Carbon Monoxide - Why is CO Staying in Houses?

If you have a woodstove or an oil furnace that backdrafts, you're probably aware of it. Wood smoke is visible and fuel-oil furnaces provide a telltale whiff of sulfur to the nose. Unfortunately, gas appliances don't offer such ready hints. In any event, if backdrafting has been occurring often in fossil-fuel burner, there should be some signs on the appliance itself. On an oil-burner, look for soot smudges around the draft control (a T in the exhaust from the furnace that has a free-swinging flap). If they're extensive, you've been having at least some spillage. On a gas furnace or water heater, look at the outlet from the unit. There will be a cone-shaped hood there that admits extra air to the chimney to stabilize draft, and soot will show up around it if the appliance has been backdrafting significantly. Even if you find no signs that your furnace or water heater has been backdrafting, it's a good idea to perform the simple test described in the sidebar. [ed: located after this article]

If one or more of your combustion appliances is backdrafting for longer than about 30 seconds at start-up, you should fix the problem. As long as the eater is correctly tuned, the situation is not life threatening. But poor air quality may be already causing health problems - sore throats, irritation to the nostrils and eyes and asthmatic symptoms - and may be making the indoor environment overly humid. If you ignore the problem, the performance of the burner will deteriorate, and the situation could well become deadly.

Experts take two approaches to solving backdrafting problems. One method goes after the mechanism of backdrafting, the other eliminates sources. The potential for backdrafting can be controlled by balancing indoor and outdoor air pressure with a fan that pushes air into the house. Jim White points out that air will get into the building anyway, so it may be best to heat it and push it in intentionally.

Source control, as advocated by Lstiburek and Nelson, is done by a procedure called aerodynamic uncoupling. Fortunately, this isn't as complex as it first sounds. It simply means that anything that burns fuel gets its supply air from the outdoors and dumps its waste back out there. For the purposes of combustion, the furnace, water heater, gas dryer, and woodstove or fireplace all operate as if they were outdoors. The first three can be exhausted with a power venter, which can be plumbed to handle all of the burners at once. Fireplaces, if they're used, should have outside air inlets inside the fire chamber and well sealed glass doors. Woodstoves are difficult to retrofit with outside air supplies, but models built for mobile home come so equipped. One advantage of utilizing aerodynamic uncoupling over pressure neutralization is that major exhaust devices (like indoor barbecues) are no longer a problem, since there is no pollution to be drawn in, In addition, in colder climates, infiltration (air moving into the house through the walls) is preferred to exfiltration (air moving out through the walls) because water vapor in the indoor air can cause problems if it condenses inside a wall.


Though sophisticated instruments are required to accurately measure the potential for backdrafting in a house, you can perform a worst-case test of your own with nothing more than your finger as a test instrument. There are two stages to the test, the procedures for which we've adapted from recommendations by CMHC, Gary Nelson, and Joseph Lstiburek.


Close all outside doors and windows. Shut the door between the furnace room and the rest of the building, and turn on the furnace fan (no need for the burner). Using a portable smoke source (an incense stick or cigarette) inside the furnace room, look for air movement under or around doors. If the smoke trails toward the furnace, you've got return ductwork leaks that need repair.


On a mild, still day, close all exterior doors and windows. At a time when the furnace hasn't run for several hours, turn on every exhaust device in the house (kitchen and bath fans, dryer, water heater, attic and whole-house fans) and open all possible doors between the furnace room and the rooms where the exhausts are located. Keep all other interior doors closed. Go to the furnace room and have someone else turn on the furnace at the thermostat. Feel at any draft control on an oil furnace or the draft hood on a gas furnace for the heat of a backward flow in the chimney. Redo the test on gas water heaters and gas dryers after they've had a chance to cool thoroughly. If spillage continues for longer than about 30 seconds on any combustion appliance, you could have a problem.

"BACKDRAFTING YOUR LAST GASP?", pp. 92-96, Mother Earth News, November/December 1988

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Expert Advice
Carbon Monoxide Detectors w/ PPM Displays and Battery Backup

Although all home carbon monoxide detectors use an audible alarm signal as the primary indicator, some versions also offer a digital readout of the CO concentration, in parts per million.

Typically, they can display both the current reading and a peak reading from memory of the highest level measured over a period of time.

The digital models offer the advantage of being able to observe levels that are below the alarm threshold, learn about levels that may have occurred during an absence, and assess the degree of hazard if the alarm sounds.

They may also aid emergency responders in evaluating the level of past or ongoing exposure or danger.