A Guide To Residential Ventilation Exhaust Fans

Most of today's new homes are highly insulated and practically air tight. While this is great as far as cost effective energy bills go, the down side to tightly insulated homes is poor indoor air quality. As windows and doors are typically kept shut throughout the day, moisture, and stale air that can carry contaminants and pathogens circulates inside homes and gives rise to various problems including mold and mildew, health problems, ruined furniture, peeling paint etc. This is why it is important that you take every step to improve the quality of your indoor air, which can be achieved through mechanical ventilation.

An excellent way to ventilate your home thoroughly, efficiently, and cheaply is to install exhaust fans in your home. This article explains what exhaust fans are, their different types, how to select the best one for your home, and also the benefits of exhaust ventilation fans.

What Are Exhaust Fans

An exhaust fan is a mechanical ventilation device that helps to draw out stale and impure air from your home and bring in fresh air, thereby improving the quality of indoor air. Exhaust fans are typically ducted to the exteriors of your house, through which bad indoor air can effectively be removed from your living space.

Types Of Exhaust Fans

Exhaust fans are classified into various types, mainly depending on the type of mount and the location where you need to install the fans. The different types are:

1. Ceiling Mounted Exhaust Fans: As the name suggests, ceiling exhaust fans are those which are installed in the ceiling. Such fans expel stale air from your home upwards through the roof. The fan is connected to ducting, which is exhausted outside the home via an external vent, like a roof cap or soffit exhaust vent.

2. Inline Exhaust Fans: Unlike ceiling exhaust fans that are installed directly into the ceiling, inline exhaust fans are typically mounted in-between ducting, hence the name inline fan. For instance, if you wanted to ventilate an area that did not have clearance or space for a ceiling mount fan, you would make use of inline exhaust fans to ventilate such areas. The exhaust fan would be placed in between the ductwork and the stale air would travel through the ducts and ultimately be expelled from your home. Since inline fans are not mounted directly to the ceiling, they are very quiet. When installing an inline fan, to reduce noise, we recommend using an insulated flex duct that is at least eight feet long from the intake port on the ceiling to the inline fan.

Inline exhaust fans are ideal for exhausting areas or rooms where you cannot, or do not wish to install the exhaust fan directly. Since these types of exhaust fans are mounted in remote areas, they are also referred to as remote mounted exhaust fans. Inline exhaust fans can either be single-port (exhausting from a single area) or multi-port (exhausting from multiple areas).

3. Wall Mounted Exhaust Fans: These exhaust fans are installed on walls. Since they are installed on exterior walls of the home and not on interior walls, the stale air has a direct route to the outside of your home and thus no duct work is required in installing these exhaust fans.

4. Combination Exhaust Fans: Exhaust fans are also available as combination units. You have the choice of a fan-light combination where the exhaust fan provides illumination as well, or heat-fan-light combination wherein you get a heater, light and ventilating fan all in a single device.

5. Exterior Remote Mounted Exhaust Fans: While most other exhaust fans are installed inside your home and push stale air out, exterior remote mounted fans are installed outside your home and pull out stale indoor air instead of pushing it out. The main benefit of these exhaust fans is that regardless of however noisy they are, most of the noise remains outside your home.

6. Kitchen Range Exhaust Fans: These fans are mounted inside the range hood over your kitchen stove. Such fans not only help to rid your kitchen of stale air but also help to expel bad odors and reduce moisture levels in your cooking area.

These several types of exhaust fans can be used for complete ventilation of your home including intermittent local ventilation for baths, kitchens, dryer rooms; continuous whole house ventilation throughout your home, and for exhausting hard-to-air spaces such as crawl spaces, attics, and basements.

Benefits Of Exhaust Fans

Exhaust fans are very effective at ventilating your home and other living spaces. Without proper ventilation, the air inside your home can get filled with harmful contaminants and disease causing pathogens.

Pollutants such as pesticides, harmful gases, smoke, pet dander, lead, asbestos, dust mites, paint fumes, grease etc get released into indoor air due to daily activities such as cooking, smoking, burning fuel, bathing, renovating etc. In addition to these pollutants, activities such as bathing, cooking, and washing also release excess moisture in the air and make indoor air extremely humid. If not ventilated adequately, these added pollutants and increased moisture levels can decrease the quality of indoor air greatly, thereby leading to various problems such as:

Health problems including asthma, allergies, nose bleeds, skin rashes, headaches, nausea, and other breathing disorders. In fact, according to the Environmental Protection Agency, a large percentage of the over 20 million annual asthma cases in the US alone can be attributed to bad indoor air quality.

Split, warped and rotted furniture due to excess humidity.

Cracked and peeling paint on the walls.

Formation of fungus, mold spores, and mildew, which in turn lead to severe health problems.

Thus, by using exhaust fans to ventilate your home efficiently and completely, thereby improving indoor air quality, you can rid yourself and your home of all these problems.

Recommended Sizing Of Exhaust Fans

To ventilate your home effectively, it is important that the exhaust fan you choose has the capacity to exhaust the intended space completely. To ensure this, you must select the right sized fan for your needs. Here's a look at how to size exhaust fans properly.

1. Location of the Exhaust Fan and Air Changes Per Hour:

Where you intend to install the exhaust fan will have a direct bearing on its size. As per the Home Ventilating Institute (HVI), different locations in your home require varying Air Changes Per Hour (ACH) in order to be ventilated properly. Here are the ACH requirements recommended by HVI.

8 ACH for bathrooms

15 ACH for kitchens

6 ACH for rooms other than bath and kitchen

ACH refers to the number of times the air should be completely changed in an hour. Thus, an 8 ACH recommendation for bathrooms means the exhaust fan should have the capacity to completely change the air in the bathroom 8 times in one hour.

All exhaust fans are rated in CFM, which refers to Cubic Feet per Minute. To determine how large an exhaust fan you need (in other words, CFM rating of the fan) here's what you need to do.

Sizing Bathroom Exhaust Fans:

First and foremost, measure the dimensions (length, width, and height) of the room and then calculate the volume of air in the room by multiplying all these 3 numbers. For instance, if your bathroom has dimensions of 6 x 10 x 8, then the air volume in the bathroom is 480 cubic feet. Thus, the exhaust fan needs to ventilate 480 cubic feet of air in order to achieve 1 ACH. But since the recommended ACH for bathrooms is 8, the fan will effectively need to ventilate 480 x 8 cubic feet, which equals 3840 cubic feet in one hour. Fan ratings are per minute, thus by dividing 3840 by 60, you can achieve the desired CFM rating for the fan which in this case is 64.

A simpler way of determining the CFM rating for bathroom exhaust fans is to simply multiply the length of the bathroom by its width. For every 1 sq. ft. of floor area, you need 1 CFM. Thus, in the above example, area would be 6 x 10 = 60, thus recommended fan size would be 60 CFM.

However, if your bathroom is more than 100 sq. ft. in size, you have to add the different fixtures in your bathroom to reach the desired CFM rating. The recommended CFM for different fixtures is:

· Shower - 50 CFM

· Toilet - 50 CFM

· Bathtub - 50 CFM

· Whirlpool tub - 100 CFM

Thus, if your bathroom is more than 100 sq. ft. in size and has a toilet and shower, you will need an exhaust fan with 100 CFM rating. If a bathtub is also present the CFM rating will increase to 150 and so on.

Sizing Kitchen Exhaust Fans:

When sizing an exhaust fan for the kitchen, you have to take into account the location of your kitchen cooking range (if without range hood) or the size and location of the range hood if there is one. Here are the recommended CFM ratings for kitchen range hood exhaust fans:

Generally speaking, for every 10,000 BTU of the range, it is recommended a minimum of 100 CFM. So if your range is rated at 50,000 btu's, you would consider getting a fan with at least 500 CFM.

2. Understanding Static Pressure and Measuring Equivalent Duct Length:

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

Static Pressure: Inside every duct, there is a constant pressure being exerted at any point from all directions. When an exhaust fan moves air through the duct, the air counters resistance from this pressure which is known as static pressure. Thus, an exhaust fan has to have the ability to overcome the static pressure in a duct so as to effectively duct stale air to the outside of your home. This can be done by calculating the equivalent duct 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 has to overcome. Ducts may have one or more elbows, turns, or wall caps which add to the static pressure in a duct. Thus, you have to calculate the equivalent duct run and not the actual duct run so as to size an exhaust fan properly.

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

Smooth metal duct: Actual duct length x 1

Flex aluminum duct: Actual duct length x 1.25 (for 4"diameter duct)

Actual duct length x 1.50 (for 6"diameter duct)

Insulated flex duct: Actual duct length x 1.50 (for 4"diameter duct)

Actual duct length x 2.00 (for 6"diameter duct)

Wall caps and roof caps: 30 feet for each cap (for 4"diameter duct)

40 feet for each cap (for 6"diameter duct)

Elbows and turns: 15 feet for each (for 4"diameter duct)

20 feet for each (for 6"diameter duct)

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

This ventilation guide is provided as a service from R.E. Williams Cont. Inc. Please be aware, that building codes and local regulations differ from region to region, they also can change.; therefore, R.E. Williams' Cont. Inc. assumes no liability for omissions, errors or the outcome of any home improvement project. You should always exercise reasonable caution, follow your current codes and regulations that may apply, and if in doubt on any procedure consult with a licensed professional.

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