Choosing and Using a Ventilation System

One of the first steps in ventilation system design is to determine the capacity needed. The best guidelines currently suggest that a general ventilation system should be able to provide a continuous air- exchange rate of either 15 cubic feet per minute (cfm) per person or 1/3 of an air change per hour (ACH), whichever is greater. If the average occupancy of a house is 4 people, then 60 cfm (4 x 15) of continuous ventilation should be sufficient.         

To determine ACH, you first need to calculate the volume of the house. A 1,200 sq. ft. house with 8' high ceilings will contain 9,600 cu. ft. (1,200 x 8). One-third of an air change per hour would be 3,200 cu. ft. per hour (9,600 ÷ 3). This translates into 53.3 cfm (3,200 ÷ 60) of continuous capacity.       

Sometimes it’s advantageous to oversize a ventilation system. For example, a system may be sized for the needs of three occupants, but what happens when there are a dozen family members over for dinner? The solution can be as simple as installing a higher-capacity system and running it on slow speed when there are only a few people at home, then high speed when company arrives.        

Many ventilation systems are designed for intermittent operation. For example, suppose a neighbor’s wood smoke is a problem at night during the winter months. You may choose to only operate your ventilation system for 8 hours a day when the outdoor air is clear. In such a case, the capacity of the system should be sized to overventilate when it’s running. If this is done, the average 24-hour rate should meet the above recommendations. Of course, if a house is unoccupied during the day—when people are at work or school—the average ventilation rate can be reduced accordingly. These factors should all be taken into account when determining the equipment capacity.        

The 15 cfm and 1/3 ACH figures were arrived at by placing a person inside a closed room (made of fairly healthy materials) and exchanging the air in the room at different rates. Average people off the street were asked to stick their head in the room and report how fresh it smelled. For 80% of the people, an exchange rate of 15 cfm or 1/3 ACH was enough for the room to smell fresh. At lower ventilation rates, many people said the room seemed stuffy. Their reactions were not to pollutants typically found in houses because the room was made of fairly healthy materials but, instead, to the metabolic by-products released by the person sitting in the room. So, these guidelines are in actuality a 'body- odor standard', not a health standard.         

If a house is bu...