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Thread: Optimal CFM settings (post Air Sealing of Attic, etc)

  1. #1
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    Optimal CFM settings (post Air Sealing of Attic, etc)

    I recently had my upstairs attic air sealed, removed existing fibreglass insulation (R-30) and replaced it with Cellulose (R-60). All ductwork in the attic was sealed with 2 inches of closed-cell spray foam. Now the upstairs humidity is 20% higher (right now it's about 62%) than what it was previous to the air sealing, which I've read on green building advisor is 'normal' but that I should check the settings of my equipment and adjust as needed. I live in the Transition Zone (Northern VA). 3 vents are sweating, 1 in the bedroom, Walk-in-closest and bathroom. (No bathing in them all day)

    Equipment:

    Trane TEM6A0B30 VSAH
    BAYHTR1510 Heat Strip Kit
    Trane 4TWX6030 Seer 16 Heat Pump (XL16), 2.5 ton.
    Trane XL824 TStat

    I recall that the HVAC equipment installer enabled 'Enhanced Mode' on the AH unit (Switch 5 and 6 on) years ago but I do not remember any other settings for the fan speed were adjusted, nor was anything on the Tstat adjusted as well.

    Do you think the blower speed is not set correctly to reduce the humidity? I've read that it should be about 1000CFM for a 2.5/t.

  2. #2
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    Before changing any settings a static pressure test needs to be performed to give a base line reading. Then you can measure after changes to ensure your not exceeding the min / Max

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  4. #3
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    What is the temp/%RH in the home and attic? Air tight attics respond to the amount of moisture that is forced toward the attic space through the roofing materials. Shingles load with moisture in the form condensation on the bottom of shingles evenings. During the heat of the day, the sun's heat drives the moisture through many of the less than perfect vapor barrier under the rain barrier. This accumulates in the attic space. A small amount of exhaust or dehumidification controls this problem.

    Aggressive air tightening makes mechanical fresh filtered air ventilation critical to purge indoor pollutants and renewing oxygen. The moisture in adequate fresh air change and from normal occupancy requires 2-6 lbs. of dehumidification every hour dependent on the outdoor dew point and number of occupants to maintain <50%RH.

    A well setup a/c with a cooling coil cold enough to reduce the dew point of the indoor air flow through the cooling coil 6-8^F, a/cs will maintain <50%Rh with +40% duty cycle. Evenigs-mornings and rainy days require supplemental dehumidification when the outdoor dew points are +55^F.

    Whole house dehumidifiers like the Santa Fe Ultra or Broan with Merv 13 filter ventilation option can be connected to the well setup a/cs to provide the desired temperature/%RH throughout the home and attic.

    High indoor air quality and comfort with simple equipment that is simple and reasonable to maintain.

    Most homes with adequate fresh air change typical number of occupants without will be damp evenings and rainy days during mild season "grass climates" conditions.

    Does all make sense?
    Keep us posted.

    Regards Teddy Bear
    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"

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  6. #4
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    Quote Originally Posted by pecmsg View Post
    Before changing any settings a static pressure test needs to be performed to give a base line reading. Then you can measure after changes to ensure your not exceeding the min / Max
    HVAC tech coming here next week to measure that (among basic maintenance)

  7. #5
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    Quote Originally Posted by teddy bear View Post
    What is the temp/%RH in the home and attic? Air tight attics respond to the amount of moisture that is forced toward the attic space through the roofing materials. Shingles load with moisture in the form condensation on the bottom of shingles evenings. During the heat of the day, the sun's heat drives the moisture through many of the less than perfect vapor barrier under the rain barrier. This accumulates in the attic space. A small amount of exhaust or dehumidification controls this problem.
    I just discovered a problem with the Trane XL824, it's RH% is off by nearly 8%.

    Upstairs temp: 72F with 58% RH.
    Attic Temp: 99F with 61% humidity. (6 meter temps, checked closer to the attic floor is about 10F cooler)

    Aggressive air tightening makes mechanical fresh filtered air ventilation critical to purge indoor pollutants and renewing oxygen. The moisture in adequate fresh air change and from normal occupancy requires 2-6 lbs. of dehumidification every hour dependent on the outdoor dew point and number of occupants to maintain <50%RH.
    HVAC tech coming over Thursday and I'll discuss a hardware option for this to see if it's fesable.

    A well setup a/c with a cooling coil cold enough to reduce the dew point of the indoor air flow through the cooling coil 6-8^F, a/cs will maintain <50%Rh with +40% duty cycle. Evenigs-mornings and rainy days require supplemental dehumidification when the outdoor dew points are +55^F.
    Just finished manual J/S. Turns out my upstairs heatpump / A/H is oversized (should be 1.5 ton). Current air handler is also set for 3 tons (default dip switches), which probably isn't helping the humidity because of short duty cycles.

    Whole house dehumidifiers like the Santa Fe Ultra or Broan with Merv 13 filter ventilation option can be connected to the well setup a/cs to provide the desired temperature/%RH throughout the home and attic.

    High indoor air quality and comfort with simple equipment that is simple and reasonable to maintain.

    Most homes with adequate fresh air change typical number of occupants without will be damp evenings and rainy days during mild season "grass climates" conditions.

    Regards Teddy Bear

  8. #6
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    Quote Originally Posted by teddy bear View Post
    What is the temp/%RH in the home and attic? Air tight attics respond to the amount of moisture that is forced toward the attic space through the roofing materials. Shingles load with moisture in the form condensation on the bottom of shingles evenings. During the heat of the day, the sun's heat drives the moisture through many of the less than perfect vapor barrier under the rain barrier. This accumulates in the attic space. A small amount of exhaust or dehumidification controls this problem.
    Right now (6am EST): Upstairs is 73F w/ 58% humidity. Downstairs is 74F w/ 55% humidity. Attic is about 77% w/ 72F. I live in IECC zone 4A (Transition Zone, DC area)

    Attic has the traditional soffit vent + ridge vent setup.

    I guess I will probably need to do something sooner than later - because I plan on air sealing the crawlspace.

    Where would I install a whole house dehumidifier? I see Trane has a device that is a dehumidifier + fresh air vent, would that go into the attic and connect it to the existing air handler? Or do I need separate units for both?

  9. #7
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    Quote Originally Posted by Monkburger View Post
    Right now (6am EST):


    __. Upstairs is 73'F w/ 58% humidity.
    Downstairs is 74'F w/ 55% humidity.

    ___________ PERFECT __

    Attic is about 77% w/ 72F.

    I live in IECC zone 4A (Transition Zone, DC area)

    Attic has the traditional soffit vent + ridge vent setup.

    I guess I will probably need to do something sooner than later - because I plan on air sealing the crawlspace.

    Where would I install a whole house dehumidifier?

    I see Trane has a device that is a dehumidifier + fresh air vent,
    would that go into the attic and connect it to the existing air handler?

    Or do I need separate units for both?
    WHAT Does the COMMISSIONING REPORT State?

    https://www.energystar.gov/sites/def...0_Rev%2011.pdf
    Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ KEEP IT SIMPLE & SINCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities

  10. #8
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    Quote Originally Posted by Monkburger View Post
    Right now (6am EST): Upstairs is 73F w/ 58% humidity. Downstairs is 74F w/ 55% humidity. Attic is about 77% w/ 72F. I live in IECC zone 4A (Transition Zone, DC area)

    Attic has the traditional soffit vent + ridge vent setup.

    I guess I will probably need to do something sooner than later - because I plan on air sealing the crawlspace.

    Where would I install a whole house dehumidifier? I see Trane has a device that is a dehumidifier + fresh air vent, would that go into the attic and connect it to the existing air handler? Or do I need separate units for both?
    The test for the a/c setup is to measure the occupied space temperature/%RH after several hours of high cooling loads. A well setup a/c should maintain <50%RH during significant sensible cooling loads. If not <50%RH, check a/c function and the amount of air flowing through the a/c cooling coil. The cooling coil must be cold enough to remove moisture as the occupied space air passes through the coil, The coil should be a minimum of 30^F colder than the return air temperature.

    An example is with 75^F, 50%RH, a 55^F dew point, return temperature, the supply air should have <49^F dew point, 54^F, 90%RH range temp/%RH. This range will remove 3 lbs. of moisture per ton, per hour of continuous operation for most a/cs. During partial sensible cooling loads, dehumidification declines to near zero evenings and rainy days. Unfortunately the latent (moisture) load is steady 24/7 depending on the outdoor dew, amount of fresh air ventilation/infiltaration and the number of occupants.

    A healthy home needs a fresh, filtered air change in 3-4 hours to purge the indoor pollutants and renew oxygen. The moisture from the fresh air and occupants determines the indoor dehumidification. A home depending on the a/c to remove the moisture from fresh air and occupants, will rise to outdoor levels plus with only the a/c removing moisture during sensible cooling. Reheat or a small whole house dehumidifier is required to supplement a well setup a/c to maintain <50%RH during evenings and rainy days. The dehumidifier is most economical because of it is designed to max the moisture per KWH and free reheat from the process. Units like the Santa Fe Ultra/Trane/Broan are the highest efficiency and offer a fresh, filtered air ventilation option.

    https://www.trane.com/residential/en...dehumidifiers/

    Post the a/c return and supply temps/%RH for our comments.

    Regards Teddy Bear
    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"

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  12. #9
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    Quote Originally Posted by teddy bear View Post

    Post the a/c return and supply temps/%RH for our comments.
    9:44am, outside temps are 81F, 74% RH.

    Tstat was set to 71F (schedule)

    Supply temps 59F, RH is 73%
    Return #1 (Master Bedroom) - 70, RH 61%
    Return #2 (Hallway) - 71F, RH 58%

    Checked everything with a calibrated handheld probe.

  13. #10
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    Quote Originally Posted by Monkburger View Post
    9:44am,

    outside temps are 81F, 74% RH. 72^F dew point

    Tstat was set to 71F (schedule)

    Supply temps 59F, RH is 73% 50^F dew point

    Return #1 (Master Bedroom) - 70, RH 61% 56^F dew point

    Return #2 (Hallway) - 71F, RH 58% 55^F dewpoint

    Checked everything with a calibrated handheld probe.


    Your a/c is removing significant amounts of moisture. I would expect after several hours of cooling by the end of the afternoon that you would have lower %RH.

    The comfort level of a home that is 70^F, 61%, a 50^F dew point is cool and damp. The low indoor temperatue is a below the peak outdoor dew points which increases moisture condensation inside the walls and attic ceiling. Consider that you have higher outdoor dew points many days, which increase the moisture load inside the home. Also the amount of fresh air infiltration/ventilation from wind, operation of clothes drier, and kitchen hood increases the moisture load causing higher indoor moisture load. Your should have a fresh air change in 3-4 hours to purge indoor pollutants and renew oxygen.

    I suggest raising the indoor temperature to +75^F, <50%RH, <50^F dew point, which most would find more comfortable. The a/c will run less and will require slowing the a/c air flow to maintain a lower coil temperature of by 3-4^F. During evening hours and rainy days, supplemental dehumidification is required.

    Keeping a home cold like yours is making your exterior surfaces condense moisture which migrates to cool dry wall surfaces. The moisture evaporates into the home. During extend days of high outdoor dew point and cold drywall, we will see mold growing through the drywall in the coldest areas. Warming the interior temperatures decreases the moisture migration.

    During low outdoor dew points, try raising the indoor temperature to 75^F to get a feel for higher temps and lower %RH. As I post this, my temp is 79^F, 45%RH, a 56^F dew point. Feels very comfortable. Getting 80 cfm of fresh, merv 13, filtered air, 24/7.

    Keep us posted.

    Regards Teddy Bear
    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"

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  15. #11
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    Quote Originally Posted by teddy bear View Post
    Keep us posted.

    Regards Teddy Bear
    A couple of discoveries:

    - Vapor barrier on the crawlspace floor is not secured together and is damaged. RH% is much higher than outdoor temps. Will have to fix this, due to Radon Gas accumulation (just below RR)
    - Busted downspout was causing rainwater to leak into the foundation vent on the one side of the house, and lay on the vapor barrier.
    - Plastic foundation vents will not close all the way. Will have to totally airseal the crawlspace.
    - Got a quote for a WH dehumid + fresh air intake to tie into the ducts of the first floor. Will need to address the other issues before that, though.

  16. #12
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    Quote Originally Posted by pecmsg View Post
    Before changing any settings a static pressure test needs to be performed to give a base line reading. Then you can measure after changes to ensure your not exceeding the min / Max
    Had ManJ performed, as well as TESP. ManJ now says 1.5 tons upstairs (instead of 2.5 tons) because of the insulation work.

    HVAC guy checked the A/H dipswitches, was defaulted to 1100cfm (3 ton). Changed it to 2.5 ton, normal, which is ~340cfm/ton.


    With those settings:

    - At the return side of the A/H, it's 0.65wc, and at the supply side, it's 0.11wc. two 16x16x1 returns.
    - Some supplies are putting out over 400cfm (600cfm, according to a testo anemometer.)
    - HOBO Data Logger was attached to the ECM, and recorded about 7 minutes of average runtime, with 3 cycles an hour when outside temps were 90F with dewpoints in the low 70s.

  17. #13
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    Quote Originally Posted by Monkburger View Post
    Had ManJ performed, as well as TESP. ManJ now says 1.5 tons upstairs (instead of 2.5 tons) because of the insulation work.

    HVAC guy checked the A/H dipswitches, was defaulted to 1100cfm (3 ton). Changed it to 2.5 ton, normal, which is ~340cfm/ton.


    With those settings:

    - At the return side of the A/H, it's 0.65wc, and at the supply side, it's 0.11wc. two 16x16x1 returns.
    - Some supplies are putting out over 400cfm (600cfm, according to a testo anemometer.)
    - HOBO Data Logger was attached to the ECM, and recorded about 7 minutes of average runtime, with 3 cycles an hour when outside temps were 90F with dewpoints in the low 70s.
    What were the return and supply temps/%RHs during the runs? Fan "auto" or "on"? This will confirm the amount of moisture removed per hour. Do you have the ability to lower the air flow one more notch? This will increase moisture removed and increase the length of the run time and high negative return pressure. Also the high air flow supplyies cold be restricted to rebalance the cold air supply.

    Need infor to comment on your a/c setup.

    Closing the vents and improving moisture barrier on crawlspace floor, good. Whole house dehumidifier with merv 13 air filter and a fresh air change in 3-4 hours with a Santa Fe Ultra/Broan/Trane, the icing on the cake. Connect the fresh air and return from the open part of the home to the dehu return. The dehumidifier supply connects to the a/c supply and 4" supply to the crawlspace. I feel better already.

    Keep us posted.

    Regards Teddy Bear
    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"

  18. #14
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    Quote Originally Posted by teddy bear View Post
    What were the return and supply temps/%RHs during the runs? Fan "auto" or "on"? This will confirm the amount of moisture removed per hour. Do you have the ability to lower the air flow one more notch? This will increase moisture removed and increase the length of the run time and high negative return pressure. Also the high air flow supplyies cold be restricted to rebalance the cold air supply.

    Need infor to comment on your a/c setup.
    Yes, I can adjust it. Do I have to worry about the coil freezing at 300cfm or equipment damage?

    Also: I will take measurements of the supply/return temps/RH in a few hours, it's very very hot in the attic atm.

  19. #15
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    Quote Originally Posted by Monkburger View Post
    Yes, I can adjust it. Do I have to worry about the coil freezing at 300cfm or equipment damage?

    Also: I will take measurements of the supply/return temps/RH in a few hours, it's very very hot in the attic atm.
    Measurements at the inside return and supply grills will do.

    Teddy Bear
    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"

  20. #16
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    Quote Originally Posted by teddy bear View Post
    Measurements at the inside return and supply grills will do.

    Teddy Bear
    Air temp 74F, set tstat to 72F to get some runtime measurements. Outdoor temp is 57F and dropping quick, though.

    Fan is set to Auto (as usual).

    Current Tstat, 74F, 51% humidity (it's getting very cool outside, low temps are in the 40s tonight)

    After about a minute of runtime:

    Return #1 (Bedroom) 72F 52%
    Return #2 (Hallway) 72F 53%

    Supplies: 52F 72% (checked 5 of them and they are all within 5% of one another for temp, rh)

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