Negative Pressure Ventilation can be dangerous in your home!

You may have heard the saying “Build Tight, Ventilate Right” and thought, well, that’s for newbuild homes, I can’t go back and seal my house now, it’s been completed for 30 years!  (or 2 or 5 years for that matter).  This is partly true, but we’ve detailed some ways to seal finished homes in our post here.  The thing is, without the Build Tight, you can still get good air quality if you Ventilate Right for your home. 

There are basic “flows” in the universe that are true everywhere, because energy will seek to equalize, meaning move from areas of high energy to areas of low energy.  Heat will move naturally from hot to cold.  Water will flow from high elevation to lower elevation.  A liquid or gas will flow naturally from an area of high pressure to low pressure.  Let’s examine this third example.  In order for air to move in or out of your house, you will need to have an area of high pressure and an area of low pressure.  Otherwise, there will be no air flow!  Sure, wind blowing around the house will naturally generate pockets of high or low pressure, which may act locally on different external rooms.  For moderately sealed homes with all the windows and doors closed, however, the greatest chance for natural airflow to occur is when a chimney damper is open or poorly sealed, and wind flowing across the chimney draws air up out of your house.  Otherwise, no appreciable airflow is going on unless you turn on a mechanical system.  We discussed this in our post “Do Trickle Vents Really Work?”  This lack of airflow can be good and bad.

The good thing is that pollutants from outside, as well as humidity, are not actively flowing into your home when there are no mechanical systems operating.  This is good because you don’t want these things, anyway.  Here ends the good. 

The bad is that you are not getting fresh air from the outside (which even semi-polluted air can be fresh if it is filtered), and you are not getting bad air from inside (CO2, VOCs and dust) out.  We need active ventilation and it’s mandated for healthy living!  So, we need mechanical systems (fans and the like) to draw fresh air in and get bad air out.

Mechanical systems will always create a pressure differential if they are one-way.  In other words, if you only have an exhaust fan going in your home, it is always pulling air out and there will be a slight negative pressure inside.  If you always have a fan in your open window pointed into the room, it will be bringing air in and pressurizing the room.  If you have fans working in both directions, pressure will be more balanced/neutral, but you will still accomplish the goal of getting fresh air in and bad air out.  In our post “How Does Indoor Air Pressure Affect Ventilation and Air Quality?”  we dove into the pros and cons of different pressure schemes–negative, positive, balanced and balanced with an HRV or ERV, and why we think balanced pressure is the way to go. 

Unfortunately, for most people “ventilation” causes them to think about getting the bad air out only, and this is a problem.  In such homes there is no fresh air supply, turning the house into a stagnant air pool.  When the kitchen and bathroom exhaust fans are turned on or the clothes dryer is started, a slight negative pressure is generated (yep-check out our article on heat pump dryers, where we disclose that regular dryers dump about 200 cfm of conditioned air outside!).  Air can be suctioned from cracks in the building perimeter or worse, from gas exhaust flues.

If your home uses any type of gas appliance, a negative pressure environment can be very dangerous.  Carbon monoxide can quickly overwhelm people in the home, causing injury or death.  “Backflow” or backdraft is airflow going in the wrong direction; in this case, instead of going out the chimney or combustion gas vent, toxic combustion gases can be sucked into the room where the exhaust fan or dryer is located.  This video from Australia shows how to test for a backdraft situation, and correct it by opening a window.  Here are the steps mentioned:

  • Make sure your gas water heater, furnace or other appliance is shut down and cold.
  • Turn on the exhaust fan(s) and dryer in your home.
  • Hold a smoke pen or candle next to the combustion gas vent.  
    • If the smoke rises straight up or is drawn into the vent, then the exhaust fan is not making enough negative pressure to affect combustion gas venting (good).
    • If the smoke moves away from the combustion gas vent and toward the exhaust fan, then a negative pressure situation is developing.   Open a window until the smoke rises straight up or toward the combustion gas vent.  Take note of how far the window must be opened for this to happen. 

There is another device to help avoid combustion gas backdraft; it is called a Power Vent for your water heater.  Essentially, a fan in the vent duct of the heater propels gasses outside, instead of just relying on a stack effect.  Typically, power vented water heaters are installed where it’s not possible to vent vertically.  These types of vent systems are less likely to backdraft because of the vent assist (empirestateplumbing.com), but at the same time, they also draw more air from the space, adding to the negative air pressure problem.

Similarly, you don’t have to be using the fireplace to get harmful combustion residues sucked into your home.  If the chimney damper is not closed or well-sealed, air can be drawn down your chimney across layers of soot and ash, bringing it into your living room.  

Likewise, the walls between an attached garage and your home are usually not airtight, and in a negative pressure situation, car exhaust and fumes from any paints and chemicals stored there can be drawn into the home.

For an eye-opening demonstration on toxins going into a negative pressure home, check out this video!  

  • Smoke going down the chimney: 5:45 minutes
  • Exhaust and fumes from the attached garage: 6:20 minutes

The most dramatic increase in negative air pressure came when the demonstrator closed two interior doors in the simulated house (at about 4:20 minutes).  The pressure dove from -1.7 paschals to 5.7 paschals!  This is the force that will pull combustion gases, chimney smoke, and garage fumes right into your living space.  

Remember, it’s great to get the bad air out, but balance the flow so that you can control what comes in (fresh, filtered air)!  If you only have exhaust fans in your home, open a window to balance that air flow, and keep doors open or install In-Door Return Pathways so that negative pressures don’t increase.