Page 1 of 3 123 LastLast
Results 1 to 13 of 37
  1. #1
    Join Date
    Jun 2019
    Posts
    24
    Post Likes

    Whole House Dehumidifier (ducted/ac) proper sizing?

    Hello, I am looking to install a whole house dehumidifier system.
    My goal is keep the entire house around 40% RH from Spring through Fall. (I have zero tolerance for any humidity) Winters are not a problem. I am aware the AC should be taking a lot out already but there are plenty of days when the temps aren't high enough to let the ac's run very long or at all. Plus my wife and I have bitter ac wars. This is the only chance to compromise by keeping the humidity low enough without having to drop the temps too much.

    We have a full basement, 1st floor, 2nd floor and full walk-in attic. 1st and second floors have separate central AC and duct work systems. Basement - 1,400 sq/ft, 1st floor - 1,400 sq/ft, second floor 1,000 sq/ft. (future plans to add @800 sq/ft to second floor (part over garage and part over 1st floor)

    Originally, I assumed I would need 2 separate dehumidifiers to tie into each HVAC system but I guess the physics of it will work with just one unit tied in down in the basement (1st floor duct work)?

    Based on talk on this forum, advice from local hvac guys and my own research and personal preference I am looking at:

    Whole house ventilating unit with fresh air intake. Connected in basement to 1st floor duct work. Air out of dehumidifier connected to supply duct bypassing the ac air handler.

    I have narrowed it down to the Ultra-Aire and Honeywell units.

    My Question? What size do I need? Though we live in Southern New England it is too muggy and i would prefer to base it on requirements for the Southeast. Overkill is fine with me.

    Thanks in advance for any advice.

  2. #2
    Join Date
    Nov 2006
    Location
    Athens, Ohio
    Posts
    6,922
    Post Likes
    A larger dehumidifier will not be a bad thing. The Ultra-Aire units are built in the U.S.
    Honeywell now manufacturers theirs overseas.
    AOP Rules: Rules For Equipment Owners.

    Free online load calculator: http://www.loadcalc.net/


    There = not here. Their = possessive pronoun. They're = they are
    It's = contraction of it is. Its = the possessive form of it
    Too = also. To = expressing motion. Two = 2
    Then = after that, next. Than = indicates a comparison.
    Questions should end with a question mark "?" Statements end with a period "."

  3. #3
    Join Date
    Aug 2001
    Location
    Pavilion, NY
    Posts
    3,447
    Post Likes
    Teddybear will likely be along soon and you will be mesmerized by his knowledge and great looks

    'The more you know, the more you realize you don't know'
    ...

  4. #4
    Join Date
    Sep 2002
    Posts
    26,364
    Post Likes
    If you go with Ultra Aire you will have the almost immeasurable benefit of having a guy here who knows more about humidity, whole house dehumidifying, and Ultra Aires to help you more than you will ever need.

    He will be along directly I expect.

    PHM
    ----------




    Quote Originally Posted by noHHH View Post
    Hello, I am looking to install a whole house dehumidifier system.
    My goal is keep the entire house around 40% RH from Spring through Fall. (I have zero tolerance for any humidity) Winters are not a problem. I am aware the AC should be taking a lot out already but there are plenty of days when the temps aren't high enough to let the ac's run very long or at all. Plus my wife and I have bitter ac wars. This is the only chance to compromise by keeping the humidity low enough without having to drop the temps too much.

    We have a full basement, 1st floor, 2nd floor and full walk-in attic. 1st and second floors have separate central AC and duct work systems. Basement - 1,400 sq/ft, 1st floor - 1,400 sq/ft, second floor 1,000 sq/ft. (future plans to add @800 sq/ft to second floor (part over garage and part over 1st floor)

    Originally, I assumed I would need 2 separate dehumidifiers to tie into each HVAC system but I guess the physics of it will work with just one unit tied in down in the basement (1st floor duct work)?

    Based on talk on this forum, advice from local hvac guys and my own research and personal preference I am looking at:

    Whole house ventilating unit with fresh air intake. Connected in basement to 1st floor duct work. Air out of dehumidifier connected to supply duct bypassing the ac air handler.

    I have narrowed it down to the Ultra-Aire and Honeywell units.

    My Question? What size do I need? Though we live in Southern New England it is too muggy and i would prefer to base it on requirements for the Southeast. Overkill is fine with me.

    Thanks in advance for any advice.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  5. #5
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    9,622
    Post Likes
    Is this a great bunch of guys or what!

    40%RH is aggressive humidity control, what is the temperature you want? This will determine the require size of the dehumidifier. Let us use 75^F, 40%RH is 48^F dew point or 53 grains of moisture per lb. of air. Outside air in the NE US gets 78^F dew point or 150 grains of moisture per lb. of air.
    You have 4,600 sq.ft. of living space. Moderate air tightness may be an air change in 4 hours or <200 cfm in natural infiltration/ventilation with a max of
    150 grains/lb. outside air reduced down to 50 grains/lb.=100 grains of moisture tobe removed per lb.
    200 cfm /14 cfm/lb. of air=14 lbs. /min.
    14 lbs/min X 100 grains dehumidification X 60 mins per hour / 7,000 grains/ lb.moisture = 12 lbs. of moisture per hour during peak latent load.
    Next comes the moisture from the occupants living in the space. Figure about .25 lbs. per hour from occupants and activities. Four occupants add 1 more lb. per hour.
    Total 13 lbs. per hour of dehumidification.
    We start by making sure the a/c can remove +13 lbs. of moisture per hour.
    @ 75^F, 40%RH, a 48^F dew point.
    It is critical to get the a/c setup to remove this amount of moisture per hour.

    This is a big increase verses the typical 50%RH most target. We will need 4 lbs. of moisture per ton of cooling verse the normal 3 lbs.per ton. The way to do this adjust the a/c to have a 7^F colder cooling coil than normal. A 40^F cooling coil is needed to provide 44^F dew point cool air to maintain 48^F dew point space for the 75^F, 40%RH. You do not need to understand all of these number. It is here more for all the techs who are interested in how we get the numbers.
    If we can get the a/cs to do their part which is to maintain <40%RH which is on the edge, than we size the dehumidifier[s] to maintain 40%RH during low/no sensible cooling loads and high outdoor dew points. I would start out with a 155 lbs. (pints) per day on the most critical area of the home. The large dehus are a little noisy for home use. You may find a 40%RH most of the time the critical areas with 50%RH in the rest of the home very acceptable. If 40%RH through out, you will an additional 100 pints per day. This should also cover a 100 cfm of mechanical fresh air ventilation.
    Now you get the ideal.
    Tell me the ideal temperature that you want and how critical 40%RH throughout the home is. Example basements are usually cooler than the rest of the maybe 45-50%RH will be comfortable. While the warmer areas must be 40%RH.
    This is easier talk about than to deliver.
    Keep posted on the issues.
    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"

  6. #6
    Join Date
    Sep 2002
    Posts
    26,364
    Post Likes
    I agree: 40% RH is a Serious target. <g>

    Although in this case I think it was set somewhat arbitrarily rather than out of some clinical-grade necessity.

    What are the risks / complications / contra-indications in regard to the indoor humidity being 'too low' in the summer?

    What %RH might be considered to be 'too low' ?
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  7. Likes kangaroogod liked this post
  8. #7
    Join Date
    Jun 2019
    Posts
    24
    Post Likes
    Thread Starter
    Thank you everyone for the quick responses. Since following this forum I had a feeling teddybear would have a very thorough and informative answer as well. Thank you!
    40% may be a little agressive and 45%-50% is fine. I understand 50% is roughly the border for mold growth and air quality so I figured I would shoot a little lower.
    Our house has 2x6 construction (but not air tight) so the temps inside can be in the 76-80 range when it's 85-90 outside (ballpark). The temps come down to a decent level fairly quickly
    but some humidity is still there. Plus there are plenty of days when the inside temp is only 68-72 but it's muggy inside. No chance of my wife letting me run AC on at that point.
    She wants 85 and muggy in the house and I want 68 and dry. Somehow we are still living together.
    Therefore, I was hoping if I can keep the humidity below 50% then maybe the temps could stay up around 74 or 75 and still be fairly comfortable.

    It sounds like I do need to include the square footage from the basement and all floors when sizing. Given the sq/footage, the layout (3 floors including basemnt) and the separate first
    and second floor AC systems, does it make sense to eventually go with two separate dehumidifiers? One sized for the basement and first floor and a second sized for the second floor?

    I have been looking at the Ultra-Aire and Honeywell units. I also looked at AprilAire but some questioned the longevity compared to Ultra-Aire and Honeywell.
    Ultra-Aire seems quite a bit more pricey than the Honeywqell units. Any one know of justified resons?

    Unit 1 (basement and first floor, 2,800 sq/ft) Ultra-Aire - XT105H or 120V, Honeywell - DR120A3000?
    Unit 2 (second floor 1,000 to 1,700 sq/ft) Ultra-Aire - XT105H, Honeywell - DR90A2000?

    or could the Honeywell DR90A2000 on both floors be enough?

  9. #8
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    9,622
    Post Likes
    Quote Originally Posted by Poodle Head Mikey View Post
    I agree: 40% RH is a Serious target. <g>

    Although in this case I think it was set somewhat arbitrarily rather than out of some clinical-grade necessity.

    What are the risks / complications / contra-indications in regard to the indoor humidity being 'too low' in the summer?

    What %RH might be considered to be 'too low' ?
    ASHRAE suggests 40%-60%RH as a range. Beyond that it is a personal issue. The biggest issue for some maybe "too expensive". The fact remains lower %RH means less sensible cooling because of evaporative cooling on the surface of the skin. Materials store well at the 40%RH level. Mold/bacteria/dust mites/spider are dead long term.

    Providing 40%RH is not difficult as long as the a/c is able provide air about 5^F dew point lower than the desired dew point in the space. With a 75^F, 40%RH, a 49^F dew point, we need <45^F dew point a/c supply. This means 40-41^F coil temp. The a/c setup will be at the low air flow.
    We find a/c operating in this range occasionally, mostly because of small ducts. This means enough duct insulation to avoid condensation and keeping ducts in conditioned space.
    I have a SE FL customer that operates at 45%RH, 84^F in big house. Ceiling fans and is very comfortable. This totally eliminates any condensation issues on drywall and ducts.
    We may come up with some new benefits and/or problem doing this NE US.

    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"

  10. #9
    Join Date
    Sep 2002
    Posts
    26,364
    Post Likes
    In Florida I used to run the dehumidifier on constant but I found that I would be 'freezing' at 76-77 F. whenever the RH was below about 47-48%. I found 50-52% to be about right for me.

    I guess I could try easing the space temps up and the RH% down - maybe I could settle out at 80 at 40% RH. <g>

    Correct me if I'm wrong but I don't think 50% is the mold / no mold threshold. I think 60% is the upper limit. Which is where ASHRAE got it from. <g>

    PHM
    -----------



    Quote Originally Posted by teddy bear View Post
    ASHRAE suggests 40%-60%RH as a range. Beyond that it is a personal issue. The biggest issue for some maybe "too expensive". The fact remains lower %RH means less sensible cooling because of evaporative cooling on the surface of the skin. Materials store well at the 40%RH level. Mold/bacteria/dust mites/spider are dead long term.

    Providing 40%RH is not difficult as long as the a/c is able provide air about 5^F dew point lower than the desired dew point in the space. With a 75^F, 40%RH, a 49^F dew point, we need <45^F dew point a/c supply. This means 40-41^F coil temp. The a/c setup will be at the low air flow.
    We find a/c operating in this range occasionally, mostly because of small ducts. This means enough duct insulation to avoid condensation and keeping ducts in conditioned space.
    I have a SE FL customer that operates at 45%RH, 84^F in big house. Ceiling fans and is very comfortable. This totally eliminates any condensation issues on drywall and ducts.
    We may come up with some new benefits and/or problem doing this NE US.

    Regards Teddy Bear
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  11. #10
    Join Date
    Jun 2019
    Posts
    24
    Post Likes
    Thread Starter

    Cool

    Quote Originally Posted by Poodle Head Mikey View Post
    I agree: 40% RH is a Serious target. <g>

    Although in this case I think it was set somewhat arbitrarily rather than out of some clinical-grade necessity.

    What are the risks / complications / contra-indications in regard to the indoor humidity being 'too low' in the summer?

    What %RH might be considered to be 'too low' ?
    You are absolutely correct. I made up the 40% based on 1. Being well below the recommended 50% for mold and air quality and 2. My personal comfort zone. I am happiest at 35%-45%. (Did I mention how much I hate humidity?!!&#128513

  12. #11
    Join Date
    Jun 2019
    Posts
    24
    Post Likes
    Thread Starter
    Thanks for the clarification on mold. I don't why I thought below 50% was the target for mold. 60% is much easier to maintain. I can certainly be fairly comfortable at 50% while making it a little easier to maintain.

    Any thoughts on going with 2 separate/smaller units and the models I am looking at? or would one be better?

    I plan to use the fresh air intake and if needed I will add an air exchanger as well.

  13. #12
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    9,622
    Post Likes
    With almost 5,000 sq.ft. home and part of that basement, the 155 pint Ultra-Aire has a very high 8 pints per kwh efficiency. The 70h U A is 4.5 pints per KWH.
    If the 155 can be ducted to both a/cs, it would be my choice. Half the operating cost.
    Hope this helps.
    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"

  14. #13
    Join Date
    Jun 2019
    Posts
    24
    Post Likes
    Thread Starter
    How accurate are the sq/t specs listed for the units? The XT155H lists a max at 3,500 sq/ft but I may need up to 5,000 sq/ft. Even if I could find a way to connect the 1st and 2nd floor duct work (seems difficult) would it be powerful enough? If I compare different combinations what makes the most sense?

    Option 1 - 1 unit all floors - XT155H - 3,500 sq/ft (both floors) 8 Amps, 7.3 pints/kwh $2,438

    0r

    Option 2 - 2 units - 70H - 1,800 sq/ft (second floor) 5.1 Amps 5.0pints/kwh $1,248
    120V - 3,000 sq/ft (basement/1st flr) 5.8 Amps, 7.6 pints/kwh $2,078
    Total - 4,800 sq/ft (both floors) 10.9 Amps 12.6 pints/kwh $3,326


    or

    70H - 1,800 sq/ft (second floor) 5.1 Amps 5.1 pints/kwh $1,248
    XT105H - 2,500 sq/ft (basement/1st flr) 4.9 Amps, 8.8 pints/kwh $2,438
    Total - 4,300 sq/ft (both floors) 10.0 Amps 13.9 pints/kwh $3,686

Page 1 of 3 123 LastLast

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •