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  1. #14
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    Quote Originally Posted by hvacrmedic View Post
    Well, I don't think there's a problem there. If you block the supply the TESP will rise. Block the return, the TESP will rise. If you block the airflow within the air handler, anywhere between the specified test points, then you have a problem. A dirty secondary hx for instance in a furnace, or a dirty evaporator coil inside an air handler (or dirty blower wheel-thanks Core_d) will negate the TESP chart. It is by definition the total EXTERNAL static pressure that the chart is referring to.
    Yes this would be tesp. So tech measures the static. In this case the hot water coil and the dx coil are about 5' and 10' downstream. Tesp says 1.0" Tech says "cool I've got my 3000 cfm." But whoops, one of the coils is dirty, (can't see the dirt) and is causing a high supply static. Actual cfm is 2200. If he's not thorough, he misses it, if he's sent there to get tesp to compare to a fan curve, then that data is invalid, but he doesn't know that. He can't know the cfm without measuring it. Or at least having a good DP location somewhere.
    I see that on chiller barrel drop too. Where the strainer must be clean or you show much higher gpm than actual.
    I think that what I'm coming to here is that the fan static profile is a useful thing to know, AFTER the job is complete. So as a record of fan data when it was moving x cfm. This would be good info.


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  3. #15
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    Quote Originally Posted by icy78 View Post
    Yes this would be tesp. So tech measures the static. In this case the hot water coil and the dx coil are about 5' and 10' downstream. Tesp says 1.0" Tech says "cool I've got my 3000 cfm." But whoops, one of the coils is dirty, (can't see the dirt) and is causing a high supply static. Actual cfm is 2200. If he's not thorough, he misses it, if he's sent there to get tesp to compare to a fan curve, then that data is invalid, but he doesn't know that. He can't know the cfm without measuring it. Or at least having a good DP location somewhere.
    I see that on chiller barrel drop too. Where the strainer must be clean or you show much higher gpm than actual.
    I think that what I'm coming to here is that the fan static profile is a useful thing to know, AFTER the job is complete. So as a record of fan data when it was moving x cfm. This would be good info.


    Sent from my SM-G930V using Tapatalk
    It's important to read the instructions. Measure at the right points and there isn't a problem, usually.

  4. #16
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    Yea i agree it could be misleading. But fan curve charts are accurate. Especially if the start up contractor could ducment such a thing. Not that that ever happens...

    Sent from my SM-S320VL using Tapatalk

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  6. #17
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    You are asking the right questions.

    Yes, total static pressure is the difference between supply and return. Anticipate a negative return and positive supply readings. And yes you are correct, without measuring the volumetric flow, the static is meaningless. Manufacturers provide fan curves and tables for volume vs static pressure, but IMHO that helps diagnosis, and does not provide an excuse to avoid field traverse.

    Fans can be configured for a flat curve where flow is varies greatly across a small pressure range such as the forward curved direct drive fans in residential systems. Fans can be configured with a steep curve that maintains flow across a large pressure range and are often used for high pressure applications like dust collectors and inflatable bounce houses.

    The Loren Cook Company publishes a handbook that includes fan types, their classification, construction, selection and related information.
    http://www.lorencook.com/PDFs/Catalo...ok_Catalog.pdf

  7. #18
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    icy,
    You are really getting good. There are several good comments from others as well on this subject.

  8. #19
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    Quote Originally Posted by hvacrmedic View Post
    It's important to read the instructions. Measure at the right points and there isn't a problem, usually.
    Yes I agree on the instructions etc. I'm referring mostly here to existing commercial systems that I, or anyone, is troubleshooting for , well let's say, insufficient cooling. My MO usually is to establish cfm first. At least the last few years. I would love to get that by static readings but am finding that is a waste of time. So I now have flow hood, pitot tube, velocity grid, hotwire anemometer, and the back of my hand.

    Now I don't know about new resi systems but it seems to me that would be the same situation. Altho I've done resi equipment in commercial setup and done the tesp and had eg. 0.7" and measured the cfm and been below 2000 or whatever max was for that system. So it worked there i guess. That's all new equipment and duct work tho.

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  9. #20
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    Quote Originally Posted by icy78 View Post
    Yes this would be tesp. So tech measures the static. In this case the hot water coil and the dx coil are about 5' and 10' downstream. Tesp says 1.0" Tech says "cool I've got my 3000 cfm." But whoops, one of the coils is dirty, (can't see the dirt) and is causing a high supply static. Actual cfm is 2200. If he's not thorough, he misses it, if he's sent there to get tesp to compare to a fan curve, then that data is invalid, but he doesn't know that. He can't know the cfm without measuring it. Or at least having a good DP location somewhere.
    I see that on chiller barrel drop too. Where the strainer must be clean or you show much higher gpm than actual.
    I think that what I'm coming to here is that the fan static profile is a useful thing to know, AFTER the job is complete. So as a record of fan data when it was moving x cfm. This would be good info.


    Sent from my SM-G930V using Tapatalk
    Just to be clear, you are aware that higher TESP means lower CFM right? If you think in terms of the fan laws it's just the opposite. A lot of people get confused trying to sort out the difference.

    The blower curve assumes no change in the blower or its cabinet, the only changes are with the externally connected system.

    The fan laws OTOH assume no changes in the externally connected system, the only changes are to the blower wheel geometry or speed. If both are changing good luck with that.

  10. #21
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    Quote Originally Posted by hvacrmedic View Post
    Just to be clear, you are aware that higher TESP means lower CFM right? If you think in terms of the fan laws it's just the opposite. A lot of people get confused trying to sort out the difference.

    The blower curve assumes no change in the blower or its cabinet, the only changes are with the externally connected system.

    The fan laws OTOH assume no changes in the externally connected system, the only changes are to the blower wheel geometry or speed. If both are changing good luck with that.
    Maybe we're talking apples and oranges. If a fan has .5" tesp and is moving x cfm, and I change the drive to get 2x cfm. Well then the tesp is greater by a factor of 4 if I remember right. But maybe what you are saying is that if a fan has a tesp of .5 and you add restriction and the tesp goes up to whatever, then yes , the cfm goes down.

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  11. #22
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    Quote Originally Posted by icy78 View Post
    Maybe we're talking apples and oranges. If a fan has .5" tesp and is moving x cfm, and I change the drive to get 2x cfm. Well then the tesp is greater by a factor of 4 if I remember right. But maybe what you are saying is that if a fan has a tesp of .5 and you add restriction and the tesp goes up to whatever, then yes , the cfm goes down.

    Sent from my SM-G930V using Tapatalk
    Right. The coils in the duct that you mentioned, 5 and 10 ft downstream, won't cause an error in cfm calcs unless you measure on the downstream side of them.

  12. #23
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    Q Fan Curve.pdf
    I tried to put an attachment on here for the first time and doesn't look I did it right but you can probably get to it.

  13. #24
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    I can't read that attachment Wayne but my numbers are as follows. approx 3200 CFM read at the return, 2.28 brake horsepower at 1.05 inches static, at 2841 rpm. How does that look?
    The total supply of 2200, as measured by flow hood, is considerably less which is why I question the hood reading. Ductwork is fairly well sealed running at 0.26" static .

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  14. #25
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    When plotted on the fan curve using return CFM it looks really good. According to the fan curve at 3200 CFM horsepower draw is just over 2 hp and total ESP is just under 1.5 inches.
    The reason I think your return CFM is accurate is that you are measuring CFM entering the fan and duct leakage prior to the traverse doesn't matter. From your description of the duct you have a good return duct traverse plain.
    Without seeing the entire supply system there is no way to assess the potential error to expect using the flow hood. According to the SMACNA duct leakage manual hood measured airflows as much as 20% lower than the traverse is not unusual.
    I will send you an email of the fan curve.
    You are delivering about 89% of design airflow. To reach design by calculation you need 3196 RPM. That will probably over load the motor but you can get over 90% of design without motor over load but in my opinion you are good where you are unless there is an issue with comfort.

  15. #26
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    Wayne, you're awesome. Thanks for the help!

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