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  1. #1

    Confused To lower humidity, set fan to "ON" or "AUTO"?

    A little background: I live in Maryland, which has a very humid climate (last night humidity got up to 94%, a couple days ago it was 100%). My house was built in 1984. In August 2012 I had R-49 insulation put in the attic, and had it air sealed, and in November 2012 I got a Trane 16 SEER 2-stage variable speed HVAC system. The humidity in the house has been pretty high recently, reaching highs of around 70% with the thermostat set to 75 and the fan set to ON. Half the doors in the house don't want to close anymore, and some of the wood flooring is sticking up, like it's forming some kind of bubble.

    On to my question. To reduce the humidity in my house, should I set the fan to ON or CIRC (circulate, I think it runs 30% of the time) or AUTO?

    Thank you very much for any help!

  2. #2
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    This is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise, commentary, or ask questions of the OP here.

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    Last edited by beenthere; 07-17-2013 at 03:51 PM. Reason: Non Pro * member, no warning, caught himself and reported it

  3. #3
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    This is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise, commentary, or ask questions of the OP here.

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    Last edited by beenthere; 07-17-2013 at 03:55 PM. Reason: Non Pro * member

  4. #4
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    Auto

  5. #5
    Quote Originally Posted by Random1634 View Post
    Sounds like things weren't sized properly for your house. People love to throw in an oversized unit or not take in the upgrade on the r-factor and just use rule of thumb for standard houses. It sounds like you need a smaller tonnage unit that will run longer so it can remove more humidity.
    I suspect that is what happened. Before I had the new HVAC system installed, I read on the internet that it's good to do something called a "Manual J" to make sure it's the right size. I requested that the installer do this because we had just had the attic insulated up to R-49 and air sealing done. The installer said that doing a Manual J would be overkill, it would be better to just put in the same size unit that was being replaced, a 3 ton unit, which all the houses around here seem to have. I deferred to his professional judgement.

    I suppose in the future I should insist that the installer do a Manual J first, particularly if I've just upgraded the insulation, to make sure that it's sized correctly and I don't wind up with humidity problems again.

  6. #6
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    Quote Originally Posted by Random1634 View Post
    This is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise, commentary, or ask questions of the OP here.

    You can find the rules for posting and qualifications here.

    Further infractions may result in loss of posting privileges.
    Wrong.

    Don't spew info you don't know or have

    setting the fan to on, will allow at shut down all the moisture still left on the coil and in drain pan to be picked back up into the air and redistributed in the envelope...instead of going out the drain
    Last edited by beenthere; 07-17-2013 at 03:57 PM.
    It's hard to stop a Trane. but I have made one helluva living keeping them going.

  7. #7
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    Quote Originally Posted by Dr Gaius Baltar View Post
    I suspect that is what happened. Before I had the new HVAC system installed, I read on the internet that it's good to do something called a "Manual J" to make sure it's the right size. I requested that the installer do this because we had just had the attic insulated up to R-49 and air sealing done. The installer said that doing a Manual J would be overkill, it would be better to just put in the same size unit that was being replaced, a 3 ton unit, which all the houses around here seem to have. I deferred to his professional judgement.

    I suppose in the future I should insist that the installer do a Manual J first, particularly if I've just upgraded the insulation, to make sure that it's sized correctly and I don't wind up with humidity problems again.
    Yes manual J load calculation should have been performed

    What style house/layout and squ. footage?
    It's hard to stop a Trane. but I have made one helluva living keeping them going.

  8. #8
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    Gaius Baltar...cylon traitor!

    Auto is best for humid climate operation.

    Say hi to Caprica 6...
    • 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.

  9. #9
    Join Date
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    This is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise, commentary, or ask questions of the OP here.

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    Last edited by beenthere; 07-17-2013 at 03:55 PM. Reason: Non Pro * member

  10. #10
    Quote Originally Posted by Southern Mech View Post
    Yes manual J load calculation should have been performed

    What style house/layout and squ. footage?
    From the appraisal:

    2594 square feet of livable area (1473 above grade)

    Type: Detached
    Design style: Split Foyer
    Attic: Scuttle
    Foundation: Full Basement
    Exterior
    Foundation Walls: Masonry
    Exterior Walls: Siding
    Roof Surface: Comp. Shingle

    It's 2 stories, one a finished basement. High ceilings in the living room. Does that pretty much describe it? I'm not sure what else to say about its layout.

    @Shophound
    Head Six says hi back

    @Random1634
    I began setting the fan to ON because I had a problem with the master bedroom being 11 degrees hotter than the rest of the house (73 at thermostat, 84 in master bedroom). This problem predated the new insulation and HVAC system, and is what prompted me to get them last year, along with a hot air return in the master bedroom and a vent on the top of the master bedroom door.
    After the insulation was installed, I began running the old (25 year old AC, 8 year old heater) furnace fan on ON in the late summer 2012, and stopped during the winter because it was spreading cold air after the heater turned off. I turned it back to AUTO for the winter.

    Installing the new insulation and above-door vents reduced the temp in master bedroom (when thermostat was at 73) from 84 to 82. Running the fan on ON 24/7 reduced it further to about 80. Getting the new HVAC system, with the new hot air return in master bedroom, reduced it further to about 75, nearly eliminating the problem. Of course, now I have the humidity problem, but setting it back to AUTO only increases the temperature to 77-79, still comfortable.


    I've been experimenting with trying to get the humidity down for a few days now. Up till last week, I had the thermostat set to 75 and fan ON, and that was resulting in humidity between 63% and 73%. After reading on the internet about how to reduce humidity, I tried setting it to 73 and fan AUTO, which reduced the humidity to between 53% and 63% (63% at night, 53% during day). But when I called the installer who put in this Trane HVAC system, and asked his advice on what I should do to reduce humidity, he told me that switching the fan back to ON would reduce humidity. This contradicted my experience, which is why I decided to ask you guys. The installer also said that getting a whole-home dehumidifier would be expensive and probably overkill.

  11. #11
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    Quote Originally Posted by Dr Gaius Baltar View Post
    A little background: I live in Maryland, which has a very humid climate (last night humidity got up to 94%, a couple days ago it was 100%). My house was built in 1984. In August 2012 I had R-49 insulation put in the attic, and had it air sealed, and in November 2012 I got a Trane 16 SEER 2-stage variable speed HVAC system. The humidity in the house has been pretty high recently, reaching highs of around 70% with the thermostat set to 75 and the fan set to ON. Half the doors in the house don't want to close anymore, and some of the wood flooring is sticking up, like it's forming some kind of bubble.

    On to my question. To reduce the humidity in my house, should I set the fan to ON or CIRC (circulate, I think it runs 30% of the time) or AUTO?

    Thank you very much for any help!
    If you have a Trane VS air handler or gas furnace, your installing contractor (or another heating and AC person) should be able to set the 'ramp' profile of the blower to help with the humidity. Another thought would be to lower CFM/ton fan speed... tends to run a colder coil and condense more water from the air.

    Get out someone that understands indoor air quality... they should be able to help you.
    GA-HVAC-Tech

    Galatians 2:20-21; Colossians 1: 21-22 & 26-27; 3:1-4; Romans Ch's 5-6-7-8

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    Quality work at a fair price with excellent customer service.

  12. #12
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    Location
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    If your two stage system runs in low stage most of the time, you are probably not getting enough air to the master bedroom, although you note considerable improvement since insulation, new HVAC, and added return air.

    What might be a next step is an air balance on the ducts. If you have rooms or entire zones that are too cold when the a/c runs, you might be able to reduce airflow there so air into the master bedroom increases. This is best done with manual dampers at the supply take-offs near the trunk vs. throttling air at a supply register.

    One other possibility...give Six a little rest...she may be heating up the place too much!
    • 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.

  13. #13
    Here is the equipment I had installed:
    Trane 2 Speed 3 Ton AC Condensing Unit Model # 4TTX6036G1000A
    Trane 21" Cased Evaporator Coil Model # 4TXCC044BC3 with TXV
    Trane XV80-80% 2 Stage/Variable Speed Gas Furnace Model # TUD2C100A9V5
    Trane Digital Two-Stage, Digital Programmable Thermostat Model # TCONT802A

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