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Negative Pressure and Backdrafting

Last updated September 24, 2026 · Monitor My Air

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The simple idea

Air always moves from areas of higher pressure to areas of lower pressure. When you run a strong exhaust fan, a clothes dryer, or a range hood, you are pumping air out of the house. That air has to be replaced by an equal amount coming in from somewhere. If your house cannot pull in enough replacement air easily, the whole home drops into a slight vacuum. That is negative pressure.

Imagine sucking on a juice box. Pull hard enough and the sides cave in because you are removing more than can flow back in. Your house does the same thing, except instead of caving in, it finds sneaky paths to pull air through, including some paths you really do not want it using.

How it plays out in your home

Under negative pressure, your house grabs replacement air wherever it can: through crawlspace vents, wall cracks, and, most dangerously, backward down the flues of gas appliances. When exhaust gases that should go up the chimney get pulled back down into the living space instead, that is called backdrafting.

• A powerful kitchen range hood running while a gas water heater fires can reverse the water heater's flue.
• The reversed flue can pull carbon monoxide and combustion moisture into your home.
• Negative pressure also drags in humid outdoor air and damp crawlspace air, feeding mold.

This is not a rare edge case. Modern homes are tighter and modern exhaust fans are stronger, so the imbalance is easier to create than it used to be.

The takeaway

Every fan that pushes air out of your home creates a pull somewhere else. Left unmanaged, that pull can reach into a flue and bring combustion gases and moisture back inside. Respect the balance between air leaving and air entering, protect your combustion appliances, and negative pressure becomes a manageable force instead of a hidden hazard.

What professionals know

Backdrafting is a depressurization failure of atmospherically vented combustion appliances, which rely on buoyancy-driven draft to expel flue gases. Their draft is weak, often only 1 to 3 pascals, so even modest house depressurization can overwhelm it. Building science guidance generally holds that worst-case depressurization at the appliance zone should stay under about 3 to 5 pascals for atmospheric appliances; beyond that, spillage and flue reversal become likely.

Exhaust devices readily exceed this. A large range hood can move 600 to 1200 CFM; a downdraft cooktop or a clothes dryer adds more. In a tight home with limited leakage area, removing several hundred CFM can drive the enclosure to negative 5, 10, or more pascals, sufficient to reverse a water heater or furnace flue. Combustion spillage introduces carbon monoxide, NO2, and water vapor directly into occupied space.

The engineered solution is makeup air: interlocked, sometimes tempered, outdoor air supply sized to offset large exhaust flows, required by code above certain hood ratings. Sealed-combustion (direct-vent) and power-vented appliances are inherently resistant because they draw combustion air from outside and use a fan to force flue products out, decoupling them from house pressure.

Diagnostically, this is verified with a worst-case depressurization test: all exhaust equipment on, house closed, interior doors positioned to isolate the combustion appliance zone, then measuring zone pressure with a manometer and confirming draft with a smoke pencil or draft gauge. Persistent negative pressure also raises radon entry and increases latent moisture load by pulling humid infiltration air across the envelope.

Related answers

• The Stack Effect: Why Air Moves Through Your Home
• Ventilation Basics: Getting Fresh Air Without Wrecking Your Air

Education, not medical advice or a professional inspection of your property.