Depressurization - What is it?

As homes become increasingly air-tight, one issue that needs to be discussed is depressurization.  Essentially, depressurization is a negative pressure that develops when an exhaust device (fan) is turned on in a home.  As the exhaust fan pushes air outside, the inside pressure begins to drop.  The degree of the pressure drop is determined by the size of the house, the airtightness of the house and the size and/or number of exhaust fans running.

Source: HRAI - Residential Mechanical Ventilation


The two large main culprits responsible for creating negative pressures in homes are the kitchen range and clothing dryer.  Exhaust fans that expel less than 150 cfm (bathroom fans) generally do not pose a health or building envelop risk.  When using an Air Exhanger (HRV or ERV) they are intended to operate at a balanced state and therefore do not depressurize while ventilating.


Creating a negative pressure in a home is especially dangerous when there is a wood burning fireplace present.  Wood stoves or fireplaces require indoor air (oxygen) to burn.  When the house is at a lower pressure than outside the chimney, the house can easily backdraft, bringing smoke or CO back into the house. Bringing smoke or (even worse) fire back into a house is very dangerous.


Similarly having naturally drafted combustion appliances in a depressurized home creates the same dangers. These ‘spillage susceptible’ combustion appliances include standard water heaters, mid-efficient furnaces or boilers, gas pizza ovens as they all vent up through a "B" vent chimney.  Each pose the same risk of backdrafting carbon monoxide back into the home when depressurized.


In order to manage depressurization you may need a make-up air system (MUA).  To properly size a MAU, we can perform an HRAI W-3 calculation.

April 13, 2015 by Matthew Pedersen
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Aunt Betty

Aunt Betty said:

Very interesting!


Lance said:

I had air testing done in my house due to allergies. My house air sample tested around 0.02 fiber count….0.01 is a pass…. I had my ducking cleaned and air retested …..failed again …. tester said there is cellulose (blown in installation) in my home air, he doesn’t have a clue where it is coming from but mentioned depressurization ? I don’t have a clue what to do know. I m getting a new furnace Monday with a HEPA filter getting him to check the vents. Any suggestions?

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