Combustion air for furnaceGas appliances are common in many homes and provide warm air through furnaces, hot water through gas water heaters, and other appliances such as gas clothes dryers. However, unlike electric appliances, gas appliances require combustion air in order to operate properly. If an adequate amount of combustion air is not provided, this can cause a significant hazard to occupants within the home.

The combustion process requires both adequate fuel and oxygen to burn and without both of these items, flames will burn improperly. A side effect of improper combustion is carbon monoxide (CO), an odorless, colorless gas that is formed by the incomplete combustion of fuels. Not to be confused with carbon dioxide (CO2), carbon monoxide can be potentially lethal, especially when in an enclosed space. With an average of 430 deaths and over 50,000 emergency room visits in the United States each year, carbon monoxide poisoning is a serious hazard in any home that has a gas fired appliance.

When a gas furnace or water heater is operating properly, products of the combustion process are vented out through a combustion vent to the exterior. When the gas is burned inside the appliance, the combustion products are pushed up and out through the flue, removing it from the interior of the home. While newer direct vent furnaces and water heaters draw combustion air from the exterior, standard vented appliances draw combustion air from within the home. Direct vent appliances typically use two PVC pipes to draw combustion air and expel flue gasses, while standard efficiency appliances use a metal flue.

Combustion air for water heaterFor standard efficiency gas appliances, combustion air is drawn from the interior of the house and require a certain amount of combustion air, which is directly correlated to its BTU per hour (BTU/hr). BTU stands for British Thermal Unit and is a measurement of energy in natural gas. The higher the BTU per hour rating that an appliance has, the more combustion air it requires for proper operation. For most combustion appliances, 50 cubic feet per 1,000 BTU/hr is required for a space to have adequate combustion air. This is particularly important if there are multiple combustion appliances in the same room, such as a gas furnace, gas water heater, or gas dryer.

If a room is too small to contain adequate combustion air for these appliances, vents can be added to the door or wall to draw air from other spaces. A common solution is to install a louvered door to the room to allow air to be drawn into the combustion appliance room. A fully louvered door can work if the combined input rating is less than 175,000 BTU/hr. Other options exist as well, however combustion air must never be drawn from a bedroom or bathroom. Consulting with an HVAC contractor is recommended whenever changing or replacing combustion appliances to ensure that adequate combustion air is available.

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