Furnace backdrafting occurs when combustion gasses, which are supposed to be vented outside, instead re-enter the home. Furnaces, water heaters, and other fuel-burning appliances are designed to vent exhaust gasses like carbon monoxide (CO), nitrogen dioxide, and sulfur dioxide out of the house through a chimney, flue, or vent. However, when backdrafting happens, this process fails, causing these dangerous gasses to flow back into the living spaces. This issue poses significant health risks, including carbon monoxide poisoning, respiratory problems, and fire hazards.
Backdrafting occurs primarily due to negative pressure inside the home. When a house is under negative pressure, air from the outside (including exhaust gasses from combustion) is pulled back into the building instead of being vented out. Homes that are tightly sealed, especially energy-efficient homes with poor ventilation, are more susceptible to this issue. Other appliances such as bathroom fans, kitchen exhaust hoods, or even clothes dryers can exacerbate negative pressure by drawing air out of the house and inadvertently pulling combustion gasses back inside.
Causes of Furnace Backdrafting
- Blocked or obstructed vents or chimneys: A chimney or vent that’s blocked by debris, nests, or creosote buildup will prevent exhaust gasses from properly exiting the home, causing them to backdraft into the living space.
- Negative pressure in the home: Negative pressure is often the main culprit. When exhaust fans (bathroom, kitchen, or even laundry room fans) pull out more air than what is being brought in, it creates negative pressure, which can draw gasses back into the home.
- Improper furnace installation or design: If a furnace or water heater isn’t properly installed, or if the flue design is faulty, combustion gasses may not be vented correctly, leading to backdrafting. Common issues include improper sizing of flues or vents, improper placement of appliances, or inadequate combustion air supply.
- Leaky ducts: Leaky air ducts can contribute to backdrafting by disrupting the airflow in the home and causing negative pressure.
- Changes in atmospheric pressure: Sudden drops in atmospheric pressure due to weather conditions can disrupt proper venting, causing combustion gasses to flow back into the house.
- Downward wind drafts: Strong winds blowing over the top of chimneys or flue outlets can push gasses back down into the house.
Health and Safety Risks
Backdrafting poses severe health risks, the most concerning being carbon monoxide poisoning. Carbon monoxide is colorless, odorless, and deadly in high concentrations. Symptoms of CO poisoning include dizziness, headaches, nausea, and in severe cases, unconsciousness and death. Additionally, prolonged exposure to other combustion gasses like nitrogen dioxide and sulfur dioxide can exacerbate respiratory problems, especially for individuals with asthma or other lung conditions.
How to Prevent Furnace Backdrafting
- Regular maintenance and inspections: Ensure your furnace and other combustion appliances are inspected and maintained annually by a licensed professional. They can clean out vents, chimneys, and ducts to ensure there are no blockages. They will also check for cracks, leaks, or any other potential risks of backdrafting.
- Install carbon monoxide detectors: Installing carbon monoxide detectors throughout your home, especially near bedrooms and in the basement, can provide early detection of dangerous gasses. These detectors should be regularly checked to ensure they are working properly.
- Proper venting and flue design: Ensure that your furnace and other fuel-burning appliances have a proper venting system. A well-designed venting system ensures that combustion gasses are properly drawn up and out of the home. If you’re installing new equipment, make sure that the venting system is appropriately sized for the appliance. Flue design should take into account both the size of the chimney and the capacity of the furnace to prevent backdrafting issues.
- Seal duct leaks: Leaky ducts can create pressure imbalances that lead to backdrafting. Sealing these leaks can help maintain proper airflow and prevent negative pressure from forming in the home.
- Balance the air pressure in the home: Using mechanical ventilation systems such as heat recovery ventilators (HRVs) or energy recovery ventilators (ERVs) can help balance air pressure. These systems allow the home to “breathe” by bringing fresh air in and exhausting stale air out without causing a pressure imbalance that could lead to backdrafting.
- Install a backdraft damper: A backdraft damper is a simple device installed in the flue that allows combustion gasses to escape but prevents them from re-entering the home. This is especially useful in homes prone to wind-related backdrafting issues.
- Create combustion air zones: Ensure that your furnace and water heater are in rooms that have a dedicated supply of combustion air, separate from the air used by the rest of the house. This will reduce the likelihood of negative pressure affecting the combustion process.
- Avoid running multiple exhaust appliances simultaneously: If possible, avoid using exhaust fans, clothes dryers, or other similar appliances at the same time. This will minimize the amount of air being pulled out of the home and reduce the risk of negative pressure developing.
Furnace backdrafting is a serious issue that can lead to dangerous health hazards. However, by understanding its causes and implementing preventive measures like regular maintenance, proper ventilation, and balancing air pressure in the home, homeowners can protect themselves and their families from the risks of backdrafting. Regular inspections, CO detectors, and well-designed systems are the best defense against this potentially deadly problem.
