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Old 06-30-2014, 07:50 AM   #1
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Humidity Problem


Alright, so this is kind of a cross topic post... Not sure if it's HVAC or Building / Construction related.

I have a Trane XC95M Furance with a XL20i AC unit (non-heat pump model). I have the system set up to dehumidify the house and the RH Set point is 50%. Which works great for pretty much the whole house.

Except, the master bedroom, which is also the furthest point from the furnace. When this was installed they balanced all the ducts, checked the CFM at the registers, etc. Yet the master bedroom (on the 1st floor also) always is 2 or 3 degrees hotter the the rest of the house. Humidity will drop to 50% in the room though, but over night when the AC doesn't really run much due to lack of heat gain the room will jump up to 60%+ humidity while the rest of the house will be 51% to 55%..

Now this doesn't sound like an HVAC issue, but a wasn't sure... what could cause a condition like this? air infiltration? needs more insulation? air flow problem? not really sure where to start with this... I had our house blower door tested twice in the past and drastically reduced air infiltration. Our bedroom does have a large sliding glass door, and 3 windows, along with one wall being part of a chase for the fireplace (which i am sure isn't insulated)

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House Information: Build 1996, 1500 sq ft basement, 1500 sq ft 1st floor, 800 sq ft 2nd floor, 560 sq ft unfinished attic space on 2nd floor. Insulation: Attic blown fiberglass, walls R-19, Basement R-19. HVAC: Trane xc95m fully modulating furnace, single zone, Trane XL20i commiunicating AC unit, TCONT900 Communicating thermostat.
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Old 06-30-2014, 07:57 AM   #2
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Humidity Problem


Temporarily block the fireplace chase of any air movement, temporarily insulate the windows and sliding door and re-test over a period of a few days.

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Old 06-30-2014, 09:48 AM   #3
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Humidity Problem


Sounds like an air flow problem. Check to see if there is a damper in the duct feeding your bedroom.
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Old 06-30-2014, 10:03 AM   #4
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Humidity Problem


If the AC is not on do you run the fan for continuos circulation. You and the Missus give off humidity when you breathe and if there is no circ and the door is closed it will rise.

Also sounds like a infiltration problem to me. In the winter you can get a infrared camera and see where the leaks may be. Need a good temp diff for the camera to work.
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Old 06-30-2014, 10:32 AM   #5
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Humidity Problem


Quote:
Originally Posted by Bitters View Post
Sounds like an air flow problem. Check to see if there is a damper in the duct feeding your bedroom.
All the vents are producing the correct CFM as calculated by the installer when they did a the calcs.. So I don't think that is the issue. There is also no dampers on that specific set of ducts
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House Information: Build 1996, 1500 sq ft basement, 1500 sq ft 1st floor, 800 sq ft 2nd floor, 560 sq ft unfinished attic space on 2nd floor. Insulation: Attic blown fiberglass, walls R-19, Basement R-19. HVAC: Trane xc95m fully modulating furnace, single zone, Trane XL20i commiunicating AC unit, TCONT900 Communicating thermostat.
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Old 06-30-2014, 10:35 AM   #6
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Quote:
Originally Posted by yuri View Post
If the AC is not on do you run the fan for continuos circulation. You and the Missus give off humidity when you breathe and if there is no circ and the door is closed it will rise.

Also sounds like a infiltration problem to me. In the winter you can get a infrared camera and see where the leaks may be. Need a good temp diff for the camera to work.
We have continuous air flow set to 40%, however with Tranes "proprietary dehumidification" turned on at the stat, it turns this off when the stat is set to cool so it doesn't circulate air when the AC isn't running, only during a heat setting.

I can reproduce this also during the day, say today our town is 90% RH outside at 80 degrees... the house is 70 degrees. RH in the house is 52% same in the master bedroom, close the door come back an hour later the bedrooms RH is now 65% (AC not running during this time) but the rest of the house is still 52-54% depending on location. So I don't think it's us being in there that is the major source of the problem
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House Information: Build 1996, 1500 sq ft basement, 1500 sq ft 1st floor, 800 sq ft 2nd floor, 560 sq ft unfinished attic space on 2nd floor. Insulation: Attic blown fiberglass, walls R-19, Basement R-19. HVAC: Trane xc95m fully modulating furnace, single zone, Trane XL20i commiunicating AC unit, TCONT900 Communicating thermostat.
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Old 06-30-2014, 10:52 AM   #7
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Humidity Problem


Wet bath towels, commonly remaining in a closed room on a towel rack, can quickly change the RH.
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Old 06-30-2014, 11:00 AM   #8
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Humidity Problem


If RH is the problem it can be quickly lowered by 5 percent when the room temperature is raised just a few degrees like possibly 5F.

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Old 06-30-2014, 07:51 PM   #9
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Probably an air infiltration problem. May want to seal the room better.

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