   Actual duct length x 1 

·         Flex aluminum duct 

o   Actual duct length x 1.25 (for 4-inch diameter duct) 

o   Actual duct length x 1.50 (for 6-inch diameter duct) 

·         Insulated flex duct 

o   Actual duct length x 1.50 (for 4-inch diameter duct)  

o   Actual duct length x 2 (for 6-inch diameter duct) 

·         Wall caps and roof caps 

o   30 feet for each cap (for 4-inch diameter duct)  

o   40 feet for each cap (for 6-inch diameter duct) 

·         Elbows and turns 

o   15 feet for each (for 4-inch diameter duct) 

o   20 feet for each (for 6-inch diameter duct) 

Using the above values, you can calculate the equivalent duct length that an exhaust fan must overcome in order to push stale air outside your home and counter static pressure effectively. 

It is NOT recommended that a smaller diameter duct is used with what the fan is designed for. This will not only cause the fan to run harder, but it will also greatly reduce the fan’s CFM performance and generate more noise. However, larger diameters can be used, and will result in improved performance. 

In addition, the kind of duct material and how it’s set can certainly mean the difference between a properly functioning bath fan and one that will make you want to rip it from your wall or ceiling. If you’re installing a fan in a new home, the HVI recommends that your contractor follow the manufacturer's installation instructions. Also, as seen above, duct material, size, length and number of bends can adversely affect the product’s airflow. 

Rigid vs. flexible ducts 

It’s recommended, where possible, to use rigid ducts. It has less resistance to air flow and allows the fan to operate much more efficiently. If flexible duct is used, be sure the duct is as straight as possible. Too many twists and turns in the ducting will significantly reduce the blower’s ability to remove moist air. A good indication that this is happening is if your mirrors and windows fog up even when the fan is running. But, if your ducting must bend, gradual turns are preferred over tight corners. 

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