All fossil fuel burning appliances need ample air intake and draft to complete the combustion process in a safe and efficient manner. The most common fossil fuel heating systems we see in Colorado are forced air furnaces fueled by natural gas. Some homes have multiple furnaces and water heaters all requiring ample amounts of combustion air. Whether one or multiple, including gas boilers, the same rules apply.
So, what is combustion air? According to the Housing Industry Research Center, the definition is as follows: (http://hisearch.org/C.aspx)
Combustion Air - (1) Air that is supplied to combustion appliances to be used in the combustion of fuels and the process of venting combustion gases. Inadequate combustion air can lead to dangerous problems. (2) The duct work installed to bring fresh, outside air to the furnace and/or hot water heater. Normally 2 separate supplies of air are brought in: one high (for ventilation) and one low (for combustion).
Depending on the year of construction and the style of the dwelling, there are several ways in which combustion air is provided to the home. This can be through crawlspace vents for crawlspace appliances, exterior wall vents that allow for air supply through metal ducting for basement and main levels, or simply roof, gable, and soffit vents for attic installations.
What Xcel Energy didn't mention specifically, but implied in it's warning to consumers, is exterior vents are also commonly obstructed with leaves, plastic bags, bird's nests and lint. Frequently, interior vents have been blocked with sweatshirts, towels, and rags by the homeowner because of the "cold" coming into an otherwise comfortable living space.
Please take a moment and locate the vents that supply your gas appliances making sure they are freely open, unobstructed, and able to do the job they were designed to do.