Bath Fans and the Importance of Certified Airflow

Should a company manufacture products that perform as tested in ideal conditions or conditions based on real-life experiences? This conundrum is an age-old debate. For example, would you buy a new vehicle that advertised its fuel efficiency based on miles per gallon calculated while it was rolling downhill?

Many products, including some bathroom exhaust fans, have a design based on optimal conditions that include the perfect installation. Unfortunately, life seldom includes words like optimal or perfect. The result of most applications often is different. It is possible but happens infrequently.

The Dirty Little Secret of Bath Fan Installations

Take installing a bathroom exhaust fan and ductwork-- it is possible to achieve optimal performance--just not likely. Let’s say that an engineer specified a short run of six-inch diameter galvanized ductwork for use with the bath fan installation. The engineer did not know another trade (plumber or electrician) would interfere with the ductwork layout.

Not anticipating that situation turns a straightforward installation into one that uses a four-inch diameter flex duct with a longer linear length and elbows for the directional turns that were not in the original plan. These changes in the installation often have a dramatic effect on airflow resistance and overall fan performance.

The Reality of Static Pressure in the Real World

Engineers typically specify a static pressure of 0.10 or 0.25 inches. Actual installations commonly have a static pressure range of 0.40 to 0.50 inches. This dramatic difference begs the question: why do most residential grade ventilation manufacturers only certify their performance at 0.10” or 0.25” static pressure? Bath fans only capable of achieving that level of performance will underperform in most installations.

Underperforming bath fans contribute to foggy mirrors, odors, mold, and mildew. Any of these issues could result in a costly callback or worse. The total financial impact easily adds up to $450 per callback when including labor and lost revenue. More problems can result if the issue becomes a long-term challenge.

Table 1 shows the performance of a well-known bath fan currently sold to customers. This fan model only catalogs to a maximum of 0.25 inches of static pressure. Since airflow resistance often is closer to 0.40 to 0.50 inches of static pressure in most installations, it’s unknown how much air, if any, this particular fan will move at 0.40 to 0.50 inches of static pressure; since the static pressure exceeds its maximum range. This unknown performance issue at installed conditions is only one challenge.

Table 1

Ceiling Exhaust Fans Indoor Air Quality