Study Finds Wood Burners Prone to Spillage
Researchers have discovered that conditions typically found inside a house can cause backdrafting in a fireplace or a wood stove. Tests sponsored by the Bonneville Power Administration showed common household appliances, such as fans and clothes dryers, could create enough suction to increase levels of hazardous carbon monoxide (CO).
Air pressure in houses is complicated, because it's affected by several forces: wind blowing, a forced-air furnace circulating air, a clothes dryer running, and even the gentle force of warm air rising toward the ceiling on a cold day. When the pressure inside a house is greater than outside, it's called positive pressure. When the pressure inside is less than outside, it's called negative. Pressure is seldom uniform throughout the entire building. It can be strong in some areas, weak in others, or positive in some rooms and negative in others.
Any device that pulls air out of a house can create negative pressure. These include bath fans, range hoods, clothes dryers, woodstoves, fireplaces as well as combustion furnaces and water heaters. Downdraft cooktop fans often move 200 to 400 cubic feet per minute of air, generating strong negative pressure.
More homes now contain automatic ventilation systems. The simplest systems&emdash;and the most popular&emdash;use an exhaust fan to expel stale indoor air. This places a slight suction on the house that draws fresh air in through wall or window vents. Negative pressure is measured in units called pascals. Fifty pascals is a standard pressure used in blower door testing. It's about the same as a 20-mph wind or 0.2 in. water gauge. One pascal has been described unscientifically as the wind from a butterfly's wings. In other words, not much.
Airtight Wood Stoves?
"Wood stoves and fireplaces can be strongly impacted by negative pressure," said Mark Jackson of the Bonneville Power Administration (BPA). He based this statement on results of research he supervised. BPA contracted with Science Applications International Corporation, an independent laboratory, to conduct tests in the lab and in the field.
In the lab tests, high-quality, "airtight" wood stoves were placed in a sealed room that was maintained under a slight negative pressure. Tests were conducted at minus 2 and minus 8 pascals. CO concentration was measured for an entire burn cycle. Increases in CO indicated that flue gasses were spilling into the test chamber.
In 12 of the 19 tests performed, CO concentration exceeded 100 parts per million (ppm) at some time during the cycle. (For comparison, the Environmental Protection Agency ambient air standard for CO is 9 ppm, and the Occupational Safety and Health Administration 8-hour workplace standard is 50 ppm.) The highest concentration (2997 ppm) occurred when inside combustion air was used and the air inlet was open only 15 percent. The CO concentration exceeded 100 ppm even during seven tests when the stove was connected to outside combustion air.
CO leaked when the negative pressure in the test booth exceeded the draft created by the normal operation of the wood stove. Elevated CO levels generally occurred an hour or more into the burn cycle, when the fire started to die down. The flue temperature fell causing the draft to weaken. Flue gases&emdash;high in CO now that the fire was dieing down&emdash;were pulled into the test chamber even by slight negative pressure.
If the wood stove and flue were entirely airtight, there wouldn't be a problem. But stoves aren't entirely airtight. Leaks were measured around the door, at secondary air inlets and at flue joints. On one stove, a flue collar connection leaked significantly. Plus, the door must be opened to load fuel wood. Changes in manufacturing and installation could correct these leaks.
Two homes were selected for field tests. One had an EPA Phase II certified wood stove and the other had a fireplace with glass doors. Both homes had forced-air furnaces and were built in the late 1980s. Inside air pressure was measured under various conditions (fans on and off, doors opened and closed). A fire was burned while air pressure, stack pressure in the flue and CO were monitored. Like the lab tests, negative pressure induced backdrafting, which pulled CO into the house.
In the wood stove house, average house pressure dropped as low as minus 5 pascals and the CO concentration peaked at 120 ppm. In the fireplace house, the lowest average house pressure reached minus 7.5 pascals, and the CO peaked at 80 ppm.
The research report concluded that wood stoves with either inside or outside combustion air supplies experience leakage of flue gases into the living space when the negative pressure is minus 8 pascals. Minor leakage occurred under negative pressure of minus 2 pascals. Typical homes experience negative pressure conditions strong enough to draw flue gases from wood stoves and fireplaces.
"The only factor that matters is negative pressure," said Jackson. Many details about a house can affect air pressure, but it's negative pressure that causes backdrafting to occur.
Wood stoves, fireplaces, gas water heaters and furnaces and other combustion appliances aren't intended to be installed in a negative pressure environment. Unfortunately, few installers are equipped with the knowledge or the tools to test pressure conditions or to fix problems.
The study supports the need for more information about the pressure dynamics inside homes, especially homes with combustion appliances. And these limited tests give a glimmer of what further research may reveal.
This article appeared in Energy Source Builder #40 August 1995