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  1. #1
    Join Date
    Mar 2008
    Posts
    657

    couldn't get the suction pressure up

    fellas did an install today on a 5 ton lennox ac13 with an old lennox oil furnace (don't have model number). dropped charge after pulling vac. started unit up and it was running a 64 and a 200 ambient temp 85. went in side checked air filter and it was clean, it was an older merv. 16. i don't have the tools to check cfm. lineset was about 25 ft. long 3/8 by 1 1/8 i added 1lb. suction did not rise past 64 head rose to 225. by the way it was a txv system. what could be the prob? added another 12 oz and nothing.

    and yes just the ac was replaced not my decision, the HO did not want to pay the xtra dough. i informed him about possible effects of this but he didn't want to hear it.

    and yes i should apply for pro membership, just waiting to get a new copy of epa card ( lost the orig.)

    thank in advance guys
    Last edited by badboyheel; 06-12-2008 at 08:41 PM. Reason: add info
    "The things you own end up owning you. "

    Tyler Durden

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,317
    Well, when you added that pound, you probably over charged it.
    You got low air flow most likely.

    You should have checked the firing rate of the furnace. Then fired it up and check temp rise and figured out your CFM from that. You might only be moving 325 CFM per ton.
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  3. #3
    Join Date
    Sep 2005
    Location
    Bay Area, CA
    Posts
    843
    Add to that you should check the charge, you didnt check the charge. You just saw what the preasures were.
    Global Warming or: None like it hot
    No pleasure, no rapture, no exquisite sin greater... than central air. -Dogma

  4. #4
    Join Date
    Mar 2008
    Location
    Grundy Co. IL
    Posts
    773
    Does that old oil furnace have a belt drive?

  5. #5
    Join Date
    Mar 2007
    Location
    NW AR
    Posts
    2,478
    Not enough info.

  6. #6
    Join Date
    Mar 2008
    Location
    Grundy Co. IL
    Posts
    773
    Quote Originally Posted by ar_hvac_man View Post
    Not enough info.
    We're getting to that., But you're right could be undersized ducts.

  7. #7
    Join Date
    Mar 2007
    Location
    NW AR
    Posts
    2,478
    Did you change the coil or leave the old one in?

  8. #8
    Join Date
    Sep 2006
    Location
    Southern California
    Posts
    86
    mismatched system gives no guarantee of proper system operation. check esp, i bet it's way too high. probablly never get the refrigerant side balanced out correctly.

  9. #9
    Join Date
    Mar 2008
    Posts
    657
    well i checked my approach temp and it was right on. if remember correctly my superheat was on the high side 25. beenthere your right i could've checked cfm using temp rise, damn. still learning and that leaked out of my brain. jay it wasn't a belt drive. i thought it was lack of air flow, and say that was the case, whats the next step if furnace blower does not produce enough airflow.
    "The things you own end up owning you. "

    Tyler Durden

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,317
    High SH isn't from low air flow. Low air flow gives a low SH.
    Do a temp rise. Find out if you are low on air flow.
    Then you have to determine why. Most 5 ton coils are very restrictive. It may be the coil causing most of the restriction. A static pressure test will tell you if its teh coil, return, or supply duct.
    You may be able to set the blower to a higher speed.
    After you have your air flow to 1750 or more. Yopu can take your enterID WD, and Db temps, and start the A/C again. Recheck your SC/approach and see what your SH and coil delta are, and check your discharge air temp.
    If its still not inline, check the TXVs powerhead.


    Is the TXV adjustable.
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  11. #11
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,006
    An "oil furnace" with a large heat exchanger coming to near the top of the furnace?

    If it is like the Thermo Pride OL 11 you will have a restriction "especially for 5-ton," between the coil & the heat exchanger.

    Check the static pressure (SP) between (after) the blower & before the coil, then look at the blower chart or graph for CFM at that ESP.

    It ought to deliver 2,000 to 2250-CFM to achieve a full 5-ton of cooling under Rated conditions. The linked page will explain my point.

    http://www.udarrell.com/oil_furnace_heating.html

    Could that furnace be similar to this one?

    - udarrell

  12. #12
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,006
    Quote Originally Posted by udarrell View Post
    An "oil furnace" with a large heat exchanger coming to near the top of the furnace?

    If it is like the Thermo Pride OL 11 you will have a restriction "especially for 5-ton," between the coil & the heat exchanger.

    Check the ESP between the blower & the coil: that is "air probes" before the blower & before the coil, then look at the blower chart or graph for CFM at that ESP.

    It ought to deliver 2,000 to 2250-CFM to achieve a full 5-ton of cooling under Rated conditions. The linked page will explain my point.

    http://www.udarrell.com/oil_furnace_heating.html

    Could that furnace be similar to this one?

    - udarrell
    I had to clarify original confusing wording, by editing the bold info
    Last edited by udarrell; 06-15-2008 at 12:50 PM. Reason: "air 'probes' test points"

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