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Negative Pressure from Bath Fans

dougeb | Posted in General Questions on

I am nearing the end of a home renovation and need to make some decisions on bath fans.

I am a homeowner working with a builder who is not very familiar with HP building so I am trying to manage as much of that aspect as possible to make the home both comfortable and efficient.

I have no clue what we will net on blower door score once our windows and WRB are installed.  If we are over 3 I might opt for aerobarrier with a goal to get us to 1 ach50.  If we are at or under 3 I might save the money.

So, as I look at bath ventilation I am wondering if I need to worry about depressurizing the house when the primary bath is in use.  That bath will have a 150CFM fan, the other two baths will be 50cfm.  There is no gas in the home, we are all electric.  I did plan for mechanical ventilation using the Lunos system (no room for ducts) but they do not correct for imbalance created outside systems.  Is the potential negative pressure caused by the primary bath or some combination of the 3 something I need to worry about?  Not sure there is anything I can really do at this point but figured I’d ask. Also, not sure how it is different than a 400cfm kitchen hood (which would not require make-up air).  Perhaps I answered my own questions.  If so my follow-up would be at what ach50 level do you need to worry about negative pressure in these amounts?

The location is Southern California in case that matters.


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  1. DCContrarian | | #1

    You don't say how big your house is but let's say it's 2500 square feet to make the math easy. With 8 foot ceilings that's 20,000 cubic feet of volume, so 1 ACH is 20,000 cubic feet per hour or 333 cfm. The house does that at a pressure of 50 Pascal. There are about 100,000 Pascal in an Atmosphere, so 50 Pascal is about 0.0005 Atmospheres. That isn't noticeable. Barometric pressure might change from day to day by 0.03 atm and it's not something you notice.

    1. maine_tyler | | #3

      I'm a bit perplexed by the second half of your paragraph. Why are you comparing how many atmospheres 50 Pa is? And barometric pressure, which has not to do with pressure differentials across building envelopes? Your implied conclusion (that makeup air isn't likely needed) seems fine, just not sure where you are going with those figures, and they seem kinda misleading.

      50 Pa of pressure differential is quite a bit despite the fact that an atmosphere has 1010325 Pa. Of course 50 Pa isn't the number dougbe is working with anyways.

      1. DCContrarian | | #6

        Tyler --

        The point I'm trying to make is that in terms of magnitude, the pressure drop the house is going to experience with all three fans running -- 250 CFM total -- is about what they got during the blower door test, 333 CFM. That by definition was 50 Pa.

        50 Pa is and isn't a lot of pressure. If you have a combustion appliance, particularly one that relies just on the natural draft, then it's enough to cause backdrafting. But if you're not worried about that, it's not a lot. Since air pressure isn't something we deal with every day it's not something we have an intuitive grasp of. I was converting into atmospheres to try and give an intuitive hook. Over the course of a day the air pressure will vary by far more than 50 Pa. A 50 Pa difference represents a difference of about 14 feet in altitude, or a flight and a half of stairs. It's about a quarter inch of water column. You won't be able to notice it without instruments.

        1. dougeb | | #7

          I appreciate the connection to measures that are more familiar. Considering the bath fans are not running all the time it does seem minor for us.

          Now, we are obviously not building to passive house standards but again I find myself wondering after reading the energy vanguard article from Tyler. That article claims that a 50 PA difference is significant for a passive house. If that 50 PA is the difference of 14 ft and we use the numbers in that article (which I believe is about 0.3ach50 based on my volume) then we would be negative maybe 3x the amount equating to roughly 42 ft of pressure. I don't think I've ever registered a pressure change going up 5 stories. So why is that significant for a passive house as claimed in the article.

          Feel free to point me to a different place if I should let this thread die and I appreciate all of the guidance and info thus far.

          1. maine_tyler | | #8


            This question shows precisely why DC's comparison is misleading and confusing.

            Here's the quote from Allison's article:
            "[50 Pa pressure difference] is a lot of negative pressure for a house, which is why Passive House designers and builders worry so much about the mechanical systems they use."

            It's not that 50 Pa differential is a lot 'for a passive house', it's that low CFM rates will induce that high pressure differential in a passive house. In other words, the passive house will hit that 50 Pa differential much easier with fans, etc. In any case, 50 Pa is still 'a lot of pressure differential.'

            The thing to keep in mind here is that:
            1) You don't know your ACH (or CFM) numbers from a blower door test yet, so we really don't know how much pressure differential your fans will produce, and
            2) You don't have any natural draft appliances (or any combustion appliances at all it sounds like).

            Point number 2 is significant and probably reason enough alone not to worry.

        2. maine_tyler | | #9

          DC, nothing personal but what follows is my beef on the matter:

          "I was converting into atmospheres to try and give an intuitive hook."

          I contend it does mostly the opposite. Whether a given pressure is 'a lot' is certainly contextual, and in many ways it's true that 50 Pa is not a lot, but the context here is precisely why I think your comparison is a bit misleading. Citing that 50Pa is some tiny fraction (0.0005) of atmospheric pressure gives the false impression that 50Pa is unequivocally a miniscule differential. But 1 atmosphere of pressure differential would be ENORMOUS, so citing a fraction of that is sort of a mis-scaled comparison. There's probably a reason why blower door tests are conducted in pascals and not atmospheres.

          As far as variances in barometric/station pressure-- whether due to weather changes or elevation-- I again see little relevance. We're talking about pressure differentials across an envelope, not gross and absolute changes in ambient environmental pressures, which the building envelope does not really 'experience' save for in the form of wind driven by said atmospheric pressure differentials (the BSI article states that wind loading is 'likely' to 'on average' generate around 5Pa differential for a house. 50Pa is about a 16mph wind).

          Lastly, I'm not clear what you mean by 'you won't notice [50Pa pressure differential] without instruments.' Like if you had one ear on one side of the envelope (with the envelope tightly sealed to the bisect of your person) and the other ear on the other? In that case, I believe one would actually notice it. If you mean one can walk 14 ft. up some stairs and not 'feel' some sort of pressure drop, then... yes... but so what? You can actually climb thousands and thousands of feet and not 'notice' it because we have membranes that are (nearly) constantly equalizing.

          1. DCContrarian | | #10

            But when Allison says 50 Pa "is a lot of negative pressure" what does he mean? I'm going to continue to argue that unless you have combustion appliances, the only impact of that negative pressure is increased negative pressure is increased infiltration. And yes, a house that is built to a tight standard is ordinarily going to have very little infiltration, and the negative pressure is going to increase the infiltration dramatically.

            But so what? The negative pressure is coming from ventilation, the desire is to get rid of some of the interior air, it needs to be replaced with exterior air. The alternative to having it replaced with infiltration air is to have it come through deliberate makeup air through a vent. What is the benefit to that air coming through a vent?

            Let me throw out another comparison to put it in perspective. My duct resistance calculator tells me that at 300 CFM, an 8" duct produces 50 Pa of pressure drop at a length of 9". So if your makeup air duct is longer than 9", or narrower than 8", it's not even the path of least resistance for air entering the home.

          2. maine_tyler | | #11

            Response to DC #10

            "I'm going to continue to argue that unless you have combustion appliances, the only impact of that negative pressure is increased negative pressure is increased infiltration."

            That's totally fair DC, and is also not counter to what I have been arguing. I'm not suggesting makeup air is needed in this situation. I'm simply saying I found the comparisons to atm and barometric pressures odd and not so illuminating to the question.

            I'm guilty as anyone of making extraneous or distracting points in the name of scientific acrobatics or just for funzies, but I think the question "why a given negative pressure IS or IS NOT a problem" needs to be addressed in close coordination with comparisons like "what does 50Pa equate to in atm, or in flights of stairs." I don't see any coordination between those points. Of course anyone is allowed to make whatever comparisons they wish for whatever reasons they wish, but these comparisons deserved some further qualification, imo.

            On your latest point re duct pressures: I'm not sure what to make of it. I'm not an HVAC specialist. I have a bit more knowledge in electrical, and so by analogy I would offer that the air will flow across all available paths, not the one of least resistance. Is your contention that an 8" makeup air duct longer than 9" would essentially provide no relief to a house under 50Pa pressure differential? Without thinking too hard about it, it seems to me like all the little cracks in the existing structure by themselves add to 50Pa at the assumed CFM, so adding another path with 50Pa resistance would essentially double the available air (?). So the makeup air duct of the configuration you describe would provide about half the makeup air, while infiltration provides the other half? (I also imagine that if interior doors partition spaces with enough resistance, it could change the location of where the makeup air comes from.)

  2. dougeb | | #2

    DCContrarian, thanks for the quick and detailed answer. Your assumptions are pretty close. Overall square footage is a bit lower but we have 10 ft ceilings for part of the downstairs so probably about 21k cubic feet.

    So it seems the change in pressure is miniscule in the grand scheme of things and certainly not something one would feel.

    Given these numbers it makes me wonder about ventilation. I had two people hard selling on trying to make a ducted HRV system work and frequently putting at the top of the list the lack of need for bath fans and the benefit of the house being fully balanced. I had wanted a ducted system it just was not possible with our plans our builder and our budget, the changes were too great for us to make so late in the process.

    So my follow-up is why do people highlight the not needing bath fans with a ducted HRV if the pressure impact at ACH 1 is so low? I understand that it's fewer penetrations but if done properly with a backdraft damper those penetrations can still be sealed? More just curious as I'm learning about all of. My curiosity and desire to do things right find me thinking about my next build my sanity and bank account thinks I'm an idiot considering we didn't even finish this one.


    1. maine_tyler | | #4

      I think people often duct their HRV to exhaust air from the bathroom so a separate unit would just be redundant.

  3. DennisWood | | #5

    I think the sell point is that if you duct your HRV/ERV system, then you can save money on a few exhaust fans, and also reduce your envelope penetrations by a few. Going with a ducted fresh air system makes the most sense to me, particularly after logging CO2 levels in our home for a few years. Dropping fresh air in the bedrooms is suggested, and I 100% agree. It's not hard to exceed 1500 PPM CO2 in a bedroom, even with a small ERV running 24/7.

    If I was doing a build, then 100% I would exhaust from the baths with a booster switch in each, 1 kitchen exhaust, and dump fresh into each bedroom, and 1 drop in main living. An HRV/ERV with ECM fans may only use 25-35 watts vs running it ducted via a forced air system (where you need to run the furnace fan to move the fresh air). We have an ECM fan furnace, but it definitely uses more than 35 watts, even at 350 CFM fan mode.

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