Whole House Attic Fans and Carbon Monoxide

Whole House Attic fans can kill you
And your family
If you don’t pay attention

Although they are great during mild weather, whole-house fans have caused fatalities and carbon monoxide poisoning in numerous cases.

The problem isn’t the attic fan — it’s the house!  More specifically, it’s how the house operates.

Gas appliances depend on combustion air to operate and a flue to safely guide the products of combustion to the outside.  If either combustion-air or the flue is inadequate, the gas appliances can and will produce lethal levels of carbon monoxide.

Your home is a big box with holes in it.  Air for you to breath and your appliances to use for combustion comes from infiltration through those holes.  Doors, cracks around and drafts through windows, and other smaller openings, all allow fresh air to enter the home.  As we weatherize and make homes more energy efficient, those holes are blocked up or made smaller.  Fresh air then becomes dependent on how many times the doors are opened.

During cool weather, doors and windows are normally closed and the home’s furnace operates to maintain temperature.  If you run your whole house attic fan for a few minutes to cool down the kitchen after baking, for example, the attic fan can pull much more air out of the house than can enter through an open window.  When that happens, the air pressure within the house goes “negative” and air is sucked down the appliance flues.  This is called flue backdraft.

Because the air that is sucked down the flue contains the products of combustion (is already burned air) there is much less oxygen and more carbon dioxide than normal.  When the “used” air gets pulled into the appliances for combustion air, the water heater and furnace generate lethal amounts of carbon monoxide in a very short period of time.

As the attic fan continues to pull air down the flue, the high concentration of carbon monoxide mixes with the surrounding air and gets re-burned, making the situation worse.  In 15 to 30 minutes the concentration of carbon monoxide may reach very lethal levels (400 to 1,200 ppm).  When that concentration is mixed with ambient air, the carbon monoxide gets diluted, but is still above safe levels.  At this point, it is a matter of exposure time before carbon monoxide poisoning symptoms become evident.

Randolph J. Harris of Fay Engineering describes this carbon monoxide poisoning scenario on their engineering website and explains how quickly an attic fan turned a small party into a tragedy.

Carbon monoxide incidents are usually caused by several factors combined. Very rarely is it caused by the “furnace failure” often noted by the gas company or fire department.

• Teenagers were having a party in a modern house. Due to the excessive cigarette smoke, they turned on the whole-house attic fan for approximately three minutes. Everyone left except for two boys who slept over in the house. One never woke up again. The cause of death … carbon monoxide poisoning.

The teenagers were having their party on an extremely cold winter night, so all the windows and doors were closed tight. The gas appliances in the home had been present for 15 years and were still operating fine after the poisoning. However, when the whole-house fan was turned on, it reversed the flow down the flue pipes; instead of up.

This also caused the furnace burner to operate improperly and produce large amounts of carbon monoxide, which dumped into the living quarters. Within three minutes, the carbon monoxide level throughout the home was 300 ppm. When the attic fan was turned off, the gas burner returned to normal operation. This carbon monoxide concentration was not high enough to cause any ill effects to the party goers by the time they left. However, the concentration was high enough for continued exposure to cause a build up in the blood of the two young men who remained.

The carbon monoxide level in their blood slowly increased until one young man died and the other was put in the hospital. Carbon monoxide incidents are usually caused by several factors combined. Very rarely is it caused by the “furnace failure” often noted by the gas company or fire department. Typically in buildings, most of the factors involve improper installation and/or lack of maintenance. Gas appliances can usually operate fairly well for years until that one time when something changes. This one additional factor is often “the straw that breaks the camel’s back”. The additional factor may allow the incident to occur, but is often not the primary cause.

The moral of the story is to make sure you have sufficient windows open BEFORE you run whole-house attic fans.  This is the only way to insure that your gas appliance flue does not backdraft and cause a carbon monoxide problem.