How to fix a bathroom fan that’s pulling in outside air

If your bathroom fan starts pulling in air from outside, it’s not only annoying but also defeats the purpose of having ventilation to improve your indoor air. You might feel cold or hot air drifting in when the fan’s off or notice moisture popping up where it shouldn’t. Usually, this happens due to a simple fix. Let’s see how you can figure it out and get it fixed so your bathroom fan works like it’s supposed to.

The info in this article might not match your exact appliance model. While we aim to give helpful advice for all kinds of DIY repairs, differences in models can mean variations in troubleshooting steps, error codes, parts, and so on. For the most accurate info, we recommend checking your appliance’s manual or reaching out to us if you’ve got questions.

Why your bathroom fan pulls in outside air

Normally, a bathroom fan pushes warm, humid air out of your home. But if you feel drafts coming in through the vent, there are several possible causes:

• Damaged or missing vent flaps: The backdraft damper, which should prevent outside air from entering, may be damaged, stuck, or missing entirely, allowing outside air to flow into the bathroom.
• Improperly installed ductwork: If the ductwork is not properly sealed or has a leak, outside air can easily get sucked in, especially if the fan is located near a vent or opening that leads to the outside.
• Excessive negative pressure: If your home is sealed too tightly or has poor ventilation, the fan could create negative pressure, drawing air from outside. This is more common in tightly insulated homes.
• Air leaks around the fan: Gaps or cracks around the fan housing or duct connections can let outdoor air enter when the fan is running.
• Wind or weather conditions: Sometimes, strong winds or specific weather conditions can force outside air into the exhaust vent, especially if it’s not properly sealed.
• Backdraft from the attic: If there’s a lot of airflow in your attic, the fan may inadvertently draw air in from there, which can include outside air, depending on your ventilation setup.

Troubleshooting steps

Damaged or missing vent flaps

If the backdraft damper (vent flap) is damaged or missing, it could allow outside air to enter your bathroom when the fan is running. Here’s how to resolve it:

1. Ensure the fan is turned off and the power supply is disconnected to prevent any accidents.
2. Use a screwdriver or drill to remove any screws securing the vent cover. Gently pull the cover off to expose the backdraft damper.
3. Check if the damper is stuck, damaged, or missing. If it’s stuck, you may be able to free it by gently wiggling it or cleaning any debris that’s causing it to stick. If the damper is broken or missing, you’ll need to replace it.
4. Install a new damper (if necessary). If the damper is missing or broken, purchase a replacement backdraft damper that fits your fan model. 
5. Once the damper is in place, reattach the vent cover to ensure everything is sealed.
6. Turn the fan back on and check if outside air is still being pulled in.

Improperly installed ductwork

Leaks or improperly sealed ductwork can cause outside air to be sucked into the bathroom. Here’s how to fix this:

1. Disconnect the fan from the power supply to avoid any electrical mishaps.
2. Check the ductwork leading to the fan and the outside vent. Look for any visible holes, gaps, or disconnected sections where air could be leaking in.
3. If you find leaks, use duct tape or a sealant specifically made for HVAC systems to seal the gaps. Ensure the connections between the ductwork and the fan are tight.
4. Inspect the vent opening outside to ensure it’s securely closed. A loose vent cover can also allow air to flow into the ductwork.
5. After sealing the ductwork and outside vent, turn the fan back on to see if outside air is still coming in.

Excessive negative pressure

If your home is too airtight, the fan could cause negative pressure, which pulls in outside air. To fix this:

1. Make sure the fan is off and the power is disconnected.
2. Check for any other ventilation issues, like sealed windows or doors that could be contributing to the negative pressure. Ensure your home has adequate ventilation, such as a fresh air intake or air exchange system.
3. Consider installing a vent or air intake system in your home to help balance the air pressure. This will allow your fan to expel air without creating negative pressure that sucks outside air in.
4. After addressing ventilation, turn the fan back on and check if outside air is still entering.

Air leaks around the fan

Air leaks around the fan housing or duct connections can lead to outside air being drawn into the bathroom. Here’s how to fix it:

1. Disconnect the power supply to the fan.
2. Look around the fan housing for any gaps, cracks, or holes that may be allowing outside air to leak in.
3. Use caulk or weatherstripping to seal any gaps around the fan housing. For duct connections, use HVAC tape or sealant to prevent air from escaping or entering where it shouldn’t.
4. Ensure the fan is properly aligned and secured to prevent future air leaks.
5. After sealing everything up, turn the fan back on and check if outside air is still being pulled in.

Wind or weather conditions

Strong winds or specific weather conditions can force outside air into the exhaust vent. Here’s how to address this:

1. Make sure the fan is powered down before inspecting it.
2. Check the vent outside to ensure it’s properly sealed. Sometimes wind can force open a loosely secured vent cover. If needed, replace or tighten the vent cover.
3. If you’re in an area with frequent strong winds, you can install a wind-resistant vent cover that prevents outside air from being sucked into the fan.
4. Once the vent is secured, turn the fan on and see if outside air is still being pulled in.

What you can do next

Fixing damaged or blocked ducts is usually pretty straightforward. Just make sure you have the right replacement ducts or connectors if the existing ones are bad. You can check out repair help resources on AppliancePartsPros.com if you need more guidance. If you’re not sure which fan part you need to address or replace, check out how to find your appliance’s model number. That way, you can identify the fan model and get the right replacement parts if needed.

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