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  1. #1
    Join Date
    Mar 2015
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    Seattle
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    Question Do you measure CFM during maintenance?

    Hi all,

    New tech here and on my maintenance calls I would like to start expanding what I do (past cleaning, filter change, and overall checkup) so I can personally get a better picture of the condition of furnaces and heat pumps, and so I can be more than a parts changer when things fail. In my limited training I've heard non-stop how the number one issue with these furnaces and heat pumps is that they are not moving adequate air or are moving too much air through the system which causes high operating pressures and reducing the life of the internal components.

    My question here is do you look at air flow during maintenance calls, and more importantly how are you doing it?

    I'd like to post a hypothetical example to see if my thinking is correct here and get pointers where I need them.

    I have recently been using CFM = BTU (output)/DeltaT * 1.08 and then comparing that to the tonnage of the furnace. E.g. 400 CFM/ton on a 3 ton system = 1200CFM. If they are moving, for examples sake, 1900CFM my thoughts are to lower the fan speed to increase our DeltaT and bring that CFM to as low as we can at the furnace itself. If after that we are still moving, say, 1600CFM, well that tells me I need to whip out the ductulator and find out if their ductwork is sized properly.

    These concepts are pretty new to me so I'd like to see how you all do this in the field vs. what I have learned on paper. I just believe it to be important in understanding the overall health of a system. Thank you

  2. #2
    Join Date
    Jan 2016
    Location
    Michigan
    Posts
    24
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    The majority of my maintenance calls are related to utility rebates, which requires it in the paperwork.

    For air conditioning, I usually use a magnehelic and pitot tubes, then reference the manufacturer's tables. I then do a quick cross check with an anemometer. (Newly aquired AAB200)

    For heating, I also use my magnehelic, but temperature rise is a good way to back into it.

    Even if it wasn't for a utility rebate, I would still measure it on the majority of calls. That said, there's always the homeowner that makes you feel rushed, so if i'm in a pinch, I do just rely on temp rise to tell me if i need to investigate further.

    I have very recently put together an excel spreadsheet that calculates the airflow, efficiency, actual BTU output, and a few other things based on the ANSI standards formulas for furnaces. I want to actually share it with whoever wants to try it out, but not sure how. I am fairly new to the site, so perhaps one of the senior members can explain how to share docs.

    Also, Tru Tech Tools has a good video library. check it out, i think there's a video on measuring pressure / airflow.

  3. #3
    Join Date
    Jun 2001
    Location
    Louisville, KY
    Posts
    12,515
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    Quote Originally Posted by raggyshakes View Post
    Hi all,

    New tech here and on my maintenance calls I would like to start expanding what I do (past cleaning, filter change, and overall checkup) so I can personally get a better picture of the condition of furnaces and heat pumps, and so I can be more than a parts changer when things fail. In my limited training I've heard non-stop how the number one issue with these furnaces and heat pumps is that they are not moving adequate air or are moving too much air through the system which causes high operating pressures and reducing the life of the internal components.

    My question here is do you look at air flow during maintenance calls, and more importantly how are you doing it?

    I'd like to post a hypothetical example to see if my thinking is correct here and get pointers where I need them.

    I have recently been using CFM = BTU (output)/DeltaT * 1.08 and then comparing that to the tonnage of the furnace. E.g. 400 CFM/ton on a 3 ton system = 1200CFM. If they are moving, for examples sake, 1900CFM my thoughts are to lower the fan speed to increase our DeltaT and bring that CFM to as low as we can at the furnace itself. If after that we are still moving, say, 1600CFM, well that tells me I need to whip out the ductulator and find out if their ductwork is sized properly.

    These concepts are pretty new to me so I'd like to see how you all do this in the field vs. what I have learned on paper. I just believe it to be important in understanding the overall health of a system. Thank you
    How are you calculating btuh output?
    Perhaps you should have read the instructions before calling.

  4. #4
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    11,757
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    I've never found a system in residential that has too much airflow.

    The simplest way to check airflow is by temp rise on an air handler with electric elements.

    The next quickest is by using static pressure and manufactures blower performance chart.

    The most time consuming is traversing the duct with an anemometer.

    On service calls I either use temp rise or TESP/blower chart for a quick check. It usually becomes apparent that you have an undersized return duct as soon as you stick your manometer probe in the return. Supply side is usually ok. 90% of duct systems I check need additional return capacity to get rated performance.

  5. #5
    Join Date
    Feb 2008
    Location
    Western Wa.
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    1,102
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    No if your delta T/ temp rise is ok why waste your time you have enough to do.
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    The higher your post count the more you know

  6. #6
    Join Date
    Jan 2016
    Location
    Michigan
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    Quote Originally Posted by jrbenny View Post
    How are you calculating btuh output?
    you can calculate how much heat went into heating the air if you know the temp rise and cfm. However, the formula is only as good as your measurements, and only calculates how much heat went into the air. Also, without taking humidity into account, it is less accurate.

    The formula is:

    BTU captured by the air = CFM x 1.08 x ∆T

    It is kind of like a reverse heat loss. also, it does not take into account jacket loss or anything like that. just the heat into the air.

  7. #7
    Join Date
    Jun 2001
    Location
    Louisville, KY
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    Quote Originally Posted by BurnerGuy View Post
    you can calculate how much heat went into heating the air if you know the temp rise and cfm. However, the formula is only as good as your measurements, and only calculates how much heat went into the air. Also, without taking humidity into account, it is less accurate.

    The formula is:

    BTU captured by the air = CFM x 1.08 x ∆T

    It is kind of like a reverse heat loss. also, it does not take into account jacket loss or anything like that. just the heat into the air.
    He's rearranging the sensible heat equation to solve airflow. He has to know btuh output to do that.

    Back to my question for the OP...

    How are you determining btuh output?
    Perhaps you should have read the instructions before calling.

  8. #8
    Join Date
    Dec 2012
    Location
    columbus, OH
    Posts
    2,950
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    No but static pressure should be done and recorded on a start up as well as circling all settings on stat and control board. That would be nice:-)

  9. #9
    Join Date
    Jan 2004
    Location
    PA
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    If you have a furnace moving 1900 CFM. Then slow the blower to its lowest and its still moving 1600 CFM. Its not a duct size problem. But an over sized furnace/blower problem.
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  10. #10
    Join Date
    Nov 2008
    Location
    Bristol Va.
    Posts
    1,186
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    Quote Originally Posted by jtrammel View Post
    I've never found a system in residential that has too much airflow.
    I found a 2.5 ton heat pump with a static of .3 and the blower in medium speed. The homeowner for 12 years felt his hp is blowing too much air.

    .3 tesp - med. speed =1280 cfm
    .2 tesp - low speed = 1030 cfm

    so yes it does happen.

  11. #11
    Join Date
    Sep 2008
    Location
    VA
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    Quote Originally Posted by jtrammel View Post
    I've never found a system in residential that has too much airflow.

    The simplest way to check airflow is by temp rise on an air handler with electric elements.

    The next quickest is by using static pressure and manufactures blower performance chart.

    The most time consuming is traversing the duct with an anemometer.

    On service calls I either use temp rise or TESP/blower chart for a quick check. It usually becomes apparent that you have an undersized return duct as soon as you stick your manometer probe in the return. Supply side is usually ok. 90% of duct systems I check need additional return capacity to get rated performance.
    X2

    With the exception of never too much air flow. A lot of mfg's install 1/3hp blowers in their smallest furnace. With a 1.5-2 ton match up its usually way too much air flow. A high static pressure typically found on most residential equipment brings it back down.
    "The only true wisdom is in knowing you know nothing" Socrates

  12. #12
    Join Date
    Feb 2016
    Location
    Roanoke, VA
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    8
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    Nope, I do not check air flow and all of that because on a system maintenance all of those issues "should" have been dialed in at time of the install. I only worry about it if the customer says something. Besides, how are you going to make any money for the boss when it takes you 2 hours to do all of that plus the drive time to get to that customer's house?

  13. #13
    Join Date
    Jan 2015
    Location
    Bradenton, Fl
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    I live in South Florida so almost everything is air conditioning. For delta T the target is a 20* temp. drop across the coil with 50% relative humidity and 75* return air. Not much help to you in Seattle as I'm thinking it's cold there now but come summer the a/c's will be hummin.

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