Depressurization/make-up air issues

One of the issues to consider when adding a range hood, or replacing one, is whether it will cause depressurization problems in the house. If the exhaust flow of a range hood (or any other fan) causes high levels of house depressurization, there are some possible negative consequences:

  1. Backdrafting or spillage of vented combustion appliances, including fireplaces and woodstoves
  2. An increase in radon entry from the soil
  3. An increase of air entering the house from attic spaces or wall cavities
  4. Drafts from the outside

The bigger the fan and the tighter the house, the more depressurization that is likely to occur.

Have you encountered this in your work? Have you seen instances where a newly installed range hood requires some monitoring or the provision of make-up air? What levels of depressurization have you seen created?

Back when I was testing houses in the eighties and nineties, we saw several houses exceed 20 Pa of depressurization. Are we seeing that today?

The gradual disappearance of natural draft appliances (those with chimneys) and the availability of consumer carbon monoxide alarms have reduced some of the dangers.

We would be interested in hearing about your experiences.

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  • Don: thx for the excellent questions, as usual ! (hope this is not redundant; it is my second try in commenting on this)

    re: combustion safety, I would stress source control as the first and most reliable measure (remove or minimize the indoor & outdoor sources of pollutants).  CO alarms are a  limited backup strategy (for extreme CO hazards).  CO alarms are a good idea if any combustion appliance is present, but would not be a good indicator of more subtle depressurization effects.  This is based my experience developing the CARB IAQ guideline on combustion pollutants and working on the CPSC CO alarm task force.

    The same logic applies to source control for radon, attached garages, etc.

    re: make up air, I am recommending pressure testing the installed supply fan system to verify that problems are avoided (and of course, avoiding real high flow fans).   This info will be in the upcoming update to the ROCIS range hood best practice guide at

    See slides 34-37 in my Healthy Kitchen Ventilation / NAPHN 2018 presentation at the bottom of that page for images & links to Dan Cuertin article (Fine Homebuilding) and European research results & recommendations. We have not recommended passive make up air systems


    Kitchen Range Hoods |
  • This is an old query that you had made six months ago,  but it is worth commenting on that second to the last paragraph.

    "The gradual disappearance of natural draft appliances (those with chimneys) and the availability of consumer carbon monoxide alarms have reduced some of the dangers."

    Consumer carbon monoxide alarms follow UL 217 and 2034... they typically DO NOT sound off unless the carbon monoxide levels are at levels that are already dangerous.  They take both exposure time (minutes/hours) and the ppm of CO and trigger if it exceeds a level that changes with time.   In general they will sound an alarm if the CO exceeds roughly 100ppm  (that's roughly).   But much lower levels of CO can cause adverse health problems if those levels are 30ppm not for minutes or hours... but for days, weeks or months.    Small children or individuals that already have respiratory problems are already at risk from those even lower levels.   Here is some reading material :

    I have perhaps another 100+ references with much more detailed info, the key on the items above is to note that CO exposure that is chronic long term - can be at VERY low levels that do not trigger many of the off the shelf CO detectors.  You need to look for detectors that can detect low levels.

    Next it's important for the members of the house to REALLY understand that if a CO detector goes off,  it means grab your ID, cellphone, car keys and get out of the house FIRST.  If multiple alarms are going off -once you are out of the house contact the fire department.  They have the respiratory gear to enter safely AND vent house if there is a risk.   VERY FEW fire departments will be unhappy with a 911 call, if the message is that occupants have alarms going off and they are leaving.   Fire departments prefer talking with people who haven't passed out and need oxygen or trip to hospital.  They ALMOST certainly prefer that to trying to help contact relatives or the corners office because of fatalities.

    Very few people really recognize the symptoms of long term exposure... and very few people can spot or recognize possible back drafting problems.  Weatherization, residential inspectors might - because they have training and sensitivity to look for the problems.  But very few homeowners can visualize the problem.  And sometimes HVAC/appliance installers don't either.   I've seen furnace with the return air duct in the basement - just four feet from a NG hot water heater... with an improperly installed flue pipe... 

    Passive houses that are extremely tight can experience CO problems EVEN if they've decided to use only electric appliances/water heaters/heat pump HVAC.    They just need to have somebody annoyed with the HRV/ERV because of the noise and turn it off permanently.  Four or five people in a smaller house during the holidays with candles can raise the CO.

    I would suggest that make up air should always be a provision...

    FWIW,   I believe LBNL and California surveys of residential uses of range hoods has shown many people don't even use them.   I've seen those numbers reported at ASHRAE conferences.

    Effects on health of prolonged exposure to low concentrations of carbon monoxide
    The effects on health of prolonged but low level exposure to carbon monoxide (CO) are unclear. Studies of carbon monoxide exposure focus mainly on sh…
    • There are more sensitive CO detectors on the market but, unfortunately, their very sensitivity makes it impossible to meet UL standards. The revised Canadian CO sensor standard, CSA 6.19, has a section for sensitive alarms. I am not sure if any product has been tested to it.

    • Correct,  but if we're worried about depressurizing issues - having at least one very sensitive CO detector to monitor the issue - if make up air is not being supplied.

    • Don Fugler wrote:

      There are more sensitive CO detectors on the market but, unfortunately, their very sensitivity makes it impossible to meet UL standards.

      As I'm sure you know, that's because those standards were written to protect municipalities (from nuisance alarms) rather than protect the public =:-o

    • They do meet UL standards, different UL standards,   but not for alarm systems that call fire departments.

      The CO detectors that are commonly bought and installed - are last ditch before "people die alarms".   The save lives,  but do not necessarily prevent CO injuries to the occupants.   If we are worried about having a healthy efficient house - shouldn't we also desire to have alarms that can warn us before the occupants are exposed to long term chronic low levels of CO poisoning.. levels that can lead to heart disease, brain injury (looking like dementia), hypertension?

      Even though they might not automatically call the fire department,  perhaps its time that we start thinking about the impacts of lower levels of CO.

      Worrying only about death should not be the only reason for choosing a detector.

    • You're preachin' to the choir. We had several in-depth discussions on this topic here, here, here, and here. Unfortunately, CO-Experts detector is no longer manufactured. I believe the ProTech 8505 is still available (see this site).

      Don't Compromise — Get a Low-Level Carbon Monoxide Monitor
      You know that saying, Don't judge a book by its cover? That certainly applies to what may be the best protection against carbon monoxide poisoning yo…
    • After we had discussions on this thread in this forum,  I started searching for and looking for CO detectors that would report the readings.  I'f figured that we'd see some movement and newer products.

      I did find some,  but the "Chingish" from sentences such as: 

      "This product is only used to reduce the accident, cannot make sure work every time. Except use our detector in the right way,  please pay more attention in your daily life and strengthen safety and security conscientiousness."

      "Sensitive level 5ppm"

      Not mentioned is contact information for warranty, etc.   I'd gess the sensitive level really is in $ not ppm.

      I guess it's time to work with state fire marshals,  UL and NFPA to include some kind of better detection and warning earlier when long term lower levels are detected.  The major "good quality" makers of the CO sensors have improved their devices enough such that it should be possible to alarm and warn when the levels are above 20 ppm after eight, ten or twenty hours....  an early sign that there is a problem.

    • We have had success tracking combustion issues using a CO2 detection device. There are many choices in this hardware as they are commonly used to control commercial building ventilation. If there is combustion spillage, there should be a rise in CO2 that is more noticeable than CO. CO production can be variable and may depend on the quality of the combustion process, or as a result of high CO2 in the combustion air.......resulting from the spillage event.

    • CO2 isn't the particular problem I was following up on.  It was the in ability of CO alarms to detect and report out low levells of CO until the appliances had gotten so bad that the CO was an immediate danger to life (short term danger).  CO at low levels isn't detected by alarms... people then miss a furnace heat exchanger that may have burnt through or have a crack in it.  etc.  Or a DIY project makes stuff bad and a professional isn't called in with a combustion analyzer...

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