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  1. #1

    Low Humidity Problem

    My RH inside is around 20% on cold days, I am in Upstate SC, where most people have a high humidity problem.

    My house is ~30 years old. 1650 sq ft. on a slab. Windows replaced 3.5 years ago with mid-upper grade vinyl, doors are at least a little leaky. The heat pump air handler seems to be original. The outside unit is ~16 years old, replaced by previous owner.

    If I could afford to, I would have the system replaced, ducts checked/repaired/updated as necessary. I am hoping to get a couple/few more years out of this system.

    Option 1, is to install a humidifier. But it seems there is more likely a problem with my system causing the air to be so dry.

    HVAC repair tech looked at the system to quote installing a humidifier, and says the air ducting is wrong and likely the culprit. The air handler is laying horizontal, the plenum has three lines, one on each side, and one coming out the end. HVAC tech says the one on the end is wrong, the plenum should be longer and the line should come off the side like the other two. The current config causes too much air moving across the coils, causing it to dry the air.

    His recommended option is to put a mechanical adjustable damper on the duct line coming off the end of the plenum to equalize the pressure there. He felt this would probably alleviate the dry air problem.

    Does this sound accurate? After reading a bit here, it seems I need to also have the ducts checked for leaks. What else should I get checked?

    Thank you.

  2. #2
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,296
    Quote Originally Posted by KenSC View Post
    My RH inside is around 20% on cold days, I am in Upstate SC, where most people have a high humidity problem.

    My house is ~30 years old. 1650 sq ft. on a slab. Windows replaced 3.5 years ago with mid-upper grade vinyl, doors are at least a little leaky. The heat pump air handler seems to be original. The outside unit is ~16 years old, replaced by previous owner.

    If I could afford to, I would have the system replaced, ducts checked/repaired/updated as necessary. I am hoping to get a couple/few more years out of this system.

    Option 1, is to install a humidifier. But it seems there is more likely a problem with my system causing the air to be so dry.

    HVAC repair tech looked at the system to quote installing a humidifier, and says the air ducting is wrong and likely the culprit. The air handler is laying horizontal, the plenum has three lines, one on each side, and one coming out the end. HVAC tech says the one on the end is wrong, the plenum should be longer and the line should come off the side like the other two. The current config causes too much air moving across the coils, causing it to dry the air.

    His recommended option is to put a mechanical adjustable damper on the duct line coming off the end of the plenum to equalize the pressure there. He felt this would probably alleviate the dry air problem.

    Does this sound accurate? After reading a bit here, it seems I need to also have the ducts checked for leaks. What else should I get checked?

    Thank you.
    What are you using to determine the Relative Humidity?

    You may wish to check with another instrument.

    http://www.homedepot.com/h_d1/N-5yc1...1#.UQRYi79QRc4
    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

  3. #3
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    7,098
    I'm in upstate SC, what county are you in? Maybe I can help. My company does blower door testing, duct leakage testing, thermal imaging, etc. my email is in my profile if you would like to contact me.

  4. #4
    Join Date
    May 2007
    Location
    Alabama
    Posts
    909
    Humidifier on a heat pump??? Don't think you need that.

  5. #5
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,350
    Either your tech lacks a basic concept of psychrometrics, or you did not translate what he told you adequately for this forum.

    I will be charitable and assume a mistranslation. First of all, the only time an HVAC system dehumidifies a house is when it is cooling and dehumidifying the air. In heating, it only heats. It adds no moisture to the air, nor does it take any moisture away.

    The only thing that causes a house to become too dry in winter is air leakage. The source of that leakage can be the HVAC system ducts or the house itself, or both. For most residential construction of similar age and design as your own, it's both. To resolve it requires a plan, and that plan can best emerge from an energy audit or blower door test. I have a house that is slab on grade and is only about 200 square feet larger than yours, and I don't need a humidifier in winter. The house is over fifty years old, and I did not spend a fortune to improve its airtightness. I just learned where to look for the leaks, and fixed them.
    • 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.

  6. #6
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    7,098
    This is what a home energy audit consists of http://www.comfortinstitute.org/whol...e-checkup.html
    Heating/Cooling Services Inc.
    www.andersonhvacservice.com

  7. #7
    Quote Originally Posted by dan sw fl View Post
    What are you using to determine the Relative Humidity?
    It is a simple indoor thermometer with humidity meter. I don't doubt that it may be inaccurate, but the dryness of the air makes it very uncomfortable and is causing some sinus issues, so it is definitely too low.

    Quote Originally Posted by jtrammel View Post
    I'm in upstate SC, what county are you in? Maybe I can help. My company does blower door testing, duct leakage testing, thermal imaging, etc. my email is in my profile if you would like to contact me.
    I am in Greenville county. Do you service this area?

    Quote Originally Posted by Shophound View Post
    Either your tech lacks a basic concept of psychrometrics, or you did not translate what he told you adequately for this forum.

    I will be charitable and assume a mistranslation. First of all, the only time an HVAC system dehumidifies a house is when it is cooling and dehumidifying the air. In heating, it only heats. It adds no moisture to the air, nor does it take any moisture away.

    The only thing that causes a house to become too dry in winter is air leakage. The source of that leakage can be the HVAC system ducts or the house itself, or both. For most residential construction of similar age and design as your own, it's both. To resolve it requires a plan, and that plan can best emerge from an energy audit or blower door test. I have a house that is slab on grade and is only about 200 square feet larger than yours, and I don't need a humidifier in winter. The house is over fifty years old, and I did not spend a fortune to improve its airtightness. I just learned where to look for the leaks, and fixed them.
    Great info, very helpful. I had some doubts about what the tech was saying, and this confirms it and makes sense, thank you.

    I will check into the energy audit and report back. Thanks for everyone's input.

  8. #8
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    7,098
    We usually don't go to Greenville just anderson, oconee , pickens and abbeville but if you'll email me your address or at least what area of Greenville I will see if we can give you a call and see what we can do. My email address is in my profile on this site.

  9. #9
    Thanks jtrammel, but I may have this sorted out. I already had an appt for another company to come check out the problem.

    This morning, the air flow was measured at all vents and at the returns.

    Supply = 902
    Return = 667

    So it looks like the return ducts are leaking. However, we used pleated filters on the returns which the tech said was restricting the flow. I pulled the filters and he measured the returns again and they measured 890.

    So, it seems the pleated filters may be causing the return lines to pull in air in the attic, and drying it out. I'm going to put the cheap fiberglass filters in and see how that affects the humidity.

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,168
    Have the ducts sealed. He should have remeasured the supply after you pulled the filters.
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