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  1. #1
    Join Date
    Jul 2005
    Posts
    28

    I have been in the heating and cooling field for about 10 years now. I have been setting my AC units up by superheat. I have a chart which tells me the superheat for a specific day based on a few variables. To set the unit up I use the Outdoor Ambiant temp, Indoor Dry Bulb, and Indoor Wet Bulb Temps. This is a chart I got while at a Heil, or ICP Training. The question I have is, Will this same chart of formula work for different brands of split systems. I would think it would however I have been installing a lot of Luxaire Equipment and when I initially charge the system based on the extra length of linset and the matched coil it seems that the system is always overcharged based on my chart. Then to get my chart value I have to remove some of the freon that I put in after pumping the system down. I understand that this chart is based on having 400 CFM per ton of ac and I always
    try to install my ductwork accordingly.


    The second question I have is why do the coil temps vary so much with different equipment brands. I just but a 4 Ton 12 Seer unit in my home. The home already had the ductwork installed so I just retrofitted it. I used a 4 Ton 12 Seer Luxaire condenser with a 5 Ton Air handler and coil. I have been instructed that this will gain me a little in the effeciency. I did not however install the TXV valve I used the recommended pin instead. I understand that this hurts the Seer a bit. It seems the system has a hard time keeping up in my home. The freon is boiling at about 44F (74Lb) which seems high. (High Pressure on the Suction side) and the High pressure seems low at 175 lb of pressure. I matched the pin however it seems I may need a pin with smaller oriface. Is this a product of larger coil or of the 12 Seer of the unit. Or is my unit over charged. I added 29oz which was what the manual said for the lineset length as well as the coil installed.

    Any help would be appreciated.. Thanks.

  2. #2
    First, you can't use one chart from one OEM for EVERYTHING!

    Second, fill in these blanks for an educated answer to your question:

    'luxaire piston system'

    SP 74/44
    LP 175/92

    ST __ SH __
    LT __ SC __

    AT __
    CT __
    IDB __ IWB __

    Third, you can't charge every system by SH either.

  3. #3
    you can charge yours by superheat but if you have a system with a txv use subcooling, flip your chart over

  4. #4
    Join Date
    Jul 2005
    Posts
    28
    How do you charge your systems Jultzya?

    I will go out to my unit and get you this information you asked for right now be back in 6 Minutes..

    Thanks

  5. #5
    Join Date
    Jul 2005
    Posts
    28
    What does your abbrievation CT stand for I got the rest of them

  6. #6
    please let the system run for atleast ten minutes before taking your readings

  7. #7
    condensor disharge air temp

  8. #8
    LOL ok...

    By using the OEM charging charts when available...
    SH for piston systems.
    SC for TXV systems.
    (always check the other measurement for a better analysis of the systems performance)

    Just make sure the system has stabilized, in order to get accurate data (10-15 minutes).

    CT (Condenser Temp) outlet air temperature.

  9. #9
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,346
    The second question I have is why do the coil temps vary so much with different equipment brands. I just but a 4 Ton 12 Seer unit in my home. The home already had the ductwork installed so I just retrofitted it. I used a 4 Ton 12 Seer Luxaire condenser with a 5 Ton Air handler and coil. I have been instructed that this will gain me a little in the effeciency. I did not however install the TXV valve I used the recommended pin instead. I understand that this hurts the Seer a bit. It seems the system has a hard time keeping up in my home. The freon is boiling at about 44F (74Lb) which seems high. (High Pressure on the Suction side) and the High pressure seems low at 175 lb of pressure. I matched the pin however it seems I may need a pin with smaller oriface. Is this a product of larger coil or of the 12 Seer of the unit. Or is my unit over charged. I added 29oz which was what the manual said for the lineset length as well as the coil installed.
    First of all, I agree with jultzya, one OEM's chart does not apply to all equipment out there.

    Nailing a charge on a piston system without an OEM's chart isn't a breeze but it's not all that difficult, either. By using a combination of factors, such as superheat, subcooling, delta T across indoor and outdoor coils, etc., you can get very close to the exact charge for a piston system, which incidentally is a critical charge system.

    Notice I included subcooling as one of the parameters to observe while charging. Yes, you do charge by superheat with a piston system, but knowing subcooling helps you to know how well your condenser is working. Knowing the indoor and outdoor delta T's after a charge is adjusted will help you know how well the system is performing. Knowing the superheat will tell you if the evaporator is being overfed, starved, or fed properly.

    In your case, if you live in a humid climate, your 5 ton evap on a 4 ton condenser may give you a sensible heat removal advantage but a dehumidification penalty. Especially with a piston on the system. Pistons do not regulate superheat well, so there are times your system, properly charged, will run a starved evap under high heat loads and a flooded evap under low heat loads. This condition can be aggravated by a mismatched coil and/or an incorrect piston orifice size.

    My recommendation to you is if you wish to keep this coil size, put a TXV on the system. You'll get superior superheat control, higher efficiency, better dehumidifying. You will have to shift your thinking from charging this sytem by superheat to charging by subcooling once you go with a TXV.

    If you wish to keep the system as is, tweak with the charge until you get good delta T's, superheat, subcooling, etc. Make notes of all readings when you obtain good performance from the system, such as ambient temp, amp draw, all delta T's, subcooling, etc.
    If no matter what you do this system still runs with its tongue hanging out, your piston orifice may be oversized. This would show up on your superheat reading as low superheat under normal operating conditions (flooded evap).
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  10. #10
    BTW, if you take a suction temp (EST/STE) at the evap as well at the condenser, it will provide you some in-site of the systems operation.

  11. #11
    Ive got it covered !

  12. #12
    Join Date
    Sep 2003
    Location
    Western PA
    Posts
    1,358

  13. #13
    Join Date
    Jul 2005
    Posts
    28
    I only have one OEM Chart. I use it for everything. Even though it is not Ideal It is much better than most all of my competidors who many of them dont even evacuate the system after the installation. I got the Information and the SH is very low. I charged it with freon exactly how it said in the installation manual, and I believe it is over charged. As I said before this happens every damn time I do what the manual says. This is why I use my ICP Oem Chart. Appreciate the help


    SP 72LB 42F
    LP 173LB 91F
    ST 44F SH 2
    LT 80F SC 11

    OAT 78F
    IDB 76F
    IWB 63F

    On my Single OEM Chart I would like a SH of about 10 So once again the system is overcharged by doing what the installation manual says. Or is my OEM Chart wrong. Does the SEER Rating of the unit change anything. I know a TXV valve will change everything however this is a fixed oriface.

    Also my unit has been running for hours..
    Thanks for the help




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