Combustion Air Ducts, Part I: Why Houses Need Them

Have you ever noticed a big insulated tube dropping down next to the floor near your furnace or boiler in the basement?

If you trace this duct down, you'll find that it connects to an opening at the exterior of the building. This is essentially just a hole in the side of the building that brings in fresh outdoor air. Homeowners, builders, and insulation contractors spend lots of time trying to seal up every little air leak in to a house, but then the building code requires this big hole that allows cold air to just dump in to the basement. Silly, right?

I'll try to help make some sense of this.

This opening is a passive intake that provides needed air to the home. There are several items in a home that remove air - here's a partial list of common items found in Minnesota homes that remove air from the house:

The stack effect in a home, wind, and radon mitigation fans may also remove air. The most common and obvious problem with too much air being removed from a house is a backdrafting water heater, but there's a lot more to it than just this.

When air is removed from a house, it has to be replaced. If a house is not built tight, the air will get replaced from every little hole in the envelope in the house; the photos below show a few examples. These are the things that get corrected to make houses "tighter". The first photo below shows an outlet box at an exterior wall that hadn't yet been sealed. Those openings get sealed in new houses today, but this never used to happen.

The photo below shows the furnace vents going through the rim joist. Daylight is visible around these penetrations, which means air leakage.

Of course, windows and doors are also a huge source of air leakage. Daylight showing through is a dead giveaway.

Unsealed openings in the exterior walls equates to uncontrolled air leakage. Every time the wind blows, air will leak in or out through these openings. Even without any air moving at the exterior, the stack effect in a home will cause air to leak in through the lower openings in the envelope of a home, and back out through the upper openings, such as attic bypasses. The image below, used with permission © 2013 E Source, gives a visual example of the stack effect.

The line of neutral pressure plane will be different in every home. Some of the factors that affect this are differences in indoor / outdoor temperatures, wind, the height of the home, and how much air is leaking. For the upper 'positive pressure' leaks, one of the most obvious that can be viewed from inside the house is a loose-fitting attic access panel.

Other attic air leaks, most of which can only be seen from inside the attic, are also major contributors. These include leaks around furnace vents, electrical cables, plumbing vents, chimneys, etc. When air is allowed to leak through the house uncontrolled like this, the amount of air leakage and energy loss is typically much more than it needs to be, and it doesn't happen where, when, or how it should. This can lead to condensation and frost at windows, in the attic, and even inside the walls.

To help reduce the effects of uncontrolled air leakage, houses get sealed up as tight as possible and a single hole is created to bring outdoor air in to the basement, usually right next to the furnace. This is the combustion air duct I showed at the beginning of this post.

I'll have a follow-up post next week discussing the most common installation and maintenance problems with combustion air ducts, and well as the solutions. Of course, I'll have photos of everything.

Special thanks to Steve Schirber at Cocoon Insulation for helping to write this post.

Author: Reuben Saltzman, Structure Tech Home Inspections