The use of fire has been an integral part of human evolution and civilization throughout the ages. As humans were able to learn how to control fire, we were able to spend more time during the darker hours of the day on cultural development. The use of fire enabled people to have some downtime after a busy day of hunting and gathering to create art and spend time with each other. To this day, combustion plays an important role in multiple aspects of our life.

Water heater combustion airWhen it comes to combustion in a home, we use it for cooking, heating, and also for ambiance and comfort, such as with wood-burning and gas fireplaces. Unfortunately, the use of combustion appliances in homes has contributed to a lot of fires and deaths due to carbon monoxide poisoning. As time passed, technological development brought about safer combustion equipment such as sealed combustion appliances, but there are still many homes with combustion appliances that rely on the oxygen in the home for proper combustion and draft of exhaust gasses. This means that when gas or wood is burned in a home, there needs to be enough combustion air to ensure the safe use of appliances that burn these types of fuel.

Whenever combustion occurs, oxygen is required. The amount of oxygen that is necessary for proper combustion is based on how much fuel is being used at any given time. All combustion appliances will have a rating for how much fuel they are designed to use. This rating is measured in BTUs, which is the acronym for “British Thermal Units”. A BTU is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit and is equivalent to one kWh of electricity. 1 BTU can also be described as the approximate amount of heat created by burning one wooden matchstick. You can find the BTU rating for combustion appliances such as gas water heaters and furnaces on their data plates (stickers) adhered to the equipment.

Determining the amount of combustion air needed for a gas appliance also depends on the type of appliance. Modern gas appliances are called “sealed combustion” appliances, meaning that the combustion chamber (where the fire happens) is sealed from the inside of the house, and the required combustion air is pulled through a pipe from the outside. You can tell if your gas furnace is a sealed combustion appliance if it has two plastic pipes connected to it. One pipe serves as the air intake, and the other as the exhaust flue. These types of furnaces also have a higher efficiency rating and are usually 90% or more efficient. Other appliances that are often sealed combustion appliances are tankless water heaters and direct vent gas fireplaces.

Combustion air for furnaces and water heatersThere still are many homes that still have furnaces, water heaters, and fireplaces that do not have sealed combustion chambers, and they rely on the oxygen in the home for combustion. These appliances often have metal flue pipes that are routed up and through the roof. Since these types of appliances rely on oxygen for proper combustion, a calculation is needed to ensure that there is enough combustion air. We do this by determining the BTU capacity of the appliances and then comparing that to the volume of the space of the room that they are located in.

Combustion appliances require 50 cubic feet per 1000 BTU/h aggregate input. “Aggregate” means that if two combustion appliances such as a gas-fired furnace and a gas-fired water heater are installed in the same room, each of their input ratings in BTU/h must be included when calculating adequate combustion air. Here is an example:

40,000 BTU water heater and 80,000 BTU furnace

40,000 + 80,000 = 120,000

120,000/1000 = 120

120 x 50 = 6000

In this example, the minimum size of the room that the water heater and furnace can be located in should be at least 6000 cubic feet. So that would be a room with a square footage of 750 sq ft and a ceiling height of 8 feet.

When gas appliances that are not sealed-combustion are installed in a space that is too small, such as a utility closet, or when someone remodels a basement and builds walls and a door around the appliances, the appliance will not combust properly and exhaust gas can be pulled back into the home. This is referred to as back drafting and is a major cause of carbon monoxide poisoning and death in homes. When an appliance is sealed combustion, the volume of space that the appliance is located in is not a concern (unless the air intake is not routed to the exterior).

For most people, the consideration of whether there is enough combustion air for any given appliance in a home is not exactly within their purview. That is why it is important to have a home inspection when looking to buy a home and to have licensed HVAC contractors assess combustion appliances to ensure that safe conditions exist.

Check out the video below to find out more about combustion air concerns in a home.