How to choose an Exhaust Fan
An exhaust fan is a ventilation device that helps to remove undesired smell, water vapor or smoke from an enclosed area like kitchen or bathroom. The idea is to create air flow so that fresh air replaces the contaminated or unwanted air. Exhaust fans in home typically use air ducts to remove the stale indoor air and hence improving the living experience.
Since today’s houses are packed and insulated to drive down the energy bills, exhaust fans have become more important to remove the stagnant indoor air. Moreover, unchanged air in homes gives rise to moisture, harmful bacteria and other pathogens which might result in health related problems. This article would help you to understand various technical terms and choose an exhaust fan for your home.
Types of Exhaust Fans
Exhaust fans are mainly classified according to their mounting locations.
Ceiling Exhaust Fans
These fans are mounted on the ceiling and remove stale air from the house by moving it upward. The fan creates an air draft, pushing polluted air through the fan out of the house as the fresh air enters through doors and windows. A ceiling exhaust fan is typically connected to a duct which release air through an external vent.
Attic exhaust fan is also installed in the ceiling and helps to keep the house cooler. It removes the hot air out of the house as cooler fresh air seeps in the house through small openings in doors or windows. Attic exhaust fans are sometimes very beneficial in keeping the house cooler even without air conditioners.
Wall Exhaust Fans
These fans are installed on the exterior walls of the house and therefore do not require any duct. The unwanted air is directly thrown outside the house through the fan on the exterior wall.
Kitchen Exhaust Fans
These fans are installed directly above the kitchen stove and help remove moisture and bad odor from the kitchen. It is recommended that the kitchen exhaust fans vent the air out of the house and not in attic space. Venting in attic creates a potential fire hazard since any fire in the kitchen can be pulled up in the attic by the air flow through exhaust fan.
These fans come with various air circulation capacities but the choice of the fan should be based on the British Thermal Unit (BTU) rating of the kitchen stove. A fan with 100 CFM (term explained below) is recommended for a stove range which produces 10,000 BTU of energy. Furthermore, each increase of 10,000 BTU in your stove would require an exhaust fan of additional 100 CFM rating. So a stove which produces 50,000 BTU of energy would require a fan of 500 CFM rating.
Bathroom Exhaust Fans
Unlike kitchen exhaust fans, bathroom fans are not for removing bad odor or smell from the room. These fans are required to remove moisture and steam from the bathroom. Since the residual moisture is the trigger point for molds and fungus, a bathroom exhaust fan should be installed in each bathroom in all homes.
While choosing the right exhaust fan for your home, you might come across a lot of technical terms like CFM, Sone etc. Let me just give a small explanation of these terms.
The measure of quietness of exhaust fans is expressed in terms of "sones". Higher the value of sone, louder would be the noise from the exhaust fan. Among the fans within your budget, you should aim for the fan with lowest sone score. For example, Panasonic exhaust fans typically have the rating of 1 sone score.
The volume of air moved by the exhaust fan is measured in terms of Cubic Feet per Minute (CFM). The choice of the fan should be based on the appropriate CFM rating.
The required CFM rating for a room can be calculated as follows:
First calculate the cubic feet of the room by using the formula
Total cubic feet = Length x Breadth x height [Room measurements in feet]
Now divide the total cubic feet by 60 (number of minutes in an hour) to obtain the volume of air required to be moved per minute by the fan. However, good ventilation should change the complete room air more than once in an hour (typically 8 times in an hour in bathrooms and 15 times in an hour in kitchens). So multiply the above result by 8 to get the required CFM rating of the exhaust fan.
Ducts are one of the most important parameters that affect the performance of exhaust fans. Typically exhaust fans connected through a rigid duct perform better than those connected to a flexible duct. Moreover, ducts with larger diameter and with less bends perform better than smaller diameter ducts. Straight ducts with smooth surface provide less resistance to air flow thereby increasing the fan performance. Flexible aluminum ducts are easy to install but PVC ducts would give better results by reducing the static pressure and increasing the air flow. It is recommended that for best results ducts should be supported properly to avoid any sagging, which reduces the air flow and increases the fan noise.