Wind can cause a naturally vented furnace to lose the chimney effect, which is the process that enables combustion gases to exit through your chimney. Small holes and cracks in your chimney or your home can draw outside air in as a result of this process, but, most often, exterior winds blowing directly against the chimney can alter the interior air pressure. This changes the direction of the combustion gases.
Updrafts and Backdrafts
Naturally vented furnaces and other fuel-fired appliances rely on the chimney effect to vent combustion gases outside. Exhaust gases are warmer than the surrounding air, and this difference in temperature creates a natural updraft in the chimney, which allows the gas to escape safely outside. Clothes dryers, gas or oil water heaters and bathroom or kitchen exhaust fans can make significant demands on the air supply in your home because they move air outside, creating a negative interior air pressure. Wind plays a significant role in this process, as well, because if it is blowing directly against the chimney or venting system, it creates a positive pressure outside. When the demand for outside air is greater than the amount of air in your home can provide, depressurization occurs, and your chimney has to work against the suction created. The negative air pressure in your home draws the positive pressure outside in, and the flow of gases reverse, causing a backdraft.
Signs of Draft Issues
Backdrafts or combustion air issues in your home show in several ways. A fuel smell inside your home, or delayed ignition of your furnace, indicates that winds may be coming in as the result of a backdraft. Soot accumulation around your vents, black smoke from your chimney, soot or smoke leaking from the door ports on the furnace, or noises such as pops, bangs or rumblings are also indications of venting issues.
Backdrafts should be addressed immediately because, although exterior conditions can greatly contribute to their occurrence, they usually indicate a problem with your furnace venting system. This poses health risks for you and your family. Fuel-fired furnaces produce water and carbon dioxide, and if these do not have enough oxygen from the combustion process, carbon monoxide forms and is forced into your home by the backdraft. Natural gas furnaces produce a lower concentration of carbon monoxide initially, but as it is recycled with the intake air, the amount that escapes into your home increases. Gas or propane furnaces also produce nitrogen dioxide, which can damage and scar your lung tissue -- even in small amounts.
A neutral air pressure in your home is critical to preventing backdrafts and to ensuring that the wind’s direction or force won’t affect the proper venting of your furnace’s combustion gases. Make sure that all combustion appliances, such as wood stoves, gas or oil-fired water heaters, have their own venting system, or outside combustion air supply. Have your chimney and furnace inspected yearly to ensure that each works and is sealed properly. Reduce or limit the use of exhaust fans or other high-powered exhaust systems when operating your furnaces, unless they have their own makeup air supply. Try to start fuel-burning appliances like furnaces or fireplaces when no other appliances such as a dryer are moving air from your home. If you establish the draft before turning these on, it is less likely to change direction and backdraft.
- Photo Credit Jupiterimages/Comstock/Getty Images