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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    • Yup,  I've read prior posts.    And I know Don's posts are six months old.   While the one particular brand is no longer available,  there are actually quite a few on the market.   Its still on topic,  one of the problems with hood design and build out,  is simply people not following it to see how it works over months are a few years.   While we are having the discussion - it may be worthwhile for members of the choir to get the low level detectors and track how effective various solutions really are.  E.g.  more than a once or twice CO check,  but monitoring our own installations for a longer period of time to see if the ideas really hold up and what the long term problems might be.

  • Hi Don:  good questions, as usual.

    Would be great to routinely get depressurization data --  not hard to do.

    See my NAPHN 2019 slides near the end for example of P measurements for installed MA system, at ROCIS Range Hood page).

    re: consequence #4: suggest you include Attached Garages & storage spaces, known sources of VOC and/or CO.   Home-Garage depressurization can be significant: 

    see 1)  new home data on garage depressurization from Offerman/CARB study, 2) VOC analyses by Offermann et al., Healthy Buildings 2012,, and 3) Mallach, Fugler, et al 2017 garage exhuast ventilation study,

    re: overall effect of range hood on depressurization:  I have heard or read of several problems when a very high flow range hood is run, including down draft exhausts.  I agree that the typicall run time is probably too short to be important, but multi-hour run times for baking & roasting are probably important re: depressurization. ASHRAE 62.2 also allows the range hood exhaust to be used for whole house ventilation at a low speed, but I don't know if many people are doing this yet.

    Final thought:  good design, quality control, and commissioning for range hoods are the exception, based on what I have read and heard from contractors and friends.  ASHRAE 62.2 is working on a method to test installed range hood flows, and  the CA building E standards could then incorporate those. This may be a good opportunity to also require P testing of  MA systems, if 62.2 folks are not already planning to do so.

    Mantra for basic/easy, home testing: PVCT (pressures, ventilation flows, CO2 during occupancy, and ventilation, thermal comfort  (T&RH) - see attached 2012 slide.



    Best Practice recommendations DBCP 2012.pptx

  • Hi Don, great sequence of questions, and a couple of thoughts.

    Overall, I just don't see depressurization, especially short-term and infrequent (as caused by cooking appliances), as being a major risk to health or building durability. Running a continuous exhaust fan in a Passive House with continuous, long term depressurization could have some impacts (maybe), but I'm not even convinced that's an emergency. Maybe a possible inconvenience, due to door suction, etc. 

    First, I don't see items 3 and 4 in your list as related to depressurization due to cooking exhaust use. Depressurization increases with an airtight envelope, but the flow through leaks in the envelope (#3) and the drafts from outside (#4) are the same for a given level of kitchen fan airflow. The make-up air flow is the same for a given exhaust fan flow, irrespective of the envelope airtightness. So, I think these "issues" are just "kitchen ventilation" issues, rather than depressurization issues. The tightness of the envelope shouldn't make these any better or worse.   

    Increased radon entry, that's interesting. I'm just imagining that kitchen exhaust runtime is so low that this can't really make a big difference to health risk, plus the increased radon entry occurs during periods of very high outside air ventilation (or does the entry lag the depressurization somehow?).  

    Backdrafting of combustion appliances. Again, I think this is rare, mostly due to the same elements mentioned about radon - coincident runtime of kitchen exhaust and combustion appliances is even lower than the runtime of the kitchen exhaust by itself (if random, its runtime,KitFan x runtime,CombApp). And spillage will occur when the ventilation rate is very high, thus diluting the spillage. And as you stated, these appliances should not be in super-tight homes for many reasons. 

    Cheers and thanks!


    • @Brennan: With respect to your point about combustion back-draft, in cold weather, a depressurization event especially without combustion appliance operation can result in semi-permanent flue reversal. That is when the flue is cooled below house temperature and becomes an air inlet to the lower zone of the home. Flues with exposure to the outside are especially susceptible. Subsequent combustion cycles may not re-establish draft up through the flue and so the combustion appliance will vent into the home whenever it cycles. This can occur many hours disconnected from the depressurization event that reversed the flue.

    • Dara:

      thx for the reminder. 

      As I recall, CMHC funded some studies on this, which you probably did and know well, as would Don Fugler. 

      Also, Dave Grimrsud et al at U. Minn and Niren Nagda observed and recorded this problem in at least one home.  The study may have been funded by the Gas Research Inst.; it was presented at a workshop at U. of Minn. I would suggest checking with Dave, and searching RessearchGate re: backdrafting duration and flue reversal.

      I used the Canadian and MN info when drafting the CARB IAQ Guideline on Combustion Pollutants in 90's.


    • Hi Tom

      We did some field work "in the day" on threshold effects (5 pa & 10pa) of exhaust rates and correlating those with ACPH50 values. The results of that work is now used in the HRAI Ventilation Manual as a predictive allowable exhaust calculation.HRAI Ventilation Manual I was not involved in actual published studies on flue reversal, but did a series of field investigations to follow-up combustion spillage incidents for various parties including the Ontario Combustion Fuels Safety Authority (TSSA). The common theme of my participation was to determine whether or not the house was "too air-tight" and was a factor in the combustion spillage incident. My conclusion was that non-solid fuel natural draft combustion appliances should not be permitted in any home, no matter the air-tightness.

    • Hi Dara:

      thx for the update.  Sounds like you made some progress.

      CA still has a lot of natural draft and some (illegal) unvented gas log sets, but we are now pushing to decarbonize not only the grid but also (finally) buildings. So there may need to be some incentive programs to replace old gas furnaces & water heaters with heat pumps, and hopefully with lots of shell efficiency & shading improvements.

    • This is a great sub-thread, thanks you two! I'll be looking into some of the projects/studies you've referenced. 

    • Brennan:

      re: CMHC reports: if you don't have any luck, I can track down the big pile of hard copies I collected at CARB (may be in the Cal EPA library, or in my home file storage).

    • Brennan

      Most of the research we did on combustion spillage (including surveys) was undertaken in the 1980s. Retrieval of those reports (even ASHRAE papers) is not easily done through Google or other general sources. The corporation I worked for, CMHC, lists 8 reports on spillage but all the links are now broken on their website. There will be some more recent papers that can be retrieved.

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