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Thread: RH to high

  1. #1
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    Mar 2005
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    997
    Ok guys I have a barn that has 2.5 tons of cooling. Unit has run for 14 days. No water on in the building has electric heat to help with temp control. No leaky roof,walls,floor. Where is the moisture coming from? I can't get the RH below 55% and as soon as AC is turned of RH starts to rise above 65%. stumped on this one!!!!

  2. #2
    Join Date
    Aug 2003
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    was it sized for sensible heat work? typically rh control is expensive, end of story. something has changed about the structure if you could before but cant now. rmember that everything natural tends to equalize. temp, moiture, pressure, etc. so if you open the door more than before the difference between outdoor and indoor will equalize, not instantaneously but try to for the duration of envelope failure.

  3. #3
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    Mar 2005
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    It has not worked from day one! My company put the elec heat in.

  4. #4

    Lightbulb Correct CFM?

    Alot of times a 2.5 ton Air conditioner will be hooked up to a air handler that may be designed to work with a 2 up to say a 3.5 ton A/C. If the air handler wasn't set up correctly the blower may be moving 3.5 tons of air accross a 2.5 ton cooling coil. This causes 2 problems. One, the evaporator is absorbing too much heat and doesn't get cold enough to condense the correct amount of moisture out of the air. Two, the air is moving too fast and doesn't have enough contact time with the cooling coil to allow for correct condensation. Have your HVAC company make sure the blower is moving 1000 cfm of air (which is the correct amount for a 2.5 ton system), if not the blower speed may be lowered to achieve the correct cfm. Also oversizing can be an issue. An oversized system will cool down the space so quickly that it doesn't remove any moisture since removing heat takes less time than removing moisture.

  5. #5
    Join Date
    Jun 2003
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    Madison, WI/Cape Coral, FL
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    Originally posted by drk
    Ok guys I have a barn that has 2.5 tons of cooling. Unit has run for 14 days. No water on in the building has electric heat to help with temp control. No leaky roof,walls,floor. Where is the moisture coming from? I can't get the RH below 55% and as soon as AC is turned of RH starts to rise above 65%. stumped on this one!!!!
    The moisture comes outside via infiltrating air and moisture diffusion through the structure. After exhausting the air tightning of the building and ducts, and lowering the air flow to get a very cold coil, add enough supplemental dehumidification to maintain 50%RH. This is usally 100 pints per day per 2,500 sqft. of living area. Check out thermastor.com.

  6. #6
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    Mar 2005
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    Teddy Bear Where is this printed at? (This is usually 100 pints per day per 2,500 sqft. of living area.) Also the side of the barn that does not have AC is staying around 50% RH. Trane blower set on medium low. Have not checked CFM but would think it would be around 900 cfm.

  7. #7
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    Apr 2002
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    Does the air handler blower run steady, keeps running when compressor is cycled off?
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  8. #8
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    Mar 2005
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    Yes it is in the on position. This is necessary for the electric heat to cycle on and off. I can't believe a 2.5 ton coil will have enough moisture to raise RH up 15 points.

  9. #9
    Join Date
    Aug 2003
    Location
    Central Kentucky
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    Originally posted by drk
    Yes it is in the on position. This is necessary for the electric heat to cycle on and off. I can't believe a 2.5 ton coil will have enough moisture to raise RH up 15 points.
    It may not be the moisture from the coil,it may be the fan increasing the infiltration rate of the building.

    Have you tried testing what pressure the building is operating at with the fan running & off?

    You mentioned the structure was not leaky,did you verify it with a blower door test?
    Have you set up a Google alert for Carbon Monoxide yet?
    Click here to find out how.

  10. #10
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    Fan is "on"? Where have you been? I missed that! Re-evaporating 5-6 lb. of water from a coil is enough to raise the %RH in the any building 10-15%. Try it with the fan off for a couple days. Take a look at the operating cost of your reheat verse a high eff. dehumidifier. You get 1 pint per kw with electric reheat. 5 pints per kw with a Santa Fe/Ultra-Aire plus 9,000 btus of free reheat. It don't take long to pay for the dehumidifier at those rates. This isn't going tobe easy!! TB

  11. #11
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    Apr 2002
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    Some residential stuff even has a blower that times off a minute or ninety seconds after the compressor shuts off, even that re-evaporates water.

    Typically constant fan will keep indoor humidity in a home about 10% higher than 'Auto' fan especially if it is a horizontal draw through air handler.

    Could also be a fresh air intake hooked up sucking in unconditioned air when the compressor is cycled off and the supply fan is still running.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  12. #12
    Join Date
    Mar 2005
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    997
    Have had the fan in the off position and RH still will rise. Maybe not as fast! No outside air! Rest of building with horse stalls was staying around 50% so pressure should not be a problem on conditioned side. I might try to leave in low speed and fan off for a few days to see what happens.

  13. #13
    Join Date
    Apr 2002
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    You are air conditioning an office or workshop type of area inside of a barn?

    Are air ducts for this system in the unconditioned space?

    Duct leaks will cause infiltration if the ducts are in the unconditioned space, if it was 83F and 50% RH by the horses, it would hold the same amount of moisture as 75F and about 65% RH.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

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