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  1. #1
    Join Date
    Jun 2006
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    96

    Summertime Relative Humidity

    We had a new air conditioning system installed in our home 2 years ago. It has never really provided us with a comfortable air conditioned environment.

    The ac system can produce allot of cold air very quickly, but shuts down within 10 minutes of starting, staying off for 15 minutes, then starting again. The result is a cold, clammy feeling house.

    The HVAC company says they did a manual J and manual D before sizing the unit for our 3,200 sq. ft. ranch style home and came up with 2 - 3 Ton units. One cools the bedroom wing and the other takes care of the LR/DR/Kitchen/Den.

    I'm starting to think that the system is over sized and is unable to dehumidify the air because it doesn't run for very long. The RH outside is about 85% with a daytime high of around 80F. The inside RH is about 65%.

    Is there a guide to the correct RH in a house (summer)? Does it depend on outside RH and temperature?

  2. #2
    Join Date
    Dec 2006
    Location
    Middle Tennessee
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    11,347

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    50% is a good baseline humidity for inside a house!

    call an HVAC contractor some things "may" be able to be adjusted to bring your humidity level down

    what is the model number of your airhandler?



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  3. #3
    Join Date
    Jul 2006
    Location
    Virginia
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    4,897
    call them back to check the systems and see if they can slow down the blowers most units come from the factory on high speed and the installers dont bother to change it

  4. #4
    Join Date
    Jun 2006
    Posts
    96
    Quote Originally Posted by Airmechanical View Post
    50% is a good baseline humidity for inside a house!

    call an HVAC contractor some things "may" be able to be adjusted to bring your humidity level down

    what is the model number of your airhandler?



    .
    I'm not sure who makes the air handlers. I do know that they are variable speed units (ECM). THe HVAC company tells me the minimum cfm per ton is 400, so with a 3 ton unit, that would mean 1,200cfm. When the ac runs, it is quite drafty in the house.

    It seems to me that smaller units (say 2-1/2 ton each) running 1,000cfm, but running longer would help reduce humidity and provide a more comfortable interior.

    The only time the ac units couldn't keep up was last friday evening with the outside temperature at 24C, but we had a party with 75 people and were cooking in the kitchen as well. We had the thermostat set to 21C and it maintained 23C. We don't normally have this many people in the house and everyone found it comfortable.

    Thoughts?

  5. #5
    Join Date
    Sep 2002
    Posts
    511
    What are you using to measure the Humidity level in your Home?

    24C is a 75°F outdoor air temp, with a thermostat setpoint of 70° and it was able to maintain a indoor temperature of 73.4°F with 75 people with a body temperature of 98° each.
    That is not bad at all.
    What would have happened on a 90+°F day?
    Your units are probably not oversized, the airflow may need to be adjusted however.
    Have a qualified contractor come in and check it out for you.

  6. #6
    Join Date
    Apr 2002
    Posts
    11,808
    it sounds like it ison the big side to deal with 80F outside temperature.

    Make sure you are not running the indoor fan all the time, just have it set to auto to run when the compressor runs.

    You could have the tech out to perhaps lower the blower speed.

    Make sure you are not running exhaust fans steady, especially something like a "Humidex"

  7. #7
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,789
    400 CFM per ton air flow is not a minimum value.
    They can slow it to 350 FM and you won't have a problem unless you start using it when its like 65°F outside.

    75 prople is almost a 3 ton load its self, so its not a supprise it didn't maintain set point.

    Ask to see the load calc. Sounds like they used a rule of thumb to size it.
    Like 550 sq ft per ton, which would come out to 5.81 tons, which rounds up to 6.
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  8. #8
    Join Date
    Jun 2006
    Posts
    96
    Quote Originally Posted by cobitech View Post
    What are you using to measure the Humidity level in your Home?

    24C is a 75°F outdoor air temp, with a thermostat setpoint of 70° and it was able to maintain a indoor temperature of 73.4°F with 75 people with a body temperature of 98° each.
    That is not bad at all.
    What would have happened on a 90+°F day?
    Your units are probably not oversized, the airflow may need to be adjusted however.
    Have a qualified contractor come in and check it out for you.
    Can we adjust airflow below 400cfm/ton without icing up the coil in the air handler? If not, we would need to have a smaller tonnage unit to be able to reduce airflow, correct?

    To measure humidity, I used an old hygrometer that is known to read about 15% high to test the air. It is showing 85%, which probably means it is 70% - 75% RH.

  9. #9
    Join Date
    Dec 2006
    Location
    Middle Tennessee
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    Quote Originally Posted by kayjh View Post
    Can we adjust airflow below 400cfm/ton without icing up the coil in the air handler?

    I used an old hygrometer that is known to read about 15% high to test the air. It is showing 85%, which probably means it is 70% - 75% RH.
    1.yes, but dont keep it too cold in your house and/or use a/c during cold outside ambient temps

    2.get a new digital hygrometer $30.00



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