Pressurization

Maintaining buildings under positive pressure relative to outdoors can help to maintain indoor air quality by limiting the infiltration of outdoor air that may adversely affect thermal comfort and may contain moisture and pollutants. Allowing air to move freely and unimpaired from room to room in the conditioned areas of a home is essential. Balanced pressure helps maintain consistent temperatures without placing abnormal stress on the envelope.

Many normal household items can work against your ventilation system. Some of these include the stove vent, clothes dryer, and bathroom fan. The function of each these is to remove air from the home and push it to the outside. This can create a negative pressure in the home. If the proper amount of make up air is not supplied by the design, the negative pressure in the home will draw the air from uncontrolled areas. The uncontrolled make-up air can be from a crawlspace, cracks in the foundation, leaky window seals, the water heater flue and the chimney. When air is pulled in through the water heater flue, chimney or furnace flue, carbon monoxide is also being pulled into the home, this is called "back-drafting". Uncontrolled air from the crawlspace or foundation cracks can contain radon, moisture, and other contaminants.

The proper system design will take into account household items that could create a back-drafting situation.