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  1. #1

    Duct sizes contributing to high humidity?

    My relatively new Amana APH1536 first floor package unit is not keeping my RH below 70%, particularly at night. My second floor fancy 20 SEER Am Std split system just barely gets it below 60% upstairs. Here are my house/HVAC specs:

    First floor 1850 sq ft
    Package unit: Amana 3 ton HP
    CFM: 1100 (dipswitch setting B)
    Return: 16" round flex, about 25' long, sealed with mastic
    Grille: 20"x25", centrally located in home with CLEAN MERV 6 filter
    Supply: 12 equal runs, nicely tapered toward far end from HP, sealed with mastic
    Drain pan and condensation pipe in great shape w/ P-trap
    T-stat is digital touch screen but no RH sensor
    T-stat set at 75 degrees constantly
    Fan is in Auto mode
    Encapsulated crawlspace (CleanSpace)
    Crawlspace Santa Fe Advance Dehu set at 50%
    Energy star windows
    Humidity range: 75% at 6AM 64% at 6PM
    Temp range: 73 at 6am 75 at 6PM

    Based on recommendations from my tech, I am considering adding a second 12" return duct and filter grille from my foyer area and then joining it to the existing return box. Then he would install a new 18" or 20" return duct from the existing return box to the package unit. Will this help my RH? Other suggestions???

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,376
    Aside from duct leakage or building leakage, the other common reason for poor humidity control in cooling mode is a problem with the refrigeration side of the unit. Has that beeen thoroughly investigated?
    • 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
    Jan 2004
    Location
    PA
    Posts
    68,748
    What size is he second floor 20i.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  4. #4
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,636
    The delivered %RH in the home is dependent on the a/c cooling coil temperature while cooling. The coil must be 45-50^F to provide 75^F, <50%RH during 1-2 hour cooling runs. During low/no cooling load conditions and outdoor dew points of +50^F, supplemental dehumidification are required. This is typical wet cool weather that is common in green grass climates.
    After you get your a/c setup right, reducting the Santa Fe Advance to provide supplemental dehumidification for the whole house. Ducting the air from the mainfloor of the home to the dehu with a supply back to the home and a minor supply to the crawlspace will provide <50%RH throughout the home. Or you can add an Ultra-Aire 70H to the main floors of the home.
    Get the a/c setup correctly first. I gather you are not concerned about fresh air change during calm weather?
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  5. #5
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,031
    There are a number of the A/C system problematic causes of lack of humidity control:

    1. A refrigerant overcharge
    2. Refrigerant undercharge plus icing-up
    3. liquid line restriction
    4. low evaporator airflow
    5. inefficient compressor superheat & suction may appear normal under less than a full heatload
    6. non-condensibles in refrigerant system
    7. TXV metering device: bulb loose;poorly insulated bulb; bulb lost charge; non-condensibles, poor to normal with a TXV


    I'm not going to list how you check for each problem on the list, however, a good Tech will know how...

    Heat pumps have accumulators which makes any adjustments in the charge a bit tricky. A packaged HP unit would come with the correct charge...most of the problems listed above occur with split-systems.

  6. #6
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    Does your A/C cycle on and off with the "on" cycle being short? If this is true, your unit might be oversized. The return flex could be bigger and there is a good chance our return airflow is too restrictive. Did you have a load calculation done before you put in your new units?
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  7. #7
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    What is hte location? What size is hte upstairs unit?

    That severe of a humidity problem makes me thing oversizing. If it's a well insulated home 3 tons can still be pretty large for even 1800sqft.

    I also wonder if the unit is drawing in outdoor air somehow. TO have RH that high, you must be in a very humid climate or have a very tight home and do lots of showering and cooking without ventilation.

  8. #8

    Answers to questions so far

    Awesome posts, everyone! Answers to several questions on the replies are:

    1) Location of home - North Alabama. RH issue is by far worse at night.
    2) Upstairs is 1400 sq ft
    3) Upstairs unit is a 3 ton split system Am Std, 20 SEER Communicating. It almost never needs to run at "full blast"
    4) Upstairs has a bonus room over a garage with heavy heating/cooling load. A single 1-ton commercial-style supply and register are dedicated to that room.
    5) Upstairs has no return duct, just a box - unit is in a closet right beside the grille.
    6) A load calc was done on the 1800 sq ft downstairs at the time of unit installation. Result was 3.5 tons. I talked the tech down to a 3.0 ton based my known RH problems and what I learned from this forum :-)
    7) Cycle time downstairs is not too long -- ~20 to 30 minutes each cooling run.
    8) Insulation in home: average to above-average; crawlspace insulation is like a finished basement (foam on the inside walls).
    9) Note: My walls have no OSB sheathing; they have 1" foil-faced foamboard on the outside of the walls covered by brick veneer
    10) Yes, am interested in fresh air supply during calm weather but don't know how to solve that.
    11) Leakage at the unit is always a possibility. But both my supply and return ducts are tight with mastic and good takeoffs.

    Thanks!!!

    Backpacker

  9. #9
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,031
    5) Upstairs has no return duct, just a box - unit is in a closet right beside the grille.
    You have to be sure the closet is airtight sealed from the hot high grains of moisture attic air or the heavy-loaded coil may not be operating cold enough for optimal dehumidification.

    1) Outdoor condenser’s discharge-air-temperature ______-F
    Subtract Outdoor air temperature: _______
    Outdoor Condenser Air-Temp-Split _______

    2) Need the Indoor percent of relative humidity at time of temp-tests - in the middle of the rooms or, at Return-Air inlet grilles ___
    3) Indoor Return-Air Temperature ______
    Subtract Indoor Supply-Air Temperature ______ -F
    Indoor temperature-split _______-F

    It will be a TXV with 410A.
    If U can find a turn in the small liquid line insulate the temperature-probe; get its temperature for the subcooling analysis; tubing sensors are best:
    Small liquid line temperature ____F

    To measure return air the wet bulb temp, wrap a small/thin, wet (not dripping) piece of cloth around the probe and put it into the Return Air grille & then close to the closest Supply Air diffuser near the air handler (need air movement). Wait until the temperature stabilizes - may take a little while.

    Return Air wet bulb ____F
    Supply Air Wet Bulb ____F

    Need the above information for troubleshooting & performance analysis.

    Example below:
    A Goodman 2-Ton 13-SEER condenser, @800-cfm indoor airflow; 80-F indoor dry bulb & 50% relative humidity; Indoor temp-split 18 to 19-F.
    @ 85-F outdoors; 103.9-F - 85-F outdoors or around an 18.9-F temp-split;
    @Indoor 75-F, 63-wet bulb around 50% RH - condenser temp-split is only 14.9F

    A 20-SEER system will have a much lower outdoor condenser split than the above 13-SEER example, we can look it up.

  10. #10
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    7,742
    Is your crawled pace damp from groundwater

  11. #11
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,841
    Did anyone ever do a load analysis before sizing the equipment? How about proper sizing using Manual 'S'?
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  12. #12
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,841
    Quote Originally Posted by backpacker View Post
    Awesome posts, everyone! Answers to several questions on the replies are:

    1) Location of home - North Alabama. RH issue is by far worse at night.
    2) Upstairs is 1400 sq ft
    3) Upstairs unit is a 3 ton split system Am Std, 20 SEER Communicating. It almost never needs to run at "full blast"
    4) Upstairs has a bonus room over a garage with heavy heating/cooling load. A single 1-ton commercial-style supply and register are dedicated to that room.
    5) Upstairs has no return duct, just a box - unit is in a closet right beside the grille.
    6) A load calc was done on the 1800 sq ft downstairs at the time of unit installation. Result was 3.5 tons. I talked the tech down to a 3.0 ton based my known RH problems and what I learned from this forum :-)
    7) Cycle time downstairs is not too long -- ~20 to 30 minutes each cooling run.
    8) Insulation in home: average to above-average; crawlspace insulation is like a finished basement (foam on the inside walls).
    9) Note: My walls have no OSB sheathing; they have 1" foil-faced foamboard on the outside of the walls covered by brick veneer
    10) Yes, am interested in fresh air supply during calm weather but don't know how to solve that.
    11) Leakage at the unit is always a possibility. But both my supply and return ducts are tight with mastic and good takeoffs.

    Thanks!!!

    Backpacker
    This entire post screams over sized. You state that the 2-stage 2nd floor unit rarely runs "full blast", meaning that it's operating day and night on 1st stage. Just that alone is a huge statement. If it's a 1-compressor unit, then it's running on approximately 2.0-tons on 1st stage, even during the day. What does that tell you? It tells you that 2.0-tons is plenty and maybe even then, too much for the space and tightness of your home. During the day the unit may run 20-30 minutes per cycle but I suspect those are optimistic times. I'd suspect the unit is running 10-15 minutes during the day and even shorter cycles at night. For each cycle, the first 8-10 minutes of run time is just allowing the compressor to 'organize' the refrigerant in the system. The indoor coil begins to cool but does not get down to the air temperature dew point until about 10-minutes have passed. Clearly a 10-minute cycle will then remove little or no moisture from the air.

    The simplest and least expensive remedy for your situation is to have a whole house dehumidifier installed. This will operate continuously under most summer conditions and keep the RH in the house much lower. It will also negate the need for the AC system to operate longer cycles to maintain RH, so the AC can just be used to maintain temperature. Someday, when you're ready for a new system, have an accurate Manual 'D' done by a well qualified and experienced company/person and size the equipment according to that load and Manual 'S'. You'll be surprised what the results will show.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  13. #13
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    Quote Originally Posted by backpacker View Post
    Awesome posts, everyone! Answers to several questions on the replies are:

    1) Location of home - North Alabama. RH issue is by far worse at night.
    2) Upstairs is 1400 sq ft
    3) Upstairs unit is a 3 ton split system Am Std, 20 SEER Communicating. It almost never needs to run at "full blast"
    4) Upstairs has a bonus room over a garage with heavy heating/cooling load. A single 1-ton commercial-style supply and register are dedicated to that room.
    5) Upstairs has no return duct, just a box - unit is in a closet right beside the grille.
    6) A load calc was done on the 1800 sq ft downstairs at the time of unit installation. Result was 3.5 tons. I talked the tech down to a 3.0 ton based my known RH problems and what I learned from this forum :-)
    7) Cycle time downstairs is not too long -- ~20 to 30 minutes each cooling run.
    8) Insulation in home: average to above-average; crawlspace insulation is like a finished basement (foam on the inside walls).
    9) Note: My walls have no OSB sheathing; they have 1" foil-faced foamboard on the outside of the walls covered by brick veneer
    10) Yes, am interested in fresh air supply during calm weather but don't know how to solve that.
    11) Leakage at the unit is always a possibility. But both my supply and return ducts are tight with mastic and good takeoffs.

    Thanks!!!

    Backpacker
    At night the fan won't need to be on "full blast" and actually the lower the fan speed/CFM the more moisture is removed from the conditioned air. The Santa-Fe Dehumidifier is for your crawlspace and doesn't directly dehumidify your house correct?

    I am wondering if it gets cool enough at night that you reach your thermostat setpoint, so the AC turns off but it is still too humid to be comfortable. What temperature are you setting your thermostat at night? Is your fan on "auto" or "on" position? I would say set your temperature between 65-70 F (adjust until you feel comfortable), make sure your dehumidifier is on, and make sure your fan is "on". Usually 20 SEER units will come with thermostats with humidity controls. This will drive up power usage, but you should feel more comfortable.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

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