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  1. #14
    Join Date
    Nov 2011
    Location
    USA
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    391
    Quote Originally Posted by John Smith 2 View Post
    The verbally told me just remove and replace the safeties with higher temperature values. I said that I didn't think it would be appropriate for me to do so.


    Do you mind revealing the brand name/mfg?

  2. #15
    Join Date
    Mar 2011
    Location
    North Carolina Piedmont Area
    Posts
    450
    Static pressure does not indicate true air flow. If you do not have a flow hood than you should simply be able to calculate CFM by the Equal Friction method

    Equal Friction Method:
    Area (Sq Ft.) = CFM divided by velocity FPM
    where as: area = 15,000 CFM divided by 1500 FPM = 10 Sq
    10 Sq. Ft x 144 (Sq. Inches per Sq. Ft)/24" = 60"
    Therefore, select 60"x24" supply duct


    5400 CFM ÷ 1700 FPM velocity = 3.18 Sq Ft. Duct Area


    Aspect Ratio = 48/12= 4
    Aspect Ratio 4 = 144%
    Round duct = 66%
    Difference = 78%

    Velocity FPM = airflow CFM/ area in Ft Squared

    Velocity x Area = CFM

    To Figure CFM by known Sq Ft area and Static Pressure.

    To Measure Air Volume there are two easy methods:
    1. Temperature Rise:
    In the temperature Rise Method, the heating section is ran with the indor fan speed set to cooling speed.
    The temperature of air entering and leaving the air handler or furnace is measured to find the temperature rise. A formula is computed and air volume is determined.

    2. External Static Pressure Method:
    This called total external static Pressure.
    A differential manometer is used to measure the air pressure at the inlet and outlet of the blower assembly. This pressure is the same pressure a balloon has pushing against its internal walls. Air Blowers have the same type of pressure at the inlets and outlets. The measured air is compared to design charts provided by the equipment manufacture to determine how much air volume is flowing in cubic feet per minute. (CFM).

    As the static pressure across the fan goes up, the ability to move air goes down. This static pressure at the inlet and outlet is called total external static pressure.

    The higher the Static the less air will be able to move through the air handler. The less static pressure that is present, the greater the ability to move air through the air handler.


    For every cubic foot of air, the air has weight.
    Every pound of air carries so many BTU's of heat in it.
    When a system is designed the amount of air is set and the heat in the air is measured. This is important as the system can only absorb as much heat as is available. There are two ways the amount of heat fluctuates, by the amount of pounds of air flowing and by the heat in the air. If the heat in the air the system capacity falls. If the amount of pounds of air falls, the system capacity falls.

  3. #16
    Join Date
    Nov 2008
    Location
    DFW,TX
    Posts
    2,170
    please tell us the brand you are having issues with so we can stay away from them.. thx

  4. #17
    Join Date
    Nov 2011
    Location
    USA
    Posts
    391
    Airflow has been confirmed and re-confirmed...reply #'s 3 and 13.

  5. #18
    Join Date
    Mar 2011
    Location
    North Carolina Piedmont Area
    Posts
    450
    Quote Originally Posted by Saturatedpsi View Post
    Airflow has been confirmed and re-confirmed...reply #'s 3 and 13.
    So what is the total actual CFM being delivered. What is the temperature Rise. What are the temps of the safeties. With out this info there is no way to judge whats really causing his problem..

    Trouble shooting is not part of the repair...understand the symptoms and you will find a solution.

  6. #19
    Quote Originally Posted by alcomech View Post
    So what is the total actual CFM being delivered. What is the temperature Rise. What are the temps of the safeties. With out this info there is no way to judge whats really causing his problem..

    Trouble shooting is not part of the repair...understand the symptoms and you will find a solution.
    There are 3 different tonnage sizes. Each size is delivering right at nominal cfm's per ton as determined by capture hood. Actual cfm output matches manufacturer's spec's at .2" wc. Remember I said above that heat strips are cutting out on safeties and fuse links. Both strips don't stay on long enough to take meaningful delta t measurements. As far as the cut out temperatures on the safeties and the fuse links they are what they are. I would never attempt to second guess or propose replacement of a higher temp after a units been tested and approved by ETL/UL. As stated in my original post my quest here is to find others who actually have installed like units with these unusual type heat strips as I have described above. I think I may have also stated above I removed and put in another brand unit with conventional heat strips and the units run every time perfectly.

  7. #20
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    16,076
    Quote Originally Posted by alcomech View Post
    What is the temperature Rise.
    This is a very good question, forget about the airflow for a moment and check temp rise. This will at least tell you if there is enough air moving over the heat strips, I was told back in the early 70's by my instructor, before reinventing the wheel, check for the most basic things first. Please check this and get back.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

  8. #21
    W
    Quote Originally Posted by Mr Bill View Post
    This is a very good question, forget about the airflow for a moment and check temp rise. This will at least tell you if there is enough air moving over the heat strips, I was told back in the early 70's by my instructor, before reinventing the wheel, check for the most basic things first. Please check this and get back.
    Mr. Bill, thanks for your reply.... What would you think if I added this observation that we have found. About 75 per cent of the surface area of the heat strips are not in contact with the airflow passing from the blower ports through the heater compartment. To back this up where the air does make contact with the surface area of the elements there is no discoloration on the elements. The surface area out of the air flow is found to have turned a dark brown to black color. Please give me your thoughts on this. Thanks!

  9. #22
    Join Date
    Nov 2011
    Location
    USA
    Posts
    391
    Quote Originally Posted by John Smith 2 View Post
    W

    Mr. Bill, thanks for your reply.... What would you think if I added this observation that we have found. About 75 per cent of the surface area of the heat strips are not in contact with the airflow passing from the blower ports through the heater compartment. To back this up where the air does make contact with the surface area of the elements there is no discoloration on the elements. The surface area out of the air flow is found to have turned a dark brown to black color. Please give me your thoughts on this. Thanks!
    I've never seen anything (don't believe) like you describe...how 'bout taking a pic and post it up for us?

  10. #23
    Join Date
    Nov 2011
    Location
    USA
    Posts
    391
    Another question: is this equipment brand/model/design something that's been around a while or something out of China that you decided to try?

  11. #24
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    16,076
    Does this look like your elements?

    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

  12. #25
    Join Date
    Jul 2009
    Location
    South Carolina
    Posts
    3,265
    I agree with the guys on checking the temperature rise. That should tell you everything you need to know. If I had that problem I would temporarily bypass all the limits long enough to do the test. It shouldn't take more than 15 minutes. For reference a 10kw strip at 800 cfms should give about a 40 deg. Temp rise. If you figure the CFM's by the temp. rise method then if your rise is too high you've got airflow problems & if its too low you've got air bypassing the strips. If its in the ballpark then I would think the limits are probably the problem & the factory reps recommendation is correct.
    Gary
    -----------
    http://www.oceanhvac.com
    An engineer designs what he would never work on.
    A technician works on what he would never design.

  13. #26
    Thanks Mr. Bill, Saturatedpsi and Gary for your replies... The units were sold to me by one of the largest (if not the largest) mfgr/distributors in the country, however, the units were manufactured by another company under another label. I don't want to yet mention their name because I don't want to bring upon them any bad press. I have bought hundreds if not thousands of fine air handlers and condensing units from them and probably will continue to do so. When you look at this unit from the outside and for the most part from the inside, it looks almost identical to the units manufactured here in the country by other big name brands. The one exception is the heat strips. The heat strip that you depicted is not what is in this unit we are having trouble with. The heat strip you depicted is in the brands that I replaced the troubled unit with. Notice how the strips you posted have thin curely cue wires stragically fashioned in size & shape and placed to be directly in front of the two blower ports. Every inch of these curely cue ht strips are washed in the air stream thus giving up all their heat to the supply air; unlike what is happening with the troubled units. I'm not sure the factory could up the safety temeratures after being tested and labeled, if they could I think they would have. Besides if the customer is not getting the benifit of all the heat being generated being brought out in the room and rather just heating up the furnice and ceiling cavity its really not fair to them for the wasted energy cost.

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