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Thread: Alnor LoFlo Flow Hood

  1. #1
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    Alnor LoFlo Flow Hood

    Considering getting this for residential air balancing.

    In the manual it says that the hood needs to be close in size to the registers being measured but the smallest hood is 16 x 16 and the registers I will be using it on are generally smaller.

    Is this likely to result in an error and if so how would you compensate other than doing a traverse and applying a correction factor. For most scenarios where a lot of the duct work is flex duct and limited straight runs I'm not sure how to perform an accurate traverse.

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    Make a jig out of cardboard or plexiglass is one option. Use anemometer and traverse the register or grill. These can be found on the cheap https://www.ebay.com/p/558451020. I’m not a balancer so merely suggestions I would wait for Wayne or the other regular tab guys to chime in
    Honeywell you can buy better but you cant pay more

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    Quote Originally Posted by crazzycajun View Post
    Make a jig out of cardboard or plexiglass is one option. Use anemometer and traverse the register or grill. These can be found on the cheap https://www.ebay.com/p/558451020. I’m not a balancer so merely suggestions I would wait for Wayne or the other regular tab guys to chime in
    Thanks yes good suggestions. I already do have a FP wireless rotating vane anemometer but have never been really sure how accurate it is and so no idea if I would trust it to create a correction factor for a flow hood.

    This is the one I have - https://www.fieldpiece.com/products/...d/anemometers/

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    The flow hood is much better than the rotating vane. The flow hood will give repeatable results for proportional balancing of identical diffusers. The best way to determine total airflow is the pitot traverse. The main thing that causes a flow hood to be inaccurate is the uneven distribution of the airflow across the grid. If you can distribute the flow evenly across the grid the results will be close to the actual airflow. Read airflows on the same diffuser at least two times. Take a reading then rotate the hood 90 degrees and take another. If the airflows differ by more than 10 % consider making a diverter out of cardboard. The idea is to divert the air in at least two directions toward the skirt. This will actual give a vertical circular action which will improve distribution.
    Diffusers that throw air in a horizontal circle are the worst and can't be read with a flow hood unless you make a vortex breaker that stops the vortex action. Other than that the biggest problems I had was linear diffusers which skirt wash (concentrate airflow down one side of the skirt) and the small square straight down throw diffusers used in prisons. The standard house diffusers with two way throw were not much of a problem.
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    Quote Originally Posted by WAYNE3298 View Post
    The flow hood is much better than the rotating vane. The flow hood will give repeatable results for proportional balancing of identical diffusers. The best way to determine total airflow is the pitot traverse. The main thing that causes a flow hood to be inaccurate is the uneven distribution of the airflow across the grid. If you can distribute the flow evenly across the grid the results will be close to the actual airflow. Read airflows on the same diffuser at least two times. Take a reading then rotate the hood 90 degrees and take another. If the airflows differ by more than 10 % consider making a diverter out of cardboard. The idea is to divert the air in at least two directions toward the skirt. This will actual give a vertical circular action which will improve distribution.
    Diffusers that throw air in a horizontal circle are the worst and can't be read with a flow hood unless you make a vortex breaker that stops the vortex action. Other than that the biggest problems I had was linear diffusers which skirt wash (concentrate airflow down one side of the skirt) and the small square straight down throw diffusers used in prisons. The standard house diffusers with two way throw were not much of a problem.
    OK thanks for the info. So if the airflow does not differ by more than 10% at the 90 degree reading can I assume the reading is accurate to within the percentage required for balancing.

    How does one make a diverter? Just a structure that goes from the edges of the register to the base of the flowhood?

    I wonder if the Alnor comes with a pitot tube for doing a pitot traverse. I assume if a correction factor is applied it holds true for all different sizes of registers?

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    You make the diverters out of cardboard and tape it to the hood. It takes some experimenting but not that difficult. The only cardboard device that you can sit on the base is the vortex breaker. For residential You may never need it.
    Flow hoods inherently are not as accurate as the pitot traverse but I never had a problem when using the 90 degree trick.
    Flow hoods do not come with a pitot tube. I have a digital Shortridge that accepts the ADM meter. The meter is not permanently mounted to the hood. It can be used for taking static pressures, velocity pressures and etc. I assume Alnor offers the same thing.
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    Quote Originally Posted by WAYNE3298 View Post
    You make the diverters out of cardboard and tape it to the hood. It takes some experimenting but not that difficult. The only cardboard device that you can sit on the base is the vortex breaker. For residential You may never need it.
    Flow hoods inherently are not as accurate as the pitot traverse but I never had a problem when using the 90 degree trick.
    Flow hoods do not come with a pitot tube. I have a digital Shortridge that accepts the ADM meter. The meter is not permanently mounted to the hood. It can be used for taking static pressures, velocity pressures and etc. I assume Alnor offers the same thing.
    I believe the high end Alnor comes with a detachable manometer but that does not seem to be the case for the LoFlo. I was looking at the LoFlo as it seems smaller and less $$ that the higher end version and I primarily do residential. It should be fine for measuring supply but probably not returns due to the upper limit on the allowed airflow.

    The vortex breaker is the cross placed at the bottom of the hood near the instrument correct? Again based on what I read it does not look like the LoFlo comes with it.

    Can you use a pitot traverse to measure airflow in to returns and out of registers. I've only used a pitot tube inside ducts.

    Any thoughts on hot wire anemometers - how accurate are they compared to flow hoods and rotating vane?

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    The LoFlo is better for residential because of the smaller height. You shouldn't need diverters or vortex breakers very often but you need to be aware of the possibility.
    If you have a lean budget get the LoFlo hood (analog), an incline manometer (for duct traverse), a 12" pitot tube, an 18" pitot tube, a magnehelic gage 0 to 1 inch and a magnehelic gage 0 to 2 inches.
    I haven't used the new digital flow hood. When I retired the digital hoods were unstable when used on low pressure systems such as resi.
    Hot wires are OK but I have two micro-anemometers I like better. I sold the hot wires without ever using them.
    You can't traverse diffusers. The one exception is duct mounted diffusers where you can insert the pitot into a straight duct run.
    The rotating vane is worthless.
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    Interesting suggestion about getting an analog setup. I guess I could start with that and progress to the LoFlo or use the analog setup to cross check the LoFlo.

    I had an interesting thought - I could make a rectangular duct out of ductboard to closely match a lot of the registers I want to measure - set them up against the register using some sort of adjustable stand (most of the systems I work on have the registers on the ceiling) and then use the pitot setup to measure the airflow through the ductboard duct. The long straight run should help with accuracy correct? The only thing is that it would be timing consuming to having to do traverses rather than a single reading with a flow hood. Having said that I think I read in ASHRAE 111 that for small ducts you can get good accuracy with a pitot setup by taking a single reading at the center of the duct.

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    You are correct but the time involved will totally kill your ability to make a profit. I recently had a fire station that had to have an unusually accurate outside air setting. I had them add a duct to the air intake on the unit and traversed it.
    I don't buy the single point traverse unless the duct is less than 3" diameter because velocity pressure profile can be really bad on anything larger.
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    I think I will start with an analog setup to experiment with and progress to the LoFLo in the longer term and use one to cross the check the other. It will be interesting to use the analog setup to cross check the rotating vane. From what I read in the ASHRAE 111 the main issues with rotating vane is the friction of the wheel making it inaccurate at low velocities and also due to the general inaccuracy needs to have a correction factor applied and from the wording it sounded like the correction factor is not constant so you would need some sort of table or equation.

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    I don't have an up to date digital electronic flow hood. The older one I have is too erratic when used on low pressure systems. I would use the analog on residential because they are quicker than the digital and to be competitive you will have to learn to be fast and accurate.
    Don't rush in the beginning because you need to absorb what you are learning. There aren't any secrets to balancing you just have to get good at it.
    The friction of the wheel on the rotating vane is not the problem. You can't develop a correction factor for an application because on identical diffusers with the same airflow the rotating vane will give different readings. The reason is the different directions of the airflow exiting the diffuser. I have a very expensive electronic rotating vane that isn't worth a damn. I ran extensive tests trying to program it for use on diffusers that there wasn't room to get the flow hood on. I was never able to use it on even one job.
    No man can be both ignorant and free.
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    I had a quick look and it seems like the analog hoods are discontinued. May be able to pick up one on ebay. On the other hand maybe the newer digital hoods are more accurate than the older ones you had experience with?

  14. #14
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    I hope you are correct.
    One story about the digital hoods I'll share with you. Keep in mind this was about 10 years ago.

    I got a call from a company that wanted me to take readings on some fans that cooled critical safety controls. When the guy told me where he was located I told him it will take me longer to get there than to run the tests. There is a balance company within a couple of miles of you that can do it a lot cheaper than I can. He said I know that but want a separate opinion. I knew the company close to him never used an analog hood. I suspected he had some bogus readings that are almost always high.
    When I got there I took 10 readings with the digital hood and played them all back for him and ask him what the flow was. He had no idea nor did I. I then took readings with the analog hood and because of the slight wind it varied a little but was easily averaged mentally. That convinced him I should use the analog hood. He told me the balance company down the road had taken the readings with a digital hood. His problem was his control equipment was over heating. They did the design thinking they had enough airflow to cool the equipment but didn't. Their product was literally life safety and had to work at all times.
    You won't run into that on houses but I thought you would like at least one example of why I only used the digital hood on VAV systems.
    No man can be both ignorant and free.
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    Interesting. Without knowing the details of the electronics in the design it's not clear to me why the digital hood would be more inaccurate assuming that the sensing elements such as a pitot tube are the same. The issue could be with inaccuracies in the transducers when the physical quantities such as pressure is converted to an analog signal or errors in the ADC during the digitization process. Once the signal is digitized it is unlikely that the processor would create any additional errors such as when averaging.

    Another possibility is that fluctuations in the air velocity is averaged out by the inertia of the analog gauge which would act as a low pass filter. In the digital domain such fluctuations could be captured in the data and then the residual error could be present event after digital averaging.

    It would be interesting to understand the reasons for the discrepancies. Maybe the manufacturers have been able to resolve these in the latest versions of the tools.

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    Your theory on inertia is correct along with persistence of vision. Airflow fluctuates in air systems but is a lot more exaggerated in low pressure systems. The digital hoods I have read out instantaneous values which are correct at the instant read. Consider a sine wave type airflow where one reading can be at the top of the wave, another at the bottom and more readings at random locations on the wave. You can store the readings and average what you read but all you have is the average of your readings. You don't get a true average of the actual airflow.
    No man can be both ignorant and free.
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    I’ll preface this post by stating, I have nowhere near the air balancing experience as Wayne, but I have done my fair share of testing, taking notes, and comparing different methods for “residential applications.”

    I trust the Alnor Lo-Flo as my #1 for residential supply air diffuser readings so far. It seems to read slightly low, but it’s linear and repeatable. I’ve compared it to high volume flow hoods, high end large vanes, small vanes, hot wires, pitot with auto zeroing digital dual ports, analog magnehelic and red oil tubes, Retrotec powered duct leakage tester configured for air measurements, and probably a few others I’ve forgotten about. I even set up a lab test with a small sealed return and supply ducting system.

    So, when comparing several diffusers, you can weight them. When balancing a residential system, your trying to get a certain % of the total volume to certain spaces. The actual cfm doesn’t really matter much if the meter reading is repeatable and linear, because you’re simply comparing it to the other vents. Basically “robbing Peter to pay Paul”. As long as your static isn’t affected that much.

    The total volume is more accurately read by the fewer return vents. Of course there’s leakage to be considered. Unfortunately, the Lo-Flo is not capable of reading return air volumes above 500cfm. I have found flow hoods to be extremely accurate with return air readings. Larger cfm flow hoods are less accurate for residential supply air readings.

    I found:

    the hot-wire to be accurate at higher velocity without a grille (less turbulence).

    The vanes to be ok most of the time with a grille but less so without a grille. I find the expensive large vane to be a waste of money for small residential supply air vents. It works well with the larger return air vents, but the small vane is probably a better buy for small supplies. Just don’t get a cheap off brand one from e-Bay.

    Pitot only good for in duct, Magnehelic most accurate other than very expensive auto zeroing manometer. Cheaper digital manometers under $400 are not worth connecting to a Pitot. The Magnehelic works very well and is relatively inexpensive.

    High flow hood very good at large return air measurements. Probably good with high volume large supplies, I don’t know.
    Not good with small supply measurements.

    Lo-Flo Alnor very good with small return air, and small supply air.
    Again, it reads a little low on supplies I think, but it’s very reliable IMO. I don’t even bother reaching for anything else when taking residential supply air readings. It has the small 16”x16” short hood.

    For more accurate readings I made a cardboard box with a hyperbola shaped insert. This made both the high flow and low flow hoods read very close to one another.
    "The only true wisdom is in knowing you know nothing" Socrates

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    The other thing I like about the Lo-Flo is the field correction factors that can be programmed to use on different types of grilles. Most residential homes use similar grilles, so the correction factor can be set for: factory, correction A, or correction B, in order to yield very accurate and repeatable results.
    "The only true wisdom is in knowing you know nothing" Socrates

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    I like your posts mgenius33 because we have both dug into this stuff pretty deep. I did commercial and you have covered residential and flow quantities do make a difference. I remember when you were running some experiments and read all your posts and know you didn't take anything for granted. I used the Alnor Lo-Flo hood but for a fairly short period. It was a two month trial before buying. I had 5 flow hoods and was thinking about buying a low skirt hood for office spaces where cubicle walls were tall enough to make reading airflow difficult.

    Post #14 was based on the use of the big bucks digital hoods and big bucks analog.
    No man can be both ignorant and free.
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    The lo flow hood from Evergreen is bad ass . Its not cheap though .

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