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  1. #1
    I am a homeowner with a complete Rheem system, 3 ton condensor, 3 ton coil and 75k gas furnace. This was a complete changeout and the old system was the same size. My house is 1,400 sq. ft.

    The first thing I noticed was my indoor humidity was higher with the new system. The old one kept it in the 40's% while the new one does 50's and 60's%. I thought at first it might be because the new one didn't run as much but then I discovered it doesn't seem to make a lot of difference how much it runs. I read here that lower fan speed could help so I started turning it down. I have now finally arrived at 50% humidity (at night) by turning it all the way down to 293cfm per ton! If I put it any higher it lets it slip up too high. Is this unusual?

  2. #2
    Join Date
    Jun 2005
    Fort Worth TX
    sounds like mabey yor fan speed is mis configured have the install crew check it
    Never Say Die

  3. #3
    Join Date
    Apr 2004
    was your previous system a 3 ton system?

    We dont get hung up on sizing by square foot but 3 ton seems high.

  4. #4
    Yes, it was. It is a tad high but since I like it 70 degrees they thought it would do a better job for me.

    The houses in this neighborhood are all about the same size. You see a lot of 2 1/2 ton systems but when you walk into their houses it's 78 degrees too and that isn't cool enough for me.

  5. #5
    Join Date
    Apr 2004
    You do realize that cooler air cannot hold as much moisture and the end result is a higher relative humidity, dont you?

    The same air in your home at 70 degrees and 55% RH would be at 75 degrees 46% RH. The amount of moisture didnt change in either case, the grains are the same.

    Now, oversizing the unit isnt helping either and if you are running it at 293 cfm per ton and able to maintain 70 degrees you are oversized. I would suspect there is a duct leakage issue in the return in either the attic, basement or crawl space that was not there before. The unit you have, has one if the better latent capacities. Alot of things can be done during a system changeout that can cause changes in comfort. What else has changed?

  6. #6
    The house is on a slab and it's a downflow system. I was losing some air through the plenum but they reworked it before they set the new furnace on it. The return was in good shape so they reused it.

    It doesn't hold the house at 70 degrees on anything higher than a 90 degree humid day. Yesterday for example it was 96 with a heat index of 104. Through the hottest part of the day it held at 73 and then went back down in the evening. On days like that it will run for 12 hours easily.

    The default airflow is 400cfm per ton. But when I use that the air being delivered is not cold enough and the humidity stays in the high 50s and 60s. One time I took the temperature of the air from the furthest supply vent and it was 70 which will not do very much good. With the airflow like I have it now it stays in the 50s to 60 at the most.

    The old system was R22. The furnace was as old as the house and the air had been replaced once.

  7. #7
    Join Date
    Apr 2004
    All I can say is the refrigerant has no bearing whatsoever on the humidity levels. The saturation temperatures are the same. 400 CFM per ton is nominal airflow for any system whith the exeption of maybe a high velocity system.

    What furnace is it matched with?

    Granted at 293 cfm per ton you have reduced the capacity to probably just that of a 2.5 ton or lower system so I guess thats a non-issue. I would however be concerned that with a 70 degree return at 293 cfm per ton you might experience some frosting of the coil which will effect overall performance and reliability of the system. There should be a TXV which may help protect the superheat levels but I have to think that the coil is at or near freezing.

    At what CFM was the system charged? If it was charged at 400 CFM per ton and now you reduce the airflow to 293, you might be overcharged for those conditions. BTW, where did you get 293 cfm per ton? Just curious.

  8. #8
    Join Date
    Aug 2002
    Office and warehouse in both Crystal River & New Port Richey ,FL
    How are measuring the cfms?

    Note:Old system ,dirty coil,blower,may have been running at less cfms per ton.

  9. #9
    I have the GLL-07EBRKR furnace. It has four possible fan speeds for cooling that can also be adjusted either 10% + or - and you can do a 15% -. So it makes for a lot of possible speeds. I have tried 400cfm -10% and -15% and both were still too high. So I just went down from there and tried 300cfm and it worked pretty well. The next step down was 293cfm.

    It occurred to me that it could freeze up. It was charged at 400cfm. So far so good though. It is interesting also that the air flow is very similar to the old system.

  10. #10
    Join Date
    Apr 2004
    Low airflow would certainly account for the lack of air at the final register. Too low of a static will not allow air to reach that furthest register, meaning you are reading more or less the room temp mixed with some random duct air.

    If you reduce the air too much, you may also only be using part of the indoor coil. Unlike an A coil, there are several "A's" and you may not be utilizing the entire coil.

    I would be interested in return WB readings measured at the inlet of the indoor unit compaired to the House WB temps where you are taking your temperature and humidity readings. It would be best to use the same tool in both locations.

    I wouldnt set the airflow below 15% of nominal. In your case 1020 CFM. To many variables and unknowns can be a result. TXV can start to hunt, degrading sensible and or latent capacities, inconsistant flow through the coil, water blow off (dripping water into the plenum) among other things. You may want to make sure the plenum is dry, a small condesate leak into the plenum may be enough to mess with your RH levels.

  11. #11
    I will work on that because I would be interested in them myself. I have to go now though.

    I just went out and raised the fan to 1020cfm and will let you know how that goes too.

    I get good airflow throughout the house. The furthest register blows hard enough to move the curtains above it.
    All of them have a similar air flow which is good and strong.

    I want to thank you for all of your help. Know that if you were in my area I would gladly pay you to check it out for me.

  12. #12
    Join Date
    Apr 2004

    nobody knows where you are.

  13. #13
    I thought my location was in my user information but I see it is not. I don't care if you know where I am Oklahoma city, Oklahoma where it got up to 103 today.

    At noon the air coming in my furthest register was 59, the return temp was 69 and the temperature in the house was 71.

    At 6pm it all moved up 2 degrees . . 61 / 71 / 73

    The humidity in the house is 62% but will probably come back down as the temperature comes down.

    Not too bad for an over 100 degree day.

    I don't have a tool to do wet bulb measurements but I will get one. I am curious to know what they are myself.

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