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  1. #1

    Help with difficult diagnosis.

    Newbie here and I have a doozy of a problem.
    Here is the scenario : 1800 sq ft, condo in coastal SC. Only exposed walls are the front and back (about 700 sqft total), the top, bottom and sides are all adjacent condos with functional HVAC and normal temps. In the summer months, owner reports significant humidity problems and condensation on vents (bathroom, microwave and clothes dryer). I placed a data logger in the residence to find temps are maintained properly per thermidistat settings, but the humidity cannot be kept below 70%. No evidence of plumbing leaks, etc.
    Ran a blower door to find exchange rates of about 5.
    The existing HVAC system was a 3 ton system. We replaced the whole HVAC system with a 2 ton system. Humidity levels seemed to drop by about 5-10% on average, but still stay around 65%. We then sealed some known openings in the front and back walls to try to reduce the exchange rate (haven't retested with the blower door). Preliminary findings suggest tightening the construction did not result in humidity reduction.

    Thoughts or suggestions?


  2. #2
    Join Date
    Feb 2011
    Install a dehumidifier?
    Living the dream !!

  3. #3
    Join Date
    Jun 2009
    South Carolina
    how old is the building??

    Is it ocean front.
    If so is there alot of big windows facing the ocean?

    Is it a rental or primary residence?

    May have duct leakage between the ceiling and the floor above?

    Good luck.

  4. #4
    Join Date
    May 2007
    Atlanta area
    One of the main causes of extra humidity that often goes overlooked is leaving the blower on all the time during the summer. People do this thinking that air circulation helps to dry things out and reduces the need for air conditioning (AC), but they are mistaken. Why is this?

    One of the main benefits of AC is that it reduces moisture content of the air. Dry air feels cooler than humid air, so the t-stat can be set one or two degrees higher when the humidity has been reduced. Moisture condenses on the evap coil, then drips off into the drain pan and runs outside via the pvc drain pipe. But when you leave the blower on all the time, the condensed water on the evap coil evaporates back into the indoor air and raises the humidity level. When outdoor air is added, cooled, and rehumidified, you make the indoor air's relative humidity (and sometimes even the absolute humidity) higher than that of the outdoor air. Just like Congress, you have accomplished the opposite of your intention, and you have broken your promise.

    So you end up turning the t-stat to a cooler temperature in order to maintain your comfort level. Now you are pulling more moisture out of the air (and re-evaporating it) than you were before.

    The end result is that you have a lot of cold water vapor in the air, just dying to condense on something. Some of the cooler surfaces in your house are fogged up and you have mold problems.

    This problem is made worse by oversizing your system. A bigger coil means more rapid cooling of the air with less condensation. A bigger blower means more thorough evaporation of the condensed moisture on the coil.

    Condensation is more plentiful when humid air is exposed to the evap coil for long periods so that the water vapor has enough time to change from a gas to a liquid. But it won't run out through the drain line if the blower is on.

    What's Congress's solution? Raise taxes! Add a dehumidifier!

    What's your solution? Turn off the blower.
    Last edited by Space Racer; 07-28-2013 at 11:00 PM.
    Vacuum Technology:
    CRUD = Contamination Resulting in Undesirable Deposits.
    CRAPP = Contamination Resulting in Additional Partial Pressure.

    Change your vacuum pump oil now.

    Test. Testing, 1,2,3.

  5. #5
    Thanks for the replies
    Here are a few more details I forgot to add in the OP.
    Klee - 7 years, Not ocean front, not overloaded with windows. The building is situated on the ICW. What effect does the duct leakage have?

    De-humidifier (standalone) was successful in reducing humidity, but this is not a long-term solution
    The existing system does have a dehumidify function, however, even in this function the humidity cannot be reduced before hitting the adjusted temperature setpoint.
    SpaceRacer, the blower has not been set to "on".
    I think my focus at this point are the excessive exchange rate and what seems to be a very oversized HVAC system.

  6. #6
    Join Date
    Aug 2012
    How long are the run cycles in the heat of the day? What about your air flow and ductwork, what size is your return grill? What about SH and SC.
    Trying not to be a Hack.

  7. #7
    Join Date
    Jun 2013
    N. Canada
    Any way that you can lower the airflow? This really pulls the moisture out of the air.
    But it's a touchy balance... go too far & its frozen coils.

  8. #8
    I am not an HVAC guy, but I am trying to help someone solve a problem.
    toocool, I am not certain about run times (based on logger data, it seems they run about one 20min cycle per hour). I'll have to check on the grill size. What should I be testing for on the airflow and ductwork? I don't know what "SH" and "SC" are.
    IRBH - the existing systems do have a two speed fan for dehumidification, but they keep cooling the space down to the lower temperature limit and shutting off. It is possible they are ONLY running in the dehumid mode due to the cooling from the surrounding units.

    I will offer this bit of just-realized information. An identical unit in another building had the power turned off for a few days in early July. Outside temps reached 85 (a little cool for this time of year) and the interior temperature of this non-powered condo never exceeded 77 degrees while the humidity skyrocketed to nearly 85%!!!

  9. #9
    Join Date
    May 2007
    Atlanta area
    You need to eliminate sources and be 100% certain whether they are or are not providing moisture.

    1. Eliminate the AC. Do a simple pressure test to check for leaking ducts. Close all windows and doors. Turn on the blower. (This is not the same as a blower door test.) Using a cigarette or some other device that produces visible smoke, check at the bathroom fan and kitchen fan to see if air is being sucked into the condo. There may be a noticeable breeze. Alternately, after covering these (and any other) fans and sealing them with tape, crack open a window and check for a breeze there. Incoming air means you have a broken or leaking duct somewhere.

    If the duct is leaking and the duct is in a separate, unconditioned space, you could easily be blowing 20%-30% of your conditioned air to the outside, and replacing it with air being sucked in through various penetrations. It might be outside air or it might be air from another condo.

    If the broken duct is within the conditioned space, this test won't tell you anything, and the broken duct won't reduce condo pressure, so this kind of infiltration is probably not a problem.

    2. Turn off the AC on a dry day, ventilate condo with fresh air, close the doors and windows, cover and seal all fans, and take humidity readings at regular intervals over a 24 hour period. Are the readings similar to those taken when the AC ran? Are changes in humidity smooth or sudden? As you record your readings, identify any events that might have contributed to sudden changes.

    3. Carefully inspect the AC system and the area around it for signs of water. Verify that the drain works properly and that the amount of water in equals the amount of water out.

    4. Eliminate the plumbing. You can't simply rely on a flashlight view of the plumbing. You need to take things apart. Start under the kitchen sink and bathroom sink. Wipe all drain and supply pipes and connections with a small thin cloth or section of paper towel to check for wetness. Use a dry piece for each wipe.

    Remove all items stored there, shine a big bright light, and examine the shelf and walls. Poke them with a sharp instrument to minimize damage. Are they soft? Has water accumulated somewhere?

    While you're there, you may want to ask homeowner about roach problems and seal around floor and wall penetrations.

    5. Verify that sink, shower, and tub drains are not clogged down the line somewhere by carefully dumping a five gallon bucket of water or two down the drain and seeing if water flow slows down or comes to a stop. No, you can't just run the faucet.

    Listen to the pipes as water flows through them. To hear them better, turn off the water to the toilet tank, flush the bowl, and verify that the bowl is unobstructed by water. You can hear the water in the drain lines through the toilet bowl. This is best done with a helper.

    If a drain line is clogged, verify that backed-up water is not leaking from the pipe. This may require removing a baseboard and poking a hole in the wall or examining a ceiling below.

    How do you remove a baseboard? Use metal putty knives, not a pry bar. Be gentle.

    6. Inspect the floor under various appliances. (Refrigerator? Yes. Oven? No. Dryer? No.) Pull the appliances out if you can. Remove the dishwasher kick plate and poke the floor under the dishwasher.

    7. Inspect the floor under the toilet and the toilet tank. Does the homeowner use toilet cleaning products in the tank? If so, the washers under the tank-to-bowl bolts have been dissolving, and the tank may have been dripping. You will probably have to remove the baseboard behind the toilet and/or remove the entire toilet to determine if there is any water in the floor. See if you can examine the floor from below before removing the toilet.

    This isn't a difficult procedure. Bring a wax ring. When reinstalling the nuts on the bowl bolts, don't crank them down. Gently snug them up, check for level, shim, sit on the bowl to seat it, and re-snug.

    (Sidenote: for new toilet installations, use a Fernco wax-free toilet seal. Pick the one that fits your drain pipe.

    Maintain a checklist for the above and record your results. You will know weeks later exactly what has been eliminated and what has yet to be verified.

    After you have eliminated everything you can, if the problem continues, you can move on to other methods.
    Last edited by Space Racer; 07-29-2013 at 05:07 PM.
    Vacuum Technology:
    CRUD = Contamination Resulting in Undesirable Deposits.
    CRAPP = Contamination Resulting in Additional Partial Pressure.

    Change your vacuum pump oil now.

    Test. Testing, 1,2,3.

  10. #10
    Thanks Space Racer!
    I have a ManJ question, Table 5 (Infiltration Evaluation) suggests Summer air changes per hour at .2 to .6 for this sized structure. However, when I had the blower door test performed at 50Pa, I am getting TEN times that exchange rate. The blower door tests resulted in air changes at 5 per hour (1500 cfm).
    If I am doing this correctly, at 5 per hour, the latent heat values go sky high!

  11. #11
    Join Date
    Jun 2002
    nw ohio
    confirm the clothes dryer vent does not dump into a crawl space or attic due to vent coming loose somewhere.
    I have seen this happen several times.
    compressors never die; they're always murdered!

  12. #12
    Ok leave the 3 ton unit add humidistat in line with strip heat or gas heat use this as a reheat that will add to dehumidification in run time and the heat will also dehumidify

    Sent from my iPhone using Tapatalk

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