Make-Up Air Ventilation
by Building Inspector and Indoor Air Specialist, Dan Schilling
© Copyright 2002 Residential Inspections LLC, All Rights Reserved
What is make-up air?
Make-up-air is fresh, outdoor air, which is brought into a home or building in a designated fashion, on an as-need basis, through a specially installed vent pipe. Make-up-air prevents dangerous back-drafting of fuel burning appliances and infiltration of outdoor air in ways that could prove detrimental to a house or to our personal comfort. Make-up-air vents also improve indoor air quality and reduce sick building syndrome.
In many new construction projects, the installation of make-up-air vents is a code requirement. Until recently, no homes or buildings were built with make-up-air vents, albeit, we now know the serious consequences that can occur when make-up-air is not provided indoors. The problem revolves around negative air pressure in a home. Negative air pressure occurs whenever air is being sucked out of a building without a way for replacement air (make-up-air) to re-enter. Every time you operate a bathroom or kitchen exhaust fan you are sucking air out of your home. The same is true whenever furnaces, water heaters, fireplaces, whole house fans and clothes dryers are being operated. These appliances can suck from 50 to 1450 cubic feet of air per minute out of your house. If your house were a balloon, it would get smaller each time one of these components was operated. Your house cannot contract like a balloon, so the result is negative indoor air pressure.
Negative air pressure can literally poison your indoor environment. Negative air pressure can cause poisonous radon gas infiltration by actually pulling it out of the earth from underneath your house. Radon is the second leading cause of lung cancer following smoking. Negative air pressure can also poison your indoor air with carbon monoxide and other combustion byproducts by causing back-drafting. Back-drafting is a term used to describe reversed air flow down chimneys and vent pipes of fuel burning appliances like furnaces, water heaters and fireplaces. Combustion pollutants that would normally migrate naturally up and out of chimneys or vents are instead sucked back into the house. The carbon monoxide levels retained in the house are typically not high enough to set off an alarm, but they are nonetheless poison and may cause occasional headaches, fatigue and flu-like symptoms, as well as, long term health problems. Finally, back-drafting can waste fuel through inefficient combustion and cause premature corrosion of chimneys, vent pipes, and the interiors of furnaces and water heaters.
Evidences of back-drafting are seen often by home inspectors. These can include stains at the draft doors and hoods of furnaces and water heaters, as well as, soot stains above fireplace openings. These evidences are a clear indication of what the occupants of the home have been inhaling. Sometimes the evidences are quite visible but the lack of visual evidence does not preclude the need for precaution.
The solution is to provide a regulated amount of fresh air into your home with the use of a make-up air vent. After installation, these vents work automatically on an as need basis. Whenever the indoor pressure drops, the vent allows fresh, oxygenated air to re-enter the house. When possible, the temperature of this incoming air is also tempered through a long run of ducting, or within the air return duct preceding the furnace and/or air conditioner.
The inlet is located outside the house like a clothes dryer vent, only designed to allow air in. While many heating contractors install these vents with barometric or spring loaded dampers, I do not recommend them because they only open when the blower is running. A make-up air vent should not be restricted by a barometric or spring-loaded damper so it will function correctly whether or not the blower is running.
With so many ways air can be exhausted from a home, the best way to know for sure that you will not be affected by negative indoor air pressure is to have a make-up-air vent installed. Make-up-air maintains balanced air pressure and will improve the overall air quality indoors.
If you already have a make-up air vent, be sure there is no damper inside and if there is, secure it in the open position. Also keep the exterior air inlet clean.
If you do not have a make-up-air vent, call a heating and cooling contractor to have one installed. It will be a small investment for the benefit.
with those you care about.