Originally posted by opti
noticed a little mold/mildew around one of my vents, and tried to find a water leak. I couldn't find any, but while trying to diagnose it I noticed my humidity level in the house is pretty high - about 50-55%. I'm in Dallas, and the outside humidity is about 32% currently, but my inside humidity is about 55%. My system is 2Bryant 2.5 ton systems (zoned one per story), in a 2 story 2600 sq ft home. The house is about 5 years old. The humidity doesnt' really change much if I run the system for extended periods or just normally. I called a service company, they came out and did a check. They weren't real sure about the humidity, but they said that an issue I had was low air flow. They claimed for my 2.5 ton system I should be getting 1000cfm, they were only seeing 700. They said my 20x20 return with a 14" duct was too restrictive, and for $1000 they'll add another 2 10" returns to increase the airflow. They weren't real sure if this was my humidity issue or not, but felt that it would probably fix it. He said everything seemed fine across the coil, and that the CFM issue was the only real thing he could find. Of course, i'll get a 2nd opinion before committing to that kind of work on a fairly new system. Any ideas on my humidity problem? It's usually 34% or so at work with a pretty old system on 73 degrees. No matter what I set mine to I just can't get below 50%. I'm worried about mold issues, and why mine won't dehumidify in general. They lowered the blower speed from high to med-high to try and get it colder across the coil, but it didn't seem to help. I think he was saying because of the airflow issue he couldn't charge it as much as he thought it should be
50%RH is acceptable for mold control. Occasional 55%RH is not a problem. 90^F, 40%RH is much wetter than 75^F,50%RH. Check for the ^F dew point or grains of moisture to compare moisture content. Dallas outdoor dew points about 70^F, inside should be 50^F. Sucking in hot outside 70^F DP air makes your house wet. Watch for rainy cool weather! the a/c does not run much and the home gets wetter. Slowing the air flow makes the cooling colder removing more moisture. Cold air makes the grills and drywall around colder, causing condensation from the surrounding humid house air. Clean the grill/drywall soap/water. If wet for several days, mold will return. There must be times when your home is wetter than 55%RH, like this spring. A/C removes 100 pints of water on hot day. On a wet cool day, only a fraction of the moisture is removed. A 100 pint whole house dehumidifier will maintain <50%RH without any a/c operation. We manufacture the Ultra-Aire and Santa Fe whole house dehumidifiers.
Commercial buildings with internal heat gain from lights and equipment being cooled by near freezing cooling coils may provide lower relative humidity. RH meter are very inaccurate at very low or high %RHs.