What is Backdraft in Your Home and How to Stop It

As depicted in a 1991 Hollywood thriller, a backdraft is a serious matter if a fire ever breaks out inside a building. Fire needs oxygen to burn and actually sucks air toward it, creating a windstorm that helps it spread. This phenomenon is also familiar to firefighters combatting wildfires.

The type of backdraft you might experience in your home doesn’t have to be as devastating as a house fire or wildfire to be a serious concern. HVAC specialist Reid Larson explains: “Backdrafting pulls combustion gases—like carbon monoxide—back into the home instead of venting them safely outside. This can pose serious health risks and and can be life-threatening.”

Here’s how to identify backdrafting in your home and what to do about it.

Why You Should Be Concerned

A “backdraft” in your home is created when an appliance or device uses combustion (an open flame) that isn’t properly vented, allowing the exhaust to be pulled back into the home. This, says Larson, can create a dangerous situation.

Gas range tops and ventless fireplaces produce exhaust levels that are low enough not to require venting; however, this is not true for gas water heaters, gas or oil furnaces, or traditional fireplaces. These must have properly sized and blockage-free flues in order to vent properly.

Making matters worse, even properly installed equipment can backdraft when your home becomes negatively pressurized, meaning that there is air pressure pulling the exhaust into the home rather than pushing it out the vent. “Common causes of backdrafts include improperly installed or sloped venting, blocked or damaged chimneys, or poor airflow in the room where the appliance is located,” advises Larson. “Anything that disrupts the path of exhaust can lead to backdrafting.”

Backdrafting creates inefficiencies in your home’s mechanical systems and an increase in utility bills, but the main danger is carbon monoxide. This combustion byproduct is odorless, colorless and deadly, which is why installing carbon monoxide detectors is so important.

How to Identify Backdrafts

A properly installed CO detector will alert you to dangerous levels of carbon monoxide, but there are also ways to find potential backdrafts before they become a life-threatening problem.

One easy way to test the draw on your exhaust is to light a stick of incense or wooden match and hold it near the flue. The smoke should be wicked up and away. If the smoke isn’t getting out, then dangerous combustion gases aren’t either.

Another warning sign to watch for is hot air around the entrance to an exhaust vent. In this photo, the gasket on the top of the water heater is melting due to prolonged exposure to hot vent fumes pulling back into the living space of the home. If you see something like this in your home, it’s imperative that you figure out the source of the issue as soon as possible.This is why you should always check your furnace maintenance in the winter.

How to Prevent Backdrafting:

Check the flue vents

An improperly sized flue vent (either too large or too small), or one blocked by an animal nest, will not draw the exhaust properly. In such a case, even the slightest negative pressure in the home (such as turning on a bathroom fan) can potentially cause a backdraft.

Make vent inspections part of your home maintenance routine, looking for cracks or breaks in the flue and—if you’re noticing animal activity on the roof—check for nests on the top of your chimneys. (Animals enjoy the warm draft from the exhaust air.)

The exact size of the flue will depend on what vents into it and the height of your chimney. If you have questions about this ratio, or are noticing backdraft and know that a simple line crack or animal nest isn’t to blame, consult a contractor or home inspector to help zero in on the problem.

A home inspector reveals the signs your house is in trouble.

Provide adequate ventilation

A more subtle cause of backdrafts is a home that’s been sealed too tight. Sealing up cracks and holes is a great way to reduce energy bills, but it also inhibits the flow of air through the house. That makes it harder for exhaust fumes to escape. Don’t be afraid to seal up your home, but if you’re planning a major modification to your thermal envelope, work with your venting needs in mind. You may beed to install an energy recovery ventilator (ERV) to keep air moving.

Save money with a home energy audit.

Evaluate changes to the HVAC system

Similarly, if you make changes to your home’s mechanical systems, be aware of the ripple effect this can have. “Orphaned” water heaters are a common source of backdrafting. When a homeowner replaces an older furnace with a more efficient model that doesn’t use the chimney to vent, that often leaves the gas water heater as the only device venting into the chimney. The chimney is oversized for the water heater alone, and the exhaust fumes can’t make it all the way up.

Open windows and doors

Similarly, a large device that draws air out of the home, such as a whole-house fan, can create backdraft if it isn’t installed with ventilation in mind. That’s why it’s essential to open windows and doors during operation, especially if your house is well-sealed.

Backdrafting is a potential hazard, but with a little thought and standard maintenance, it’s a danger that you can avoid.

FAQs

Can backdrafting happen in new, energy-efficient homes?

Yes, says Larson. “Airtight, energy-efficient homes often lack the natural airflow older homes have. Without enough makeup air, combustion appliances may not vent properly—creating ideal conditions for backdrafting.”

Do carbon monoxide detectors help?

Absolutely. Carbon monoxide detectors are essential safety devices. They provide a critical warning if backdrafting causes carbon monoxide to build up in the home—and they can save lives.

About the Expert

• Reid Larson is the co-owner and HVAC specialist at Super Duper Plumbing and Heating. He’s the guy that other HVAC companies call when they can’t solve something.