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  1. #1
    Join Date
    Aug 2011
    Location
    Woodbridge, VA
    Posts
    9

    Trane hum after tstat install, and insufficient dehumidification

    I recently moved into a new home (constructed 1989). It has a lot of finished square footage at 3800 sq. ft., including a finished basement. HVAC are Tranes:
    -3 ton heat pump with propane auxilliary for the basement and main level (air handler replaced in 2004; outside unit original)
    -2 ton heat pump with electric auxilliary for the upstairs (air handler and outside unit both original)

    Both systems were recently checked by a HVAC tech who reported that refridgerant levels, pressure differentials and cooling all appear nominal or better. The coil in the upstairs air handler was clogged with buildup, which the tech cleaned out. For the upstairs unit I also had him install a float switch and p-trap in the primary drain as there was just an s-trap there before.

    Each system had its own original Trane thermostat which I just replaced with Honeywell RTH7500 units. I followed the instructions carefully and double-checked the wiring. The downstairs unit appears to function normally. But the upstairs air handler (installed in attic) now has an audible hum after installing the tstat, where there was no hum before (or at least I couldn't hear it). It doesn't seem to have affected the unit's operation as the fan kicks on as expected and it cools like it did before. There are 60 and 30 amp breakers on the side of the air handler. Turning off the 60 amp breaker makes the hum stop.

    I checked the wiring again and it all looks correct according to the manual. Here's how I did it:

    Tstat terminal <-- Unit wires
    C <-- B
    G <-- G
    Y <-- Y
    O/B <-- O
    Rc <-- jumped to R
    R <-- R
    Aux <-- W
    E <-- X2
    L <-- not used
    <-- T (tstat has no receptacle for thermistor wire)

    The downstairs unit is wired the same way. The tstat is used to differentiate what sort of system and auxilliary you have, which I did and double-checked. I set up the upstairs unit for the following:
    System type: heat pump
    Heat pump changeover valve: cooling (O wire connected to O/B)
    Emergency heat cycle: electric furnace
    Changeover: manual

    Any idea what could be causing the hum?

    Second question: The house is generally quite humid. Since the tech told me everything checks out regarding both units' operation, could the humidity simply be a result of this being an older house? I can think of several contributing factors:
    -the windows are original (wood)
    -some of the exterior doors don't seal very well
    -the blown insulation in the attic is covering up the soffet vents (I need to rake the stuff back to uncover them)
    -I live in a humid climate (northern Virginia)

    The house I came from is in the same general area but was a recent construction. I never measured the indoor humidity there but it was comfortably dry. In the house I'm in now it's upwards of 70-75% according to the humidistat on the little dehumidifier I bought from the hardware store.

    Are there any simple steps I can take to improving my system's dehumidification, or do I just need to deal with it until I can afford to replace windows and doors? Should I consider retrofitting dehumidifiers to the air handlers themselves (big expense)?

    Thanks for the space!

  2. #2
    Join Date
    Aug 2011
    Posts
    568

    Hmm

    Quote Originally Posted by nukefission View Post
    I recently moved into a new home (constructed 1989). It has a lot of finished square footage at 3800 sq. ft., including a finished basement. HVAC are Tranes:
    -3 ton heat pump with propane auxilliary for the basement and main level (air handler replaced in 2004; outside unit original)
    -2 ton heat pump with electric auxilliary for the upstairs (air handler and outside unit both original)

    Both systems were recently checked by a HVAC tech who reported that refridgerant levels, pressure differentials and cooling all appear nominal or better. The coil in the upstairs air handler was clogged with buildup, which the tech cleaned out. For the upstairs unit I also had him install a float switch and p-trap in the primary drain as there was just an s-trap there before.

    Each system had its own original Trane thermostat which I just replaced with Honeywell RTH7500 units. I followed the instructions carefully and double-checked the wiring. The downstairs unit appears to function normally. But the upstairs air handler (installed in attic) now has an audible hum after installing the tstat, where there was no hum before (or at least I couldn't hear it). It doesn't seem to have affected the unit's operation as the fan kicks on as expected and it cools like it did before. There are 60 and 30 amp breakers on the side of the air handler. Turning off the 60 amp breaker makes the hum stop.

    I checked the wiring again and it all looks correct according to the manual. Here's how I did it:

    Tstat terminal <-- Unit wires
    C <-- B
    G <-- G
    Y <-- Y
    O/B <-- O
    Rc <-- jumped to R
    R <-- R
    Aux <-- W
    E <-- X2
    L <-- not used
    <-- T (tstat has no receptacle for thermistor wire)

    The downstairs unit is wired the same way. The tstat is used to differentiate what sort of system and auxilliary you have, which I did and double-checked. I set up the upstairs unit for the following:
    System type: heat pump
    Heat pump changeover valve: cooling (O wire connected to O/B)
    Emergency heat cycle: electric furnace
    Changeover: manual

    Any idea what could be causing the hum?

    Second question: The house is generally quite humid. Since the tech told me everything checks out regarding both units' operation, could the humidity simply be a result of this being an older house? I can think of several contributing factors:
    -the windows are original (wood)
    -some of the exterior doors don't seal very well
    -the blown insulation in the attic is covering up the soffet vents (I need to rake the stuff back to uncover them)
    -I live in a humid climate (northern Virginia)

    The house I came from is in the same general area but was a recent construction. I never measured the indoor humidity there but it was comfortably dry. In the house I'm in now it's upwards of 70-75% according to the humidistat on the little dehumidifier I bought from the hardware store.

    Are there any simple steps I can take to improving my system's dehumidification, or do I just need to deal with it until I can afford to replace windows and doors? Should I consider retrofitting dehumidifiers to the air handlers themselves (big expense)?

    Thanks for the space!
    so, you changed out your own thermostat, and now doesn't work correctly?

  3. #3
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,729
    sounds like the transformer is humming/
    , if the unit is cooling let it go

  4. #4
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,538
    During peak cooling hours, your a/c should be able to maintain <50%RH regardless the t-atat. When there is low/no cooling load the a/c does not run long enough to remove the remove the 4-7 gals. of water per day needed to maintain <50%RH, you will need a good dehumidifier.
    You need help on that tstat conversion.
    A couple points for your a/c tech to max moisture removal by your a/c. Check your cooling coil temperature. It should be 25^F colder than the return air temperature to remove moisture when running.
    Your a/c blower should be in the "auto" mode to avoid evaporating moisture from the the coil to the space during the off cycle.
    Cooling cycles should be as long as possible. A 3-4^F t-stat dead band will max the moisture removed by the a/c.
    During cool wet weather when the a/c runs little, consider using a whole house dehu like the Ultra-Aire 90H to maintain <50%RH. When the outdoor dew points are less than 55^F, you should not need any supplemental dehumidification provided your home is getting an air change of fresh air every 4-5 hours.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  5. #5
    Join Date
    Sep 2002
    Location
    I don't know
    Posts
    2,905
    The new stat might be drawing more current from the transformer to power itself than the old one, resulting in more noise.

    RTH7500 isn't for dual fuel - do you have a fossil fuel kit?
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  6. #6
    Join Date
    Sep 2002
    Location
    I don't know
    Posts
    2,905
    Are there any simple steps I can take to improving my system's dehumidification, or do I just need to deal with it until I can afford to replace windows and doors? Should I consider retrofitting dehumidifiers to the air handlers themselves (big expense)?
    Make sure that the ductwork in the attic is very well sealed.

    Draft proof if possible. (chimney dampers, window frames, basement sill plate if unfinished, light switches/outlets, etc.)

    Use exhaust fans when cooking and showering.

    Call a technician to check the blower speed on both units - it might be set too high.

    If you're in and cooler climate (design temp 90F or lower) 5 tons is probably a bit much for that house.

    When it's cool and humid, use dehumidifiers but keep the windows closed. (sparingly, I might add - they aren't cheap to operate)
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  7. #7
    Join Date
    Sep 2002
    Location
    I don't know
    Posts
    2,905
    "Emergency heat cycle: electric furnace"

    If you ever use the emergency heat feature to bypass the heatpump, set the CPH setting to 5 or less; 9 is what honeywell recommends for electric heat, but it will cycle your system to death.
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  8. #8
    Join Date
    Aug 2011
    Location
    Woodbridge, VA
    Posts
    9
    Thanks for the responses.

    The upstairs unit still cools and cycles on and off like before. The only difference is the humming.

    Yes, the downstairs unit is equipped with a fossil fuel kit.

    When the tech did his checkup a week ago he noted about 20* difference between coil and return air temp for both the downstairs system and 17* for the upstairs. He characterized this as "good" in both cases.

    I have noticed that the cooling cycles are not very long, even before the new tstats. It doesn't look like I can configure the temp tolerance (dead band, if that's what you're referring to) with the RTH7500.

    Thanks for the tip on the CPH setting. I will make a note of it come winter time.

  9. #9
    Join Date
    Sep 2002
    Location
    I don't know
    Posts
    2,905
    The drybulb temp difference doesn't mean much - it varies depending on the outdoor temp, indoor temp, condition of the filter, and humidity level.

    It could be 15F one day (if it's very humid) and 22F the next.

    Reducing the blower speed can improve dehumidification, but there's a risk of freezing the coil.
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  10. #10
    Join Date
    Aug 2011
    Location
    Woodbridge, VA
    Posts
    9
    Do you think it's worth switching the tstats out for ones that have an adjustable dead band? Does an air handler have to have a variable blower in order for the tstat to adjust the speed, or can a tstat with that feature do it? I may check with the tech to see if the speed can be adjusted.

    Another theory I have about the humidity is that my house is under a lot of shade due to lots of large trees. Because of this the indoor temp is such that the A/C doesn't need to come on as much. But because the humidity is still there (due to it being humid in the summer in this area), it's not being removed from the indoor air since the A/C isn't on that much. Does this have any credence?

  11. #11
    Join Date
    Sep 2002
    Location
    I don't know
    Posts
    2,905
    Good t-stats such as what you installed don't use a fixed deadband at all - might want to get a pro grade honeywell with an adjustable cooling CPH (cycles per hour) setting.

    Most air handlers and furnaces have multi-speed blowers, so the speed can be moved down by a tap.

    On furnaces it's not uncommon to see the cooling blower speed set too high, especially if a large unit is paired with a smaller a/c. (ex: unit with 4 ton drive paired with 3 ton condenser)
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  12. #12
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,538
    Quote Originally Posted by nukefission View Post
    Do you think it's worth switching the tstats out for ones that have an adjustable dead band? Does an air handler have to have a variable blower in order for the tstat to adjust the speed, or can a tstat with that feature do it? I may check with the tech to see if the speed can be adjusted.

    Another theory I have about the humidity is that my house is under a lot of shade due to lots of large trees. Because of this the indoor temp is such that the A/C doesn't need to come on as much. But because the humidity is still there (due to it being humid in the summer in this area), it's not being removed from the indoor air since the A/C isn't on that much. Does this have any credence?
    After getting the a/c setup to remove moisture with a coil that is 25^F below the air in the home, you are dependent on long running times to remove moisture.
    It could be 15F one day (if it's very humid) and 22F the next.

    Reducing the blower speed can improve dehumidification, but there's a risk of freezing the coil.


    Adjust your blower air flow for the 45-50^F coil temp with 75^F, 50%RH inside air. Freezing will not be a problem, if you not overcooling.

    17^F coil temperatures will not provide a <50%RH home.
    During low cooling loads, a whole house dehumidifier will maintain <50%RH even when the a/c in not used.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  13. #13
    Join Date
    Jul 2009
    Posts
    247
    Just out of curiousity, when you turn the 60 amp breaker off, does the unit still function?

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