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Thread: need advice

  1. #1
    Join Date
    Jun 2005
    Posts
    32
    Had a/c serviced earlier this week because unit was severely short cycling. I live in south tx, have a 2 year old 2 story 2400 sq ft home with a 5 ton Carrier a/c. Tech came out and said unit was overcharged by 2 pounds. After he pulled the R-22 his readings were as follows:

    Head-220
    Suction-64
    Superheat-7 deg
    Subcool- 11 deg
    Wetbulb- 63 deg
    RA-72 deg
    SA-58 deg
    SL-44 deg
    DG-98 deg
    Delta temp- 14 deg
    Outdoor temp-94 deg / Indoor temp 75 deg
    Static pressure- .15
    RH inside house- 54 %

    After service I noticed run times got somewhat longer. However, I noticed that the air temp out of the registers did not feel very cool. Laser temp reading on closest register was 62 degrees with thermostat set at 76. Tech said unit was charged precisely to Carrier specs as far as superheat/subcooling goes. From all the posts I have read you guys like to see a 15-20 delta temp and I'm below that with a 2 year old properly charged system. I had ducts inspected and evaporator coil as well. Both were in good shape. Blower is on med speed which reads out to 440 cfm per ton (2200 cfm) according to Carrier chart. If I lower blower to low speed it lowers cfm to 380 Cfm per ton (1900 Cfm). Can I lower the blower speed to bring up delta temp without freezing up the coil? Would I need to get the charge readjusted?

    I appreciate any advice.

  2. #2
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,635
    Do you have a variable speed blower or a standard blower? If it's standard then you don't know what your airflow is. The chart is based on external static pressure. I don't know what duct the .15" reading is of. But that's not total external static pressure. If you have a variable speed blower then there is a good chance that you're getting the airflow that you say. And no, it would not be a problem at all to lower it to 380 CFM per ton. I assume you have a fair amount of humidity? If so then I don't understand the 440 CFM setting. 380 would be much preferred for humidity removal (though 54% isn't bad).

    I don't think you'll need the charge adjusted. That head pressure is fairly low. That implies that you might have a higher than average SEER rating (higher end AC). If so then you probably have a TXV (Freon metering device) which should compensate very well for the change in airflow. If not then there is theoretically a potential issue there. But I kind of doubt it.

    Still… I don’t know if the somewhat high airflow really accounts for the low temp drop. I don’t see you gaining 5 degrees by lowering the CFM setting. Even though it’s been checked I still wonder about the ducts. Poor insulation or a gap in the return duct could be robbing you of capacity.

    And whatever you do, put the infrared gun down and back away! Those things are horribly inaccurate… sometimes. A $15 digital pocket thermometer will blow away an infrared for accuracy any day of the week. Furthermore, the infrared isn’t even measuring air temp. Your own personal investigation will be much more accurate with a digital pocket thermometer.

  3. #3
    Join Date
    Jun 2005
    Posts
    32
    It's a 12 Seer Carrier builder's grade system. Standard blower with piston (no TXV). I got the 2200 cfm based on .15 static pressure on Carrier chart. I think it says 1950 Cfm on low speed (380 cfm/ton). You think it's worth dropping the speed down? Or do you feel there wouldn't be much of a noticable change? It was 99 degrees today with 60+% humidity..typical South TX day in July.

    Thanks for reply.

  4. #4
    Join Date
    Jun 2005
    Posts
    32
    Forgot to tell you that the .15 reading came at the return grill and the discharge above the coil. I had no increase at all between the two. Tech told me this was good. I am assuming he's telling me the truth.

  5. #5
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,635
    "Forgot to tell you that the .15 reading came at the return grill and the discharge above the coil. I had no increase at all between the two."

    I'm still trying to understand that statement. If you're saying that he read .15 at the return and .15 at the discharge then that's not possible. Now if he read negative .15 at the return (at the furnace inlet and not at the grille) and positive .15 on the discharge above the coil then that is possible. That would mean a net static of .3. But even that is not your total ESP.

    Total ESP includes the cooling coil, which would mean taking a measurement at furnace outlet before the cooling coil. That can be hard to do sometimes. A drop of .3 at the coil is not uncommon. That would give you a total ESP of .6. The absolute best total ESP reading that you might get on any system is .5 - and that's on a rare system. Even .6 is good. Most are .7 on up. Unless you know total ESP then you don't know your airflow. And even if you did know total ESP, you'd only know what the airflow through the furnace is. How much of that is actually getting delivered to the diffusers is another matter. I'm willing to bet that your airflow ranges from adequate to low - but not high. As such I don't think lowering even more is a good idea.

    There's something else going on here. Ask your tech how it is that he justifies a 14 degree split. It could simply be that you do actually have somewhat high airflow (which isn’t proven by a long shot) combined with some duct losses combined with a lot of humidity removal (the higher the humidity removal the less the split) combined with a thermometer that’s a degree off. All of those things can conspire to give you a low split. Or maybe it's what I suggested about the ducts. Maybe it's something else. I don't see it by looking at your readings (other than the low temp split of course). But there are smarter people here. Give the thread some time and maybe they'll see it.

  6. #6
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,635
    BTW: Your inital readings indicated 75 degrees inside and 94 outside. But you said it got up to 99 degrees. If your system maintained 75 degrees all day long then you can't ask for much more than that. Though the real test would be 99 degrees outside with a much higher humidity, which I'm guessing you get from time to time.

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