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Musings of an Energy Nerd

Is Your Ventilation System Working?

There’s no way to know whether your fans are working properly unless someone has measured the airflow rates

Terry Brennan described several different methods of measuring fan air flow rates at the recent NESEA-sponsored conference in Boston.
Image Credit: Image #1: Martin Holladay

What’s a “faith-based ventilation system”? It’s a ventilation system installed by a contractor who never verifies the air flow rates after the equipment is installed.

So, will this type of ventilation system work? It’s hard to say — because no one measured anything.

At last month’s BuildingEnergy 16 conference in Boston, two ventilation experts — Terry Brennan and David White — gave a presentation titled Brennan and White explained why new homes need mechanical ventilation systems, and shared important design principles underlying these systems. They ended their presentation by providing advice on (and a demonstration of) different ways to measure airflow.

Terry Brennan is a building scientist and a member of the ASHRAE 62.2 committee (the committee responsible for the residential ventilation standard). David White is a principal at Right Environments, a New York architectural firm, and an assistant professor at Parsons.

You’re not getting the airflow you think you’re getting

Most builders have no idea whether the ventilation equipment they install is working properly. Many builders assume, for example, that a bathroom exhaust fan rated at 110 cfm is moving 110 cfm of air. It almost certainly isn’t, as anyone who bothers to measure their fans’ performance soon learns.

A few years ago, GBA blogger Allison Bailes recounted the tale of a builder who installed nine bathroom exhaust fans, each rated at 110 cfm. While the builder assumed that each of these fans would be able to move at least 50 cfm — less than half of their rated airflow — it turned out that only five of the nine of the fans achieved that fairly modest goal. (One fan barely limped over the finish line — it was tested…

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  1. Adam W | | #1

    Measure air flow and then?
    1. I like the idea of testing (though I'm sure I'll get some amount of push back from the contractors). You mention "most ducts have elbows; many ducts are long; some are leaky; many are convoluted; and some have the wrong diameter." - that makes sense. But if we go ahead and do the tests and find that we're not getting the ventilation that we expect - what practical options do we have at that point? Rip out the ducting?

    2. Can an HRV exhaust in a bathroom replace the need/requirement for a dedicated bathroom fan? If so - what CFM would be required? Is it ok for the humidity of a shower to be pulled into the HRV?

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Adam Wride
    Q. "If we go ahead and do the tests and find that we're not getting the ventilation that we expect - what practical options do we have at that point?"

    A. The options are the same as for any other discovery that reveals a construction error. Basically, you can either (a) ignore the problem, or (b) fix it.

    I discussed possible fixes in my 2014 article, Bathroom Exhaust Fans:

    "Let’s say that your fan is hooked up to a funky duct system, and it’s only pulling 35 cfm. There are two solutions to this problem: you can swap out the 110 cfm fan for a more powerful model — say, a 200 cfm fan — or you can fix all the duct problems. (A good duct system has a large diameter, smooth walls, and few elbows.) Either approach will work, but the latter approach is preferable — because the system will use less energy and will be quieter.

    "Fan makers have begun to respond to reports that builders are failing the 50 cfm airflow test by offering more powerful fans. For example, ads for Panasonic’s EcoVent fan boast that the fan includes a booster switch that a builder can flip to ramp up the fan’s speed if the fan fails its airflow test. While that solution is easy for the builder, a better solution would be to fix the funky ductwork."

  3. User avater GBA Editor
    Martin Holladay | | #3

    Second response to Adam Wride
    Q. "Can an HRV exhaust in a bathroom replace the need/requirement for a dedicated bathroom fan? If so - what CFM would be required? Is it ok for the humidity of a shower to be pulled into the HRV?"

    A. Answers to your questions can be found in my article, Does a Home with an HRV Also Need Bath Fans?

  4. Adam W | | #4

    Martin - thanks!
    Thank you for pointing those out. I've found the index you built and now realize there is an article for just about everything here. Pretty incredible.

  5. User avater GBA Editor
    Martin Holladay | | #5

    Thanks, Adam
    I'm glad that you like the index.

    If any GBA readers are wondering what Adam is talking about, it's this page: How To Do Everything.

  6. Derek Roff | | #6

    Are range hoods improving?
    Following the links in this article, and some links from those links, it looks like there was no satisfactory solution to cooking-produced pollution, when some of the articles were published, four years ago. The most effective range hood discussed was described as being too loud to hold normal conversation in the kitchen. Jumping forward to this current GBA article, "Brennan advised, remember that 'a fan shouldn’t be louder than a refrigerator.'" Does the market now offer any range hood product, that can come close to meeting Brennan's advice on fan loudness, while providing reasonable effectiveness in removing the pollutants from cooking?

  7. User avater GBA Editor
    Martin Holladay | | #7

    Response to Derek Roff
    I have forwarded your question to Terry Brennan. I also invite other GBA readers to respond.

  8. Kohta Ueno | | #8

    Rooftop Exhaust Fan Measurement

    Using a Duct Blaster as an exhaust vent flow meter. This technique is used for rooftop measurements on multi-family buildings with low-slope (flat) roofs. You need a box that is open on the bottom, and you need to be able to attach the Duct Blaster to the top of the box. The box has weatherstripping at the bottom (the open end of the box); it must be large enough to install over the exhaust duct termination that penetrates the roof.

    This Building America report shows the method of using a "flow box" for measuring rooftop exhaust fan flows ("BA-1209: Multifamily Ventilation Retrofit Strategies," ). The downside of the "flow box" is that you have basically built a lightweight box kite that's sitting on top of a multistory building roof. Losing your equipment off the roof due to wind gusts has the potential to be really embarrassing!

    But more interestingly: this CEE report ("Multifamily Ventilation Assessment and Retrofit Guide," ) does a great job of summarizing several methods for measuring rooftop exhaust fan airflows, including inserting an Energy Conservatory TrueFlow into the "throat" of the rooftop exhaust. Lots of clever stuff from Jim Fitzgerald and colleagues.

  9. User avater GBA Editor
    Martin Holladay | | #9

    Response to Kohta Ueno
    Thanks for providing the links to two excellent online resources for those who need to measure ventilation system air flows in a multifamily building.

    I've taken the liberty of posting a photo from the second resource you linked to (). Jim Fitzgerald knows a lot about ventilation.


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