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  1. #1
    Join Date
    Dec 2007
    Location
    East central Indiana
    Posts
    477

    Temp Splits on Missmatches

    Greetings,

    Maybe some of you salty techs could offer some pointers for a lowly rookie.

    Last night, boss gives me a "special project." Basically, the deal is, he allots X dollars to sort of MacGyver a system for a stingy house flipper. The less I spend in materials, the more money he gives me.

    Anyway, the customer's trying to sell this remodled house, but the AC's *not functioning properly*. HIS complaint is clear: the AC's not bringing the temp down QUICK enough, so MY mission is equally clear: To get the MAX temp split out of the system, while spending MIN dollars to do so (not even concerned with humidity).

    So, boss sends me to house armed with only a thermometer, gauges, and jug of refer (a dangerous combination in the hands of a rookie). The original system was installed abt 10 yrs ago by a very reputable company in our area. The AC and coil appear very clean, but I observe the following anomalies:

    1. AC unit is 2 ton 10 SEER, but furnace is 100,000 with 4 ton drive (house is only about 1400 sq ft, med insulation, 9 runs (presumably 6")--IMO, undersized AC, oversized furnace for Indiana climate.
    2. The mother board on furnace had been replaced (by original company), and heat, cool, & cont blower speeds were all jumped to high. In spite of this, some registers seemed weak, and none really had the "jet" feel one would expect under those circumstances.
    3. The liquid line was 5/16, but the unit was clearly stubbed for 3/8.
    4. Probably doesn't matter, but the T-Stat was pretty far from the return (15 ft).
    5. Going into the A coil, about 2 ft of the suction line was bare and exposed.

    Here was the initial performance of the system when I got there. Ambient and indoor were both low 80's. Started up unit, temperatures in registers were only 3 degrees colder than indoor temp. Went outside, suction & liquid line were both *luke warm*. Put gauges on unit, suction pressure was about 45, but the schrader on the liquid line was blocked, and I didn't have any extracting tool on me. Despite knowing better, temptation got the best of me, and I started slowly adding refer and observing the temp split. But here's where it gets wierd.

    I got the suction up to about 80, and the temp split was now about 11 degrees, but for some unearthly reason, the suction line was COLD. My thermometer was inadequate for the task, but I'm guessing my superheat must have been damned near zero. The liquid line felt luke warm at best, so I'm guessing my subcooling was high. It was like I had overcharged the system. But I had been watching the temp split slowly rise from 3 to 11 as I slowly added charge, so I couldn't really understand how I managed to overcharge it.

    I was thinking, if I were truly getting 1600 cfms (from the 4 ton drive at high speed), which I almost certainly wasn't (higher than normal for a 2 ton, but probably not THAT high--my guess would be 1300 actual), I might expect an optimal split on a 2 tonner to be in the 10 degree ballpark, maybe more since the sensible/latent ratio would go up.

    Also, I was thinking that those conditions (undersized liquid line, excessive airflow, exposed suction line, etc) would give me relatively HIGH superheat at a relatively HIGH suction pressure, even though the measured conditions seemed to be backasswards.

    So, I decided to let the unit run a while, went home, ate a hamburger. Came back to the job, and saw that after about an hour running time, the unit had dropped the indoor temp about 7 degrees. Normally I wouldn't guess this is too bad, but I imagine the ambient had dropped a bit too. Went back home, decided I'd sleep on it, and try again this evening (lost a lot of sleep, as these things tend to bug me).

    Thus, my question: Does anyone here have any tips on what kind of tools I should take, or what kind of tests I should do? Since this is a special "after hours" project, I'm free to lollygag at my own pace (really love this kind of scenerio, btw). My only limitations are expense. Also, does anyone here know how sensitive sensible/latent ratios are to high cfms? The only data I have are on 350 to 450 cfms per ton, and not on gross overkills. My hunch is that ductwork issues may be at the heart of the problem.

    Any help would be greatly appreciated.

    Regards

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,590
    We'd be glad to help but this is the type of discussion for the pros forum. Based on message count, you are ready to apply. So please do and we'll move this thread over there.

  3. #3
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,759
    The higher the CFM the lower the latent capacity, and the higher the sensible capacity.

    Just a few of the Tools you should have.
    Type K thermoprobe.(strap or clamp on)
    Digital sling.
    CRT.
    Manometer, and either static pressure tips, or pitot tube.
    P/T chart.
    Recovery cyclinder.
    Tape measurer, pencil and paper.
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  4. #4
    Join Date
    Dec 2007
    Location
    East central Indiana
    Posts
    477
    Hi Beenthere,

    Thank you for your reply. By P/T chart, do you mean the ones that come on gauges, or the ones that people use for charging older units? I suspect that, given the unusual conditions of this system, it'd be nearly impossible to hit the *proper* charging pressures.

    Fellow Hoosier, Bald Loonie,

    Thank you for the advice. I submitted my application for pro membership (albeit, most likely improperly) moments ago.

    Regards,
    ECInd

  5. #5
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,759
    Standard P/T chart. Its easier to read when converting to saturation temp, then just using the gauges.
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    How many times must one fix something before it is fixed?

  6. #6
    Join Date
    Dec 2007
    Location
    East central Indiana
    Posts
    477

    Unhappy

    Hi Bald Loonie,

    I got my pro membership, but I have no idea how to post this in the pro forum.

    Beenthere,

    This system is still giving me a lot of grief. The customer seems happy, and I've been paid, but I still have the nagging sensation that the system is not running right at all.

    The boss gave me a P/T chart, and told me to charge the system like this: If the outdoor temp is 85, I should add 30 degrees to that temperature (115) and look at the P/T chart for 115 (243 psi), and that should be my head pressure, and the outdoor temp should be my suction pressure. In other words, on an 85 degree day, I should have pressures of 243/85.

    When I raised the suction pressure to 85, the high side was slightly under 260. I suspect this was due to the apparently undersized liquid line. In that case, the temp split across the evaporator was about 13 degrees, about as good as it's ever been, but I had almost no superheat and a whopping 20 degree subcool. Almost like what you would expect using the "beer can cold" method.

    When I dropped the head to 243ish, my suction dropped to about 80. Still, only about 3 degree superheat, and an even lower split across the evap (11 degrees).

    Dropping charge for a more reasonable superheat (10 degrees), I ended up with pressures 220/62, but only about a 5 degree split across the evap.

    My only guess is that my gauge readings or pipe temp readings (more likely) were off. BTW, I estimated the cfms of the system to be about 1100 based on a degree rise of about 68 degrees when I ran the furnace (about 80,000 output). So, obviously there are ductwork/blower wheel/motor issues with the furnace (since it is on high speed w/ 4 ton drive). But a 13 degree split on a 2 ton condenser at 1100 cfms with virtually no condensate removal just seems...well...pathetic

  7. #7
    Join Date
    Dec 2007
    Location
    East central Indiana
    Posts
    477

    Unhappy

    Hi Bald Loonie,

    I got my pro membership, but I have no idea how to post this in the pro forum.

    Beenthere,

    This system is still giving me a lot of grief. The customer seems happy, and I've been paid, but I still have the nagging sensation that the system is not running right at all.

    The boss gave me a P/T chart, and told me to charge the system like this: If the outdoor temp is 85, I should add 30 degrees to that temperature (115) and look at the P/T chart for 115 (243 psi), and that should be my head pressure, and the outdoor temp should be my suction pressure. In other words, on an 85 degree day, I should have pressures of 243/85.

    When I raised the suction pressure to 85, the high side was slightly under 260. I suspect this was due to the apparently undersized liquid line. In that case, the temp split across the evaporator was about 13 degrees, about as good as it's ever been, but I had almost no superheat and a whopping 20 degree subcool. Almost like what you would expect using the "beer can cold" method.

    When I dropped the head to 243ish, my suction dropped to about 80. Still, only about 3 degree superheat, and an even lower split across the evap (11 degrees).

    Dropping charge for a more reasonable superheat (10 degrees), I ended up with pressures 220/62, but only about a 5 degree split across the evap.

    My only guess is that my gauge readings or pipe temp readings (more likely) were off. BTW, I estimated the cfms of the system to be about 1100 based on a degree rise of about 68 degrees when I ran the furnace (about 80,000 output). So, obviously there are ductwork/blower wheel/motor issues with the furnace (since it is on high speed w/ 4 ton drive). But a 13 degree split on a 2 ton condenser at 1100 cfms with virtually no condensate removal just seems...well...pathetic

  8. #8
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,759
    That method is about as reliable as beer can cold.

    With 1100 CFM(550per ton) your not going to get a big temp diff.

    With that much air moving through a 2 ton coil, your heat transfer from the air to the refrigerant is inefficient, because of lack of contact time. The coils bypass factor is increased too much.

    Why not change the cooling speed to low, to get the air flow closer to what a 2 ton should have for moisture removal.
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