Challenges with Exhaust-Only Ventilation

In last month's blog, we discussed ventilation and the three methods for bringing fresh air into the home. You can read that blog HERE. At the end of the article, a comment was made about the recent ventilation code updates in Nashville-Davidson County, which stated: new homes are now required for the first time to be equipped with either a supply-only or a balanced mechanical ventilation system.

That statement needs to be corrected and clarified.

The code does not limit builders to supply-only or balanced mechanical ventilation systems. Exhaust-only is also an option when the exhaust fan can be programed to run for a certain number of minutes each hour, depending on it's flow-rate. However, there are a number of concerns with this approach, which make it not the recommended strategy in Tennessee's climate.

Exhaust-only methods usually rely on leaks in the building envelope as a pathway for make-up air to enter the house. The downside here is that this air is unfiltered and unconditioned, increasing the risk for air quality and humidity issues. In climates like Nashville's with hot humid summers, depressurization of an exhaust-only system can bring moist air into wall cavities where it can create the potential for condensation, moisture damage, and mold growth. 

Another concern with exhaust-only ventilation is that it can draw air pollutants into the home, including things like radon and mold spores from crawlspaces, dust from attics, fumes from garages, and flue gasses from fire places and combustion appliances. This is of particular concern when the bath fan, kitchen exhaust, and clothes dryer (which all put the house under negative pressure) run at the same time. E3 has measured and witnesses this phenomenon in a new construction house.

If the house is very tight, an adjustable passive vent can be used. The concern here though is that the exhaust fans must provide the proper pressure difference needed for the passive vent to work correctly. 

So, although exhaust-only ventilation is still an option, supply-only and balanced systems provide better control over humidity, indoor air quality, and energy use. 

Reference the Whole-House Ventilation System Technology Fact Sheet for more information.