LETS START WITH THE ASHRAE 62.2 BASICS
ASHRAE 62.2 is a minimum national standard that provides methods for achieving acceptable indoor air quality in typical residences. It was developed and is maintained by the American Society of Heating and Air-Conditioning Engineers (ASHRAE). The standard has three main components:
Whole House Ventilation: Exhausting stale indoor air and replacing it with fresh outdoor air.
Local Exhaust: Exhausting high levels of contaminates and moisture typically associated with cooking, bathrooms, etc.
Source Control: Preventing outside contaminates from entering the home - storing paint, gasoline, etc., inside the home.
The recommendations that follow are for most common conditions, extreme conditions require additional consideration.
Whole House Ventilation
The exhaust fan dilutes the air in the main living spaces with outside air to remove unavoidable contaminants from people, pets, cleaning, offgassing, etc. The whole house fan flow rate is determined based on the floor space and the number of bedrooms. The whole house fan provides multiple air exchanges within the home each day. The operation can be continuous or intermittent (much higher airflow cycled by a timer) if 1 sone or less. The formula for calculating how much ventilation is required is:
(total square footage of the home/100) + ((number of bedrooms+1) x 7.5 cfm)
Okay, So What Am I Looking For In a Whole House Fan?
The key with a Whole House Exhaust Fan is quiet. In most cases the exhaust fan will be located in a central location of the home and will run continuously, so you want it to be quiet. ASHRAE 62.2 calls for continuously running exhaust fan to have a sound rating of 1.0 sones or less. The reason being, if the fan is too loud, the homeowners will be tempted to turn it off - negating the whole reason why it is installed.
Local Exhaust removes high concentrations of contaminants in the rooms where they occur (kitchens and bathrooms). The local exhaust fans can operate continuously at a lower flow rate (CFM), or intermittently (cycled by a switch or timer) at a higher flow rate if less than 1 sone. If operated continuously or on a timer the local exhaust fan flow reduces the whole house exhaust fan flow requirement. ASHRAE 62.2 Local Exhaust can be solved in three ways – On Demand Local Exhaust, Continuous Local Exhaust, or Intermittent Local Exhaust.
On Demand Local Exhaust
ASHRAE 62.2 requires each room that creates contaminants (bathrooms, kitchens, art studios, etc.) must have some type of exhaust that clears the air to the outside of the home. For this scenario a whole house continuous exhaust fan would be located in a central area of the home to meet the requirements for whole house ventilation. In each of the rooms that create contaminants an exhaust fan (or range hood for the kitchen) would be utilized. Exhaust fans need to operate at less than 3.0 sones and typically utilize at least a 4" duct. Air King recommends this scenario because by locating the continuous exhaust in a central location of the home, it maximizes the effectiveness. By using separate local exhaust, it allows the room to be properly ventilated for size and need (a bathroom with a steam shower is going to need more than a powder room). This arrangement is already understood by home owners and installers and requires no new technology or training. Air King has a multitude of fan solutions for local exhaust including fan/light combination units, fire rated units and more. All Air King ENERGY STAR® Certified Exhaust Fans and ENERGY STAR Certified Exhaust Fans with Lights meet these requirements.
Intermittent Local Exhaust
In this option, the local exhaust fan is used to reduce the flow of the whole house fan. The local exhaust fan would be located in the bathroom instead of in a central location of the home and would be on a 24 hour timer switch that would cycle the fan on and off throughout the day. This involves a little more calculating. If the fan will be on less than 35% of the time you’ll need to increase the flow rate 3 times (50=150). If the fan will be on 35-60% of the time you need to increase the flow rate 2 times (50=100) and if the fan will be on 60-80% of the time you need to increase the flow rate 1.33 times (50=67). As an example, if you need 50 CFM of continuous whole house ventilation, you could use a 100 CFM fan for 12 hours a day to obtain the required ventilation. These fans are required to operate at less than 1 sone. A fan used in this manner will also need an override switch to allow the occupant to turn the fan on or off when desired. Careful consideration should be given to the location of the fan and the run time.
Continuous Local Exhaust
In this option, the local exhaust fan can provide some or all of the whole house ventilation requirement. A continuously operating fan would be located in the bathroom in addition to, or instead of, a central location of the home. This would solve both the whole house (diluting the air in the living space) and local exhaust (removing contaminants where they occur) requirement. Dual speed fans are a perfect solution for this application. A dual speed fan allows you to ventilate at a low flow continuously and then jump to a boost speed when you occupy the room such as when the shower is running and additional exhaust is needed to clear the room of steam and moisture. Air King offers a wide range of fans that operate at the threshold of hearing to provide ventilation without disturbing the occupants.
Many indoor air quality problems can be avoided by limiting the introduction of items with the potential for creating problems such as high VOC adhesives. Ventilation is not intended to overcome obvious defects such as a leaky basement; proper repairs are required.
The standard requires either an outdoor vented rangehood (or microwave) sized for at least 100 cfm & less than 3 sones (all Air King ENERGY STAR Certified Range Hoods are appropriate) or a continuously operating kitchen exhaust fan sized for 5 air changes per hour. In a large kitchen this can be a lot of airflow so a vented rangehood operated on demand by the user is a much better choice.
Air King recommends designers follow the prescriptive duct sizing recommendations in ASHRAE 62.2. This results in lower energy usage due to low resistance to flow and strong likelihood that installed performance will match design. Note that 3" ductwork is almost never suitable by these recommendations. Air King has introduced several new products with larger diameter duct connections (50 cfm 4" duct, 80 cfm 6" duct) to make compliance easy.
What is best for me?
For the best indoor air quality Air King recommends the whole house fan be located in a central location and operate continuously and, install On Demand local exhaust fans in each Bathroom and a range hood vented to the outdoors in the kitchen.
Many other details for proper design regarding solid fuel appliances, dryer exhaust, filters, air intakes, instructions for occupants, etc. can be found in section 6, refer to ASHRAE std 62.2 for additional guidance www.ashrae.org.
NOTE: The recommendations made here cover the most common situations and in our opinion represent the best way to meet the intent of the standard, derive the greatest benefit and also balance cost, complexity, operational simplicity, ease of installation and other factors. The ASHRAE Standard provides many other possibilities as well as specific exceptions that might apply to your situation (extreme cold, extreme humidity, etc.). If you have reason to suspect that your situation is outside the norm (steam sauna, indoor pool, high occupancy etc.) refer to ASHRAE 62.2 for additional information or consult a professional for help.
Back to top