Can an attic fan affect the gas appliances in your home? Government and building-science sources say exhaust fans can lower the air pressure inside a house. A practitioner source says attic fans can do the same, and that drop in pressure is where attic fan backdraft begins.
Backdrafting happens when a fuel appliance vents its exhaust back into the house instead of up the flue. Exhaust fans, including attic fans in some homes, can contribute by lowering indoor air pressure. Gas water heaters and furnaces are the appliances the sources name most directly. A qualified technician should test combustion appliances before you add an attic fan.
What Backdrafting Means in a Home
Backdrafting is when exhaust from a fuel appliance flows down the chimney or flue instead of up. Natural Resources Canada, a Canadian federal source, describes this as air flowing down the chimney rather than up. Combustion gases can then spill into the living space.
Natural-draft appliances vent through a flue without a fan, so they depend on the flue to carry exhaust out. A drop in house pressure can reverse that flow, which is the backdraft condition.
How an Attic Fan Lowers House Pressure
An exhaust fan removes air, and the house has to replace that air from somewhere. PNNL Building America says exhaust fans that depressurize a home should be evaluated for their effect on combustion appliances. A GreenBuildingAdvisor article, a practitioner source, says attic exhaust fans can end up depressurizing a home’s interior.
Sizing is a separate question from pressure. The Attic Fan Calculator for airflow sizing estimates the CFM a powered attic fan needs. It does not address combustion safety.
Which Appliances Are at Risk
Gas water heaters and furnaces are the appliances the sources name most directly. The PNNL ventilation checklist also lists natural-draft combustion appliances as items to eliminate in a ventilation retrofit.
The EPA lists backdrafting from furnaces, gas water heaters, wood stoves and fireplaces among carbon monoxide sources. EPA’s page covers US homes. Its text does not mention attic fans, depressurization or makeup air.
Attic Fan Backdraft and Carbon Monoxide Risk
A backdrafting gas appliance can allow carbon monoxide into the home, according to a PNNL Building America guide for gas water heaters. That is the health reason backdrafting matters. The link to attic fans comes from the building-science sources, not from EPA’s list.
Backdrafting can carry carbon monoxide into living space. Treat any sign of backdrafting as a reason to call a technician.
How to Check a Water Heater for Backdrafting
A smoke or incense check at the draft hood is the test the PNNL water heater guide describes. Hold the smoke near the hood. The guide says flame or smoke pushed away from the hood means the water heater is backdrafting.
Pressure limits are not settled. A GreenBuildingAdvisor table reads roughly -3 to -7 Pa for spill, while a Building Science snippet cites -2 Pa. Neither figure is checked here against a primary test standard, so this article gives no single number.
GreenBuildingAdvisor, a practitioner source, explains why a worst-case depressurization test matters. It also notes that whole-house fans are not part of the usual combustion appliance zone test.
Sizing Numbers Do Not Test for Backdraft
The airflow numbers from the tool do not show whether a fuel appliance will backdraft. Intake size is not established here as the deciding factor. Use this worked example for the tool’s arithmetic only.
- Base airflow. 1,500 sq ft x 0.7 x 1.00 (light roof) x 1.00 (6/12 pitch) = 1,050 CFM.
- Intake. 1,050 / 300 = 3.5 sq ft, and 3.5 x 144 = 504 sq in.
- Dark roof. 1,050 x 1.15 = 1,207.5. The tool shows 1,250 CFM, the next 50 CFM step above 1,207.5.
- Dark roof, 10/12 pitch. 1,207.5 x 1.20 = 1,449. The tool shows 1,450 CFM.
These are the tool’s arithmetic examples, not a recommended fan size. Neither figure tells you whether your appliances will backdraft.
Run your own roof and floor numbers in the attic fan sizing tool, then ask a technician to test your combustion appliances.
FAQs About Attic Fans and Backdrafting
What is backdrafting?
Backdrafting is when exhaust from a fuel appliance flows down the flue into the house instead of up and out. Exhaust fans can contribute by lowering indoor air pressure.
Which appliances are at risk from backdrafting?
Gas water heaters and furnaces are the appliances the sources name most directly. EPA also lists wood stoves and fireplaces among backdrafting sources of carbon monoxide.
Does a bigger attic intake prevent backdrafting?
No source read for this article shows that intake size decides backdrafting risk. A technician’s combustion test is the check to rely on.
Do open windows stop depressurization?
GreenBuildingAdvisor, a practitioner source, says that assumption fails when windows are closed. Plan for closed-window conditions and have the appliances tested.
How do I check a gas water heater for backdrafting?
Hold an incense stick or smoke near the draft hood. The PNNL water heater guide says flame or smoke pushed away from the hood means the heater is backdrafting.
Is there a set pressure limit for water heater backdrafting?
Not settled. One practitioner table reads roughly -3 to -7 Pa, while a Building Science snippet cites -2 Pa. Neither figure has a primary test standard behind it here.
Do clothes dryers and exhaust fans cause backdrafting too?
PNNL says exhaust fans and clothes dryers pull air out of a home and depressurize it. That lower pressure is the condition that can lead to backdrafting.
Sources and Further Reading
References Used in This Article
- PNNL Building America, Pre-Retrofit Assessment: Ventilation Systems, accessed 2026-11-12.
- Building Science Solutions (PNNL), Look for Signs of a Gas Water Heater Backdrafting, accessed 2026-11-12.
- Natural Resources Canada, Combustion Gases in Your Home: Things You Should Know About Combustion Spillage, accessed 2026-11-12.
- US EPA, Carbon Monoxide’s Impact on Indoor Air Quality, accessed 2026-11-12.
- Peter Yost, Combustion Safety and the Worst-Case Depressurization Test, GreenBuildingAdvisor (practitioner source, not a government page), accessed 2026-11-12.
The safety guidance here comes from government and building-science pages. Have a qualified technician test combustion appliances before you add an attic fan. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated 2026-11-12.
Shakeel Muzaffar is the Founder and Editor-in-Chief, with over 15 years of experience in digital publishing and online tool development. He oversees editorial quality and formula verification to ensure accurate, practical, and easy-to-use calculators.