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Thread: High Humidity

  1. #1
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,311

    Lightbulb

    Humidity is in the low 60's and I'm not sure how to get it lower.
    System is 5 years old, we bought the house the end of last summer and didn't notice the humidity problem (weren't checking for it either)
    Funace is a Carrier 58WAV091-14 with a 3 ton coil.
    Condensing unit is a 38BC03632.
    Metering device is a .073 piston
    DeltaT 13-18 degrees depending on outside tempature.
    Suction line feels cool (not cold enough to sweat) to touch, liquid line is about the same temp as outside ambient.
    Unit is oversized, runs about 1/2 the time on 100 degree day.
    I've already tried lowering the blower speed and that did get it from the upper 60's to the lower 60's
    I suspect the freon charge might be low but have no way to check it since I don't have a set of gauges.
    Any ideas on the best way to get the humidity down?
    Would adding a TXV be cost effective?

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,350
    Best bet is to have properly sized system.

    Failing that, a TXV will get you better performance...when the system is running. It'll still be oversized, as I'm sure you know.

    As it is with your piston system, when you change blower speeds you also need to check the charge. Piston systems are darn near if not actually critical charge systems.

    Only way to nail system performance as it stands right now is to get superheat/subcooling readings under hot/humid conditions like this and nail the charge for the airflow you have going over the coil.

    Did I mention that pistons suck?

    • 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.

  3. #3
    Join Date
    Nov 2000
    Location
    Eastern PA
    Posts
    68,950
    Add the TXV and bring in some outside air into the return. Slightly pressurizing the home with outside air that is immediately conditioned will help keep the humidity from entering the home from outside.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  4. #4
    Join Date
    May 2004
    Posts
    80
    I had this exact problem last year- oversized system, 3 ton condenser (5 year old 12 SEER) and orifice metering device. The system couldn't get second level below 60% RH and lower level below 80%. I tried lower fan speeds but still couldn't get the humidity down to a comfortable level.
    Airman1 advised me to simply downsize the coil to 2 ton and go with an externally equalized balanced port adjustable expansion valve. I maintained 1200 cfm of air to support the 3 ton condenser. Bingo! Perfect! I now can easily maintain 50% RH downstairs and 44% RH upstairs. I saved the expense of a new condensing unit. Since I'm in the HVAC business I did the work myself. This aggressive sizing method drives some of my HVAC buddies crazy. Many, many HVAC contractors I know have installed systems that are oversized. Many others oversize the coil chasing SEERS while giving up comfort. Say what you will about this kind of design but I'm here to tell you it works like a charm. By the way, my late evening room temp. is 75 degrees with 44.1% RH. It feels very comfortable. With the old coil everything felt sticky. Thanks again Airman1.
    Hope this helps.

  5. #5
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,471
    Originally posted by 54regcab
    Humidity is in the low 60's and I'm not sure how to get it lower.
    System is 5 years old, we bought the house the end of last summer and didn't notice the humidity problem (weren't checking for it either)
    DeltaT 13-18 degrees depending on outside tempature.
    Suction line feels cool (not cold enough to sweat) to touch, liquid line is about the same temp as outside ambient.
    Unit is oversized, runs about 1/2 the time on 100 degree day.
    I've already tried lowering the blower speed and that did get it from the upper 60's to the lower 60's
    I suspect the freon charge might be low but have no way to check it since I don't have a set of gauges.
    Any ideas on the best way to get the humidity down?
    Would adding a TXV be cost effective?
    First, can you slow the air flow more? The limitation on slowly in the air flow is condensation on ducts/grills and freezing the a/c coil. A 25^F delta T will get lower %RH. Before you spend big bucks on changing the a/c coil, consider a 100 pint dehumidifier. Down sizing the coil does not provide dehumidification with low or no cooling load. A 100 pint dehumidifier provides <50%RH with any cooling load at any temperature. The cost of removing moisture with a high eff. dehu like Ultra-Aire is 5 pints.kwh compared to 2 pints per kw plus the risk of overcooling.

  6. #6
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,311

    Lightbulb

    Actually I have it set on the lowest speed (red wire) and the best it will do is 18 degrees. With a 14 as the last 2 numbers in the furnace model # I'm guessing 1400 CFM on high speed but have no way of knowing what low speed airflow is.
    A dehumidifier may do the trick until the unti is old enough to justify being replaced with a correctly sized unit. At that time the funcae will be dropped to a 40,000 BTU or what ever is the smallest one avalible at the time. (Furnace runs less than 1/2 the time @ 20 degrees)
    Either that or I'll just live with he low 60's humidity, it does FEEL better now that I switched the blower to low speed.


  7. #7
    Join Date
    Apr 2005
    Location
    Tampa, Florida
    Posts
    1,634
    Adding a dehumidifier not only removes humidity, but the heat it puts out will also run the AC more, additionally pulling more humidity out. Noisy & inefficient, but it'll definitely get the job done. Now, you could use the patented Joe Homeowner(TM) method of decreasing airflow. Buy some of those 3M Filtrete filters, clog 'em up real good and never change them. Instant reduction in airflow.

  8. #8
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,296

    Lightbulb Case Solved

    Originally posted by tpa-fl
    use the patented Joe Homeowner(TM) method of decreasing airflow. Buy some of those 3M Filtrete filters, clog 'em up real good and never change them. Instant reduction in airflow.
    I always wondered
    WHY clean filters were NOT Good for Comfort.
    L.O.L.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  9. #9
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,471
    Originally posted by tpa-fl
    Adding a dehumidifier not only removes humidity, but the heat it puts out will also run the AC more, additionally pulling more humidity out. Noisy & inefficient, but it'll definitely get the job done. reduction in airflow.
    Regular residential dehumidifiers "noisy and inefficient" yes <2 pints per KW--With the Ultra-Air +5 pints per KW, when UA is ducted, because it's in the utility room.

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