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  1. #1

    Confused

    OK I have a few more questions. I had a few more bids and had a few contractors say not to replace my coil and that I should think about keeping my outside unit. They said that what might be best is redoing my duct work and adding a TXU expansion valve on the coil. One did state I could replace the outside unit to a 13 seer however my saving might not be a huge amount and it might be best to wait until failure and then replace the whole system at once..


    So my question is what do you think about my current duct work. I under stand that it is hard with seeing but by the numbers do I looked like a return/supply mismatch? Do I need larger returns?

    Return grill/filter 20x25x1
    Return duct 16 R value5.8

    Coming off the supply plume
    6 R value5.1
    6 R value5.1
    4 R value5.1
    6 R value5.1
    6 R value5.1
    8 R value5.8
    10 R value 5.1 which connects to a triangle which has the follow branch off


    6 R value5.1
    6 R value5.1
    6 R value5.1
    6 R value5.1
    4 R value 5.1

    Current Models of A/C, Coil and furnace are as follows:
    Allegiance 10 7A0048A100A0
    American Standard TXHC41A4H9A0
    Consolidated Industries HBA100ND5R


    If I am reading the numbers right my supply output right now is only about 947CFM when my unit can output around 1575CFM. And my return is about 1100 CFM right now when it should be about at least if not more then 1575 CFM

    So my unit is acting like a 2.5 ton unit? Am I way off base?


    I have had 5 different people come out and each ones tells me something different. You guys are helping me figure who is talking BS and who knows their job. Thanks


    Never spend money blindly

  2. #2
    Join Date
    Nov 2005
    Location
    Northern VA
    Posts
    512
    Any chance you could get the static pressure in the return and supply plenums near your current unit checked? That would be a more solid basis to draw conclusions from.

  3. #3
    no contrator has done that. is that something i can do? tell me how and i will

  4. #4
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,274
    Originally posted by firedoghfd
    Current Models of A/C, Coil and furnace are as follows:
    Allegiance 10 7A0048A100A0
    American Standard TXHC41A4H9A0
    Consolidated Industries HBA100ND5R

    If I am reading the numbers right my supply output right now is only about 947 CFM when my unit can output around 1575CFM. And my return is about 1100 CFM right now when it should be about at least if not more then 1575 CFM

    No actual air flow is known based on reviewing specs until the static pressures are measured as perel has stated.

    http://www.dwyer-
    inst.com/htdocs/airvelocity/AirVelocityIntroduction.cfm

    Request tech/estimator to measure prior to any meaningful bid and work scope definition.





    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  5. #5
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    The return does appear to be a little more undersized then the supplies.

    Get the static tested,and compare it to the fan chart for you Furnace ,to get the actual cfms.


    The lone filter grille is undersized ,as well.

    Poating pictures of the indoor eqipment and plenums ,would help in assisting up.

    Call around ,ask if they can do a Manaul J,S and D ,to size the system,select equipment and redesign ducts,respectively.If not keep dialing.

    These three manuals are the Industry Standard from http://www.acca.com

    If you are currently able ,with that duct system,to maintain 76 to 78 in your home,chances are ,your system is oversized.Undersized ducts cause reduced air flow ,which in turn reduces the equipments capacity,thus I'd guess it's oversized,if it's holding a resonable temp..


    FYI: The 10" comingof the plenum ,then going to several branch ducts,is likely to be problematic,likely getting much less air flow then the branches that come off the plenum.One solution ,would be if it came off the top of the plenum.Understand ,it and other ducts are still likely undersized.

    Return can be remedied by adding return(s),to another area,with ducting from a properly sized plenum.

  6. #6
    Join Date
    Jul 2002
    Location
    Yuma, AZ
    Posts
    2,361
    The tool needed is a manometer. You can make one. See http://www.komar.org/faq/manometer/.

    The pressure drop across the furnace is measured in inches of water column. ("WC) Furnaces have weak blowers. The furnace label usually says that it is designed to 1/2 "WC or .5 "WC. That isn't much, so the professionals use a manometer that is more accurate for such low readings. Nevertheless, the homemade version will establish the ballpark. The furnace static pressure is measured from the duct entering the furnace to a point BETWEEN the furnace and coil. This is no easy feat to measure.

    A new, clean A Coil will have a pressure drop of about .25 "WC. This leaves only about .25 "WC pressure to move the air through the duct system. The duct system pressure is measured from the duct entering the furnace to the pressure measured in the duct leaving the furnace.


    SEE http://www.bacharach-training.com/Tips/esp.htm

    I looked for a non-professional explanation of using this concept and came up with nothing. Not too many companies or technicians understand the airflow side of their business.

    Static pressure measurements are a simple and powerful tool in the hands of a knowledgeable technician.
    Try http://www.nationalcomfortinstitute.com/Find.cfm
    **************************
    In looking at your submitted data, where did the CFM numbers come from? You said, "If I am reading the numbers right my supply output right now is only about 947CFM when my unit can output around 1575CFM. And my return is about 1100 CFM right now when it should be about at least if not more then 1575 CFM"

    If there is no leakage, the CFM in and out of the duct system would be the same. Leakage is strongly indicated by these numbers. The duct systems are commonly undersized and the equipment oversized.

    A trustworthy source of more information is http://homeenergy.org/archives.html

    The contractor is the key to delivered efficiency and comfort. Find the right one. Good luck.
    "I have never let my schooling interfere with my education."
    Mark Twain
    More at: http://www.quotationspage.com/subjects/education/

  7. #7
    "Find the right one"

    harder done then said.


    I want one that will be honest and tell me if i really sjould update the system or just tweak it a little. everyone i've talk to has been replace everything. no question asked, here is my price. no manual j,D, or S. no looking at the system. Hell i bet if i had a 2 year system they would still be like your best option is the new 14-18SEER, new ac, new coil, new furance, new ducts, new house.



    so do you know a good honest hvac guy in houston? let me know. If he says hang on to the system another 2-3 years i promise in 2-3 year he will be the one who replaces my unit.


    used car sales and HVAC sales......why do they feel so alike?





  8. #8
    Join Date
    Aug 2006
    Posts
    10
    I personally think your return is undersized.
    taking static PSI is the thing to do.
    Finding "the Guy" is hard.

    I had similiar issues and actually found "the right guy" thru (Dare I SAY it) The Home Depot. Stores around us have a local Co doing in store sales. I spoke with the sales rep for about an hour. Only gut that knew what was going on.
    For no $ they sent out a tech that did a manual J on the whole house including static psi on the exisiting systems & redesigned the ductwork prior to a new unit sale. Of course I did not buy a new unit but they did the work for me.

    The local co was not even listed in the Yellow Pages. But they did the job right.



  9. #9
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Call around ,ask if they can do a Manaul J,S and D ,to size the system,select equipment and redesign ducts,respectively.If not keep dialing.

  10. #10
    so far most are like we will come out and "look at the system and setup". if we want a manual anything done it will cost $$$$.


    man i feel like that should be part of a bid for 4000-10,000 job. are all hvac guys lazy when it comes to bids. lol

  11. #11
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,274
    Originally posted by firedoghfd
    we will come out and "look at the system and setup".

    ... I feel like that should be part of a bid for 4000-10,000 job. Are all hvac guys lazy when it comes to bids?
    Not at all, just the Original Poster IS.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  12. #12
    Join Date
    Dec 2005
    Location
    California/Nevada
    Posts
    3,628
    in my opinion,going on the CFM numbers you said,
    your return duct is too small by about 2 - 3"

    1) not worth the money to change
    2) most systems are sized wrong anyways
    3) for .1 static pressure you are about the right size
    (many companies use .1 static pressure for residential , it should be acceptable)


    i think your supply CFM is more like 1180, most likely not worth changing either.



    i would get the AC system working properly first , then see what you have with the air flow.









  13. #13
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Originally posted by wolfstrike
    in my opinion,going on the CFM numbers you said,
    your return duct is too small by about 2 - 3"

    1) not worth the money to change
    2) most systems are sized wrong anyways
    3) for .1 static pressure you are about the right size
    (many companies use .1 static pressure for residential , it should be acceptable)


    i think your supply CFM is more like 1180, most likely not worth changing either.



    i would get the AC system working properly first , then see what you have with the air flow.








    1.Likely more then worth the money to correct it.Reduced cfms resuklt in a capacity loss of btus,could easily be losing a 1/4 ton of sensible capacity,while still paying for it on the electric meter.

    2.True,but things need to change!

    3.The .1 reffered to above is not static,it FR(friction rate ),found on a ductolator.Fr is static per 100 feet of equivalent duct,homes can have 300 to 600 equivalent feet of duct.

    .1 is used as a Rule Of Thumb,by those unable or unwilling to determine the correct FR for each job.


    Testing is required to determine the existing cfm.

    The A/c can never work properly ,if the air flow is not proper.

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