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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Replies

    • Dara, I've not heard of this issue before, and it's very interesting, indeed. But it sure seems like this could happen due to any number of issues, such as the operation of other exhaust devices, continuous exhaust fan, or even just any long time period where the gas appliance doesn't fire? Are you aware of any research or measurements (or presentations or webinar etc) documenting this phenomenon? Thanks for sharing! 

    • It is a complex issue with many factors including the house and flue configuration, relative air-tightness floor to floor., presence of a flue-damper etc etc. I am not aware of where this is written up (Don Fugler may know). This is from personal experience.In one of the cases there was a fatality.

    • Thanks for your thoughtful reply, Brennan. My point about the load impact on tight homes with right-sized equipment is only that the additional load in that case can be very large relative to equipment capacity (compared to a home that has a leaky envelope). An indirect corollary of that is that older leaky homes are much more likely to have oversized equipment than homes built to today's energy code and beyond. That's a big reason why this issue didn't get much attention 15 or 20 years ago.

      Regarding HVAC sizing... to your point, yes, it's the mechanical designer's responsibility to size equipment to the load, including transient loads. However, when a transient load is large relative to the envelope load (this can also happen with large gatherings during peak cooling hours), it's a bad idea to size the primary HVAC system to address the transient load.

      Understandably, most folks don't want to pay the cost to install a separate system to pre-condition make-up air so it's a mater of managing the owner's expectations if the primary system can't reasonably be designed to handle transient loads in real time. I've spent a fair amount of time over the years talking down clients from commercial range hoods.

      It's worth pointing out that addressing high transient latent loads in a humid climate can get very expensive very quickly. Consider a 600 to 1200 CFM hood operating just an hour a day when outside dew points are in the mid-70's. This will overwhelm a conventional air conditioning system. There's no recovering from that if the daily load is greater than the integrated daily latent capacity. Moreover, oversizing to handle a large transient latent load will only make it worse. So from my perspective, your position that these short duty cycle loads are irrelevant doesn't ring true.

      Lastly, your point about HVAC systems being explicitly designed to handle 99% of the hours is incorrect. The reason we design to the 99th and 1st percentile of historical temperature bins is simply to account for thermal lag. There's nothing in that that suggests the equipment can't meet the load during the remaining 1% of the hours!

      Although these are simply statistical constructs, it's important to understand what they really mean. For example, there will occasionally be hours when an extreme temperature event exceeds the capacity a properly sized system's capacity, but it shouldn't be anywhere close to 88 hours a year. In fact, as envelopes become tighter and more efficient, we can safely loosen the design range to the 98th/2nd percentile bins, or even 97/3, especially when there's more than the average mass in the building. I routinely make these types of adjustments in my designs. The Passive House design tool (PHPP) uses an even lower percentile bracket although I can't recall what it is.

      Perhaps too many words on sizing for the present topic at hand, but recognizing how thermal lag affects equipment sizing makes it clear that induced infiltration, which bypasses a home's insulation and thermal mass, has no such lag and thus becomes even more impactful on comfort and HVAC performance.

    • Thanks again for the thoughtful perspectives, David. I've never heard the thermal lag justification for use of 99% design conditions. Is that written in Manual J or elsewhere? Very interesting stuff, especially the linkage between using different design temperatures as envelope efficiency improves. Good stuff.

      Either way, the point is, there are always some hours with unmet load (e.g., recovery from nighttime set back, or during a party), or zones within a home with unmet load during certain periods (e.g., sun shining through windows into zone, or during cooking). I just don't feel that's the end of the world, and the system recovers as soon as the current load is reduced below system capacity. Also, an interesting note, infiltrating air has thermal lag as well, it passes through a large surface area/mass heat exchanger on its way into the living space - the envelope. Envelope heat transfer is real and does serve to temper the infiltrating air mass, and it buffers moisture from the air.   

      The humid South example is interesting, where some daily influx by a large exhaust fan is never compensated for on a 24-hour basis by a capacity limited cooling system. Again, if that's a recurring issue where the envelope load is overwhelmed by the kitchen exhaust on a daily basis, then I'd argue the system is clearly sized wrong, and the designer needs to adjust accordingly. 

      I'm especially confused by the refusal to allow a bit of over-sizing in mini-split systems in efficiency homes, which have super strong ability to modulate output, and in fact are more efficient (higher COP) at part-load conditions. Obviously, this has its limits, but a bit of oversizing, such that you could operate an exhaust fan in the home without the system falling off the rails, seems justified and realistic. But I'm not a humid South or cold North person (nor an HVAC designer), so maybe my instincts fail me ;) 

      But again, to be super clear, I don't advocate for these large exhaust fans. My original point was that the depressurization induced temporarily by even a modest exhaust fan in a very airtight house is not actually an emergency or even really a problem. And I've yet to hear an argument that justifies the concern, except maybe for the flue reversal issue that Dara B. brought up, which I need to look into a bit.  

      Cheers!

    • @Brennan, design conditions used by ACCA Manuals J & N and various other design procedures are taken from tabular data published and updated regularly in ASHRAE's Fundamentals Handbook. Chapter 14 (2017 ed.), which contains the tabular data, makes no attempt to explain the rationale for using a particular design percentile (BTW, the tables include 5 different dry bulb design percentile bins).

      My premise regarding thermal lag as it impacts the assumed outdoor design temperature is simply a recognition of reality. Whether or not the good folks at ASHRAE had this in mind when they established the various design temperature percentile standards, I don't know. In any case, I should have made clear that I consider your assumption incorrect regarding equipment not meeting the load for 1% of the hours /in my opinion/. I would be interested in hearing Hank Rutkowski's thoughts on this (author of ACCA Manual J).

      On the other hand, the idea that we should design to lower percentile bins is something that's evolved in recent years with beyond-code design. If you look at graphs that show how a tight, well insulated unheated or uncooled space responds to outside temps, the impact of thermal lag becomes obvious. I should have mentioned that diurnal temperature swing also impacts the load the equipment sees. With enough thermal mass and insulation, you can eliminate envelope loads all together. What design temperature that would yield that result?

      Your point about infiltration being tempered by the envelope is valid - to a point. But as velocity goes up, incoming air will quickly cool (or warm) components along the pathways, thus eliminating the effect. Imagine 600 CFM (1.5 tons) of induced infiltration. How much tempering to you really think will occur with that? And with ducted make-up air (which I think is now required by code for hoods that exceed 400 CFM), there's obviously little or no passive tempering.

      Your point about the designer needing to account for excess moisture is exactly my point. Your objective is clearly to play down these loads, whereas my objective is to highlight the challenges these transients loads can present. The fact is, many mechanical designers ignore transient loads or have no clue how to properly address them. Keep in mind this is a professional forum, so we must assume designers, future designers and those who manage and train designers are among the audience. Mechanical designers should always consider whether exhaust fans and other transient loads need to be addressed. Saying that it's not an emergency or even a problem in many cases is not helpful to that end. Thus my aggressive (and hopefully respectful), rebuttal.

      Lastly (with apologies for straying off-topic), I disagree with your premise that it's OK to oversize mini-splits (or variable capacity equipment in general). This is apparently a commonly held belief. The problem with that is that it robs the system of it's full dynamic range over the nominal design load (and the benefits that entails). A customer pays a premium for a system that can load-match, but gives up some (or all) of that away if the system is oversized.

      Keep in mind that variable capacity equipment can't modulate to zero (not even close), so even with a 'modest' amount of oversizing, the system will end up operating at its minimum capacity for the vast majority of hours, thus losing any ability to load match. This issue gets particularly nasty with multi-splits, which already take a serious efficiency hit due to capacity mis-match during single zone calls. And with air-zoned systems, you need the full dynamic range of the equipment (and then some) to manage single and (sometimes) two-zone calls. In short, oversizing a zoned system can lead to serious performance issues.

      I would also take issue that variable capacity equipment is always more efficient at part-load. Blanket claims like that lead to poor design decisions by specifiers and designers who are not inclined to dive deep into in-depth equipment performance data.

      If you want to dive further into these topics, please start another thread ;-)

    • @Brennan, there's virtually no lag. When an exhaust fan comes on, the pressure differential should reach stasis within a matter of seconds I would imagine.

      You wrote: " The tightness of the envelope shouldn't make these any better or worse."

      Not sure what your point was about #3 & #4.  One reason why these issues (induced infiltration) are more of a concern with tight homes is the impact on comfort.  In particular, if the HVAC system is sized properly, it won't be able to handle the additional load in real time, or even near-real-time. For example, most of the homes I work on have design heat loads of less than 20k BTUH (some less than 10k), whereas a 600 CFM fan drawing in 10F outdoor air (think large family gathering during December holidays with 2 hours of continuous breakfast cooking) would impose an additional load of about 39k BTUH.  When you add in the normal envelope load, that works out to nearly 3 times the design load! 

      In humid climates, 600 CFM can easily overwhelm an air conditioner's latent capacity. We know that well insulated homes with right-sized equipment already have latent capacity challenges. Moisture loads that cannot be met by the mechanical system is a durability issue, as you put it.

    • David, I know the pressure reaches static levels quickly. I meant, how quickly then does radon begin to emit or be drawn into the space at a higher rate. For example, initially drawn by depressurization into a basement and then rising up into the living space at a later time, after the fan was turned off. 

      Your point about the increased load is valid, but again, that occurs with or without make-up air. I'm not advocating for super-high-flow kitchen exhaust (quite to the contrary). But I am noting that the airflow is the same, irrespective of the envelope tightness or provision of make-up air (as I understand it, could be wrong...). A tight envelope doesn't make it worse. The ability of an HVAC system to meet that load is an entirely different question, and the system should be sized to deal with the loads that are present. Over-sizing has its benefits, and it was not done forever solely out of ignorance and laziness (that was probably only half of it, right?). 

      If you "right-size" and operate a large exhaust fan, you may have individual hours where load is not met. I'd say that is far from the end of the world. One hour on one holiday of the year, first world problems ;) In fact, our systems are explicitly designed under the assumption that they meet some fraction of hours < 100%, typically 99% or so, which leaves 88 hours per year with unmet loads. Etc.

      My main point is that if the fan were always operated while cooking in most households, then runtimes are generally so low as to be irrelevant for radon, moisture, heat load, etc, or very, very transient "problems" at worst. Especially if we weigh all of this against the clear and known IAQ benefits provided by adequate kitchen exhaust that is used regularly. Obviously, the best bet would be to install a lower flow (<200cfm) exhaust fan with a properly designed and situated hood with high capture efficiency. 

      Cheers!

  • I saw an interesting article on cleaning make-up air which referenced the hardware found in hydroponics shops. If you really want an effective carbon filter with a supplemental balancing fan, here is a description of where to find one: https://www.joneakes.com/jons-fixit-database/2292-STOPPING-WOOD-HEA...

    This article references Canadian sources but I am sure equivalent American ones exist.

    STOPPING WOOD HEATING SMOKE FROM COMING INTO YOUR HOUSE.
    Cliquez ici pour voir cet article en français IS THERE A FIRE IN THE NEIGHBORHOOD? December 2018 - As I was walking home from the grocery store in…
  • Luke:

    re: hood flow rate and make up air recommendations, see my slides from NAPHN 2018 (p. 34  +) and the Tools section (aww Broan's online tool for MA system design), at http://rocis.org/range-hood-tools-and-presentations.

    150 CFM might work in a super tight house with wall/corner hood installation, powererd MA, and side extensions or a very hi Capture Efficiency Hood (HVI ratings may come out this year). But I would tend to go toward 300 CFM for hi emission cooking, with split MA registers.

    re: ERV boost with a hood, the Dutch researchers (Jacob et al. at TNO) are testing this out in multifamily settings. Stay tuned.

    Range Hood Tools and Presentations | ROCIS.org
  • Luke, the depressurization caused by an overpowered fan (e.g. 600 cfm)  in a kitchen cannot be compensated by a relatively small passive duct. The fan is creating a high static pressure to drive the exhaust out of the house. The resulting house depressurization will be lower than the pressure in the duct that a fan creates and that depressurization is the pressure that will drive the flow in the passive duct. Your passive duct would have to be much bigger than the fan ducting to come anywhere close to matching the exhaust.

    Because of those size limitations, and because passive ducts do not provide acceptably comfortable air in winter conditions, most practical solutions for very large fans in houses require powered and tempered make-up air. I imagine that problems due to smaller fans might be compensated by a properly designed passive duct.

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