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Community and Q&A

$90.00 flow hood?

User avatar
Michael Chandler | Posted in Mechanicals on

I’ve been chasing a low-cost flow hood for monitoring performance of my ventilation system for some time now, hoping to get one used, or through my HERS rater, and recently discovered a comment made by Martin here on June 15th ’09 referring to a way of making a homemade flow hood “from a cardboard box and a $90 digital anemometer”. (Right under my nose)

Can you give us an updated design for such an elegant contraption and recommendations for what anemometer to buy and what other diagnostic tests we might be able to accomplish with it? the choices on these things is overwhelming.

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    To answer your question, I'll quote from my article in the August 2002 issue of Energy Design Update.

    "In an article in Air Conditioning, Heating & Refrigeration News (May 13, 2002), Howard Leonard, a trainer of HVACR service technicians in Phoenix, Arizona, described how to make a homemade flow hood using a cardboard box and a hand-held digital rotating-vane anemometer (sometimes called a velometer). Leonard made his flow hood out of a U-Haul moving box. He formed the cardboard into a funnel-shaped tube, one end of which has inside dimensions of 12 inches by 12 inches. Since the opening is one square foot, the reading from an anemometer (which measures wind speed in feet per minute) equals the airflow in cfm. For the most accurate result, the area of the opening should be a little larger than 144 square inches; it should actually measure 144 square inches plus the area of the anemometer that is being inserted into the airflow.

    "Hand-held anemometers have been dropping in price lately. Leonard uses a $90 Kestrel 1000 anemometer from Nielsen-Kellerman. ‘I like the Kestrel 1000 because you don’t need two hands to use it,’ says Leonard. ‘It’s shock-resistant, it’s water-resistant, and it’s relatively inexpensive. It’s also more sensitive at lower velocities than some of the other affordable velometers.’

    "To use the flow hood, the cardboard tube is placed over a supply register with the 12x12 dimension facing away from the register. The anemometer is set to averaging mode, and is moved in a traverse across the opening of the hood. This method is more accurate than taking a single reading in the center of the opening. To keep one’s hand out of the airflow, the anemometer can be taped to the end of a slender stick.

    "Although some people may question the accuracy of Leonard’s measurements, he maintains that his homemade flow hood is accurate enough to diagnose typical residential duct problems, and is infinitely better than no flow hood at all. ‘I am not looking for perfection,’ says Leonard. ‘I’m just looking for tools for the regular Joe out there. My game is "how can we do our job without being millionaires?"’

    "Gary Nelson, president of the Energy Conservatory in Minneapolis, Minnesota, advises builders of homemade flow hoods that the longer the cardboard tube (up to a point), the more accurate the measurement, since a longer tube gives the airflow a chance to become less turbulent. Nelson suggests using a tube that is about 3 or 4 feet long.

    "Nelson, like Leonard, likes to tinker with homemade airflow tools. Nelson has written a short description of how to measure exhaust fan flow with a cardboard box and a 60-pascal (Pa) magnehelic manometer or a digital manometer. (This technique works only on exhaust fans, and cannot be used to measure the flow from a supply grille.) A hole is cut in the top of a bottomless cardboard box, with the area of the hole in square inches equal to about half the exhaust fan’s flow in cfm. The cardboard box should not be too small; the length and the depth of the box should be at least twice the largest dimension of the hole.

    "To use the device, a piece of tubing is inserted through a small hole in a corner of the box, and is connected to the bottom pressure tap of one’s manometer. The box is placed against the wall or ceiling over the exhaust fan grille, and the pressure difference across the box opening is measured when the fan is running. The most accurate result is achieved when the hole has been sized to yield a measurement between 4 Pa and 7 Pa for a magnehelic gauge, or between 1.5 Pa and 4 Pa for a digital manometer. The fan’s airflow is calculated using the following formula: Flow in cfm = 1.07 x area of the hole in square inches x the square root of the pressure difference across the hole in Pascals. For those who prefer to invest in something sturdier than a cardboard box, The Energy Conservatory sells a plastic box suitable for the purpose."

    [Caption to illustration reproduced below: "A homemade flow hood can be made from cardboard. If the end of the hood facing away from the register measures 12 inches by 12 inches, the anemometer reading in feet per minute will be equal to the airflow in cfm."]

  2. Alex Sawyer | | #2

    Huh, cool.

  3. User avatar
    John Semmelhack | | #3


    As a follow-up to Martin's 2002 article, Gary Nelson (Energy Conservatory) now sells a product for accurately measuring exhaust flows from 10cfm-124cfm - Cost is $150, but you also need to attach your own manometer.

    If you're just looking for ballpark testing results, it seems like Howard Leonard's device would do the trick.

    Also, why not ask your HERS Rater to do the flow testing? It's very easy to convert a "duct blaster" fan and digital manometer (tools a Rater already owns) into a powered flow hood...the most accurate way to measure airflow according to this LBNL study -

  4. User avatar
    Michael Chandler | | #4

    I do have my HERS rater give me a report but we often find underperforming bath fans and stuck dampers in range hoods that it would be nice to be able to do followup testing with out having to bring in the big guns from Raleigh. I like the idea of a little fold-up flow hood that I can store in the tool trailer and use to dial the house in on move-in. Similarly, it would be great to be able to run around with a flow hood when the HVAC guy is starting the system up and see if we have 5x as much air in the master bath as called for in the man-J before bringing in the E-star person.

    If I'm going to get one visit from my auditor I'd like to be able to have the open barn doors taken care of so we can focus on the more subtle stuff.

    The eMonitor has shown me that there are a lot of opportunities to test and tweak in improving the electrical performance of a new home. I suspect that much can be gained from applying the same sort of approach to the ventilation system. I just don't think we'll get all the answers in a single afternoon. Test, find and fix, test again, find smaller and fix better, and again.

  5. User avatar
    John Semmelhack | | #5

    Sounds like a solid, proactive plan, Michael.

    Personally, I don't like to show up at a house and have to tell a builder that his "barn door is open". ;-)

  6. User avatar
    Michael Chandler | | #6

    Even less that his horse is gone...

    Completely missed Man-J flow specs, 110 cfm bath fans pushing 40 cfm, 8" damper stuck open in the range hood, triple wall flue on zero clearance wood burning fireplace instead of metalbestos w/ sealed fire stops.

    Rain drops on roses, whiskers on kittens...

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