Can a return plenum be too big? - Page 2
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  1. #14
    Join Date
    Jan 2003
    Location
    Durango, Colorado
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    101

    Wall Units and High-Velocity Systems

    Thanks for your help Kim. We considered window units for the hottest areas, but whole-house humidification was one of our critical goals. Our main floors are routinely in the 15-25% humidity range with the basement usually about 10% higher. I’ve given up playing my wood guitar except in the basement, because repaired dryness cracks always open up again. There are major problems with some wood furniture as well. We need to keep the house up to at least 35-40%, so we no longer need to worry about wood items self-destructing in our dry climate.

    We looked closely at high velocity systems in the first go-round with contractors. It looked great for our house at first, but after talking to some people who’ve had high-velocity installed we had to reconsider. My wife is so sensitive to moving air that if we sit near a ceiling outlet in a restaurant we have to ask to be reseated. She will start shivering in 70 degree air if there is any kind of constant breeze. This is kind of hard for most people (including me) to imagine, but it is a fact for her. The owners of Unico, etc. systems that we corresponded with all complained about noticing strong air movement., We felt it would be unwise to gamble on my wife’s comfort, since the whole point of a modern climate control system is to make you more rather than less comfortable.

    This is why we bit the bullet on traditional ductwork, and this is why we’re slowly tearing bits and pieces of our house apart and putting them back together again with duct inside.

  2. #15
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
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    3,304

    Humidification? De-humidification?

    Are you using the right word when you say "whole house humidification" in the context of air conditioning? Because I have only heard of AC being capable of DEhumidification. Of course my hot-humid climate in S.Texas leads me to be fixated on de-humidification most of the year.

    I am wondering if you are anticipating a central AC to help directly with raising humidity, or if you may be intending to add a humidifier to the forced air system and use that to raise the humidity in your house.

    If you haven't already, find a psychrometric calculator program (online or trial-download) and plug in various outside air degree/RH combinations and see how the RH changes when the air is brought to room temperature. I did and it changed my whole understanding of humidity issues:
    http://www.google.com/search?hl=en&q...=Google+Search

    Hope this helps -- P.Student

  3. #16
    Join Date
    Mar 2003
    Posts
    41
    i would be a bit concerned about how supper insulated the house is,,, if its to tight and the loads on the house were not run for that tight of a house you might be headed for a humidity problem,, unit short cycling,,and eventuaally mold,,,and when you are gone for long periods of time that air infiltration helps with an air change over,,,,just a thought,,,,, if it is a concern a good humidistat might do the trick or an erv (energy recovery unit),,

  4. #17
    Join Date
    Jan 2003
    Location
    Durango, Colorado
    Posts
    101

    Tightness and Over-Humidification

    Perpetual, thanks for your input. In the dry southwest central AC routinely includes whole-house humidification. There are many ways of doing this, and we’ll be using a Carel steam humidifier in the supply plenum. This unit offers simpler and safer maintenance than some of the other options we considered.

    Our humidity in the house year-round is normally 15-25%, with dry periods in June and September occasionally dipping into the single digits. This environment is frequently unimaginable for those living in the humid east. Our house is drier than most here because our entire south wall is glass. For mold to ever grow we would have to seriously over-humidify for an extended time. We plan on humidifying only to about 40%, which should not be enough to cause any moisture problems.

    Thanks for the link to PsyCalc. I'm going to download it and try it out. It looks like we can definitely learn something from the process.

    Blazer, our house was built 25 years ago before the advent of tyvek wrapping. It is extremely well insulated but not as airtight as a modern wrapped home. We also have a wood stove which, when not in use, allows outside air exchange to our main floors. The basement gets plenty of outside air via an optional flue built into our fireplace foundation that is also open to the outdoors.

    Because our external walls are 2 x 6 double tongue-and-groove cedar over tarpaper and 1/2” plywood, we are very weather and draft-resistant on windy days, at least compared to an average house around here. We ran HVAC-Calc load calculations for both super-tight and medium-tight house settings and figured we were roughly halfway in between.

    So these are the reasons we think we’re OK with regard to over humidification and mold, But we could still be wrong, and we really appreciate anyone’s input that might help us see things we’re missing or misinterpreting.


  5. #18
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
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    3,304

    General thoughts

    JJnet, thanks for explaining your position. I should say upfront that it's like another world for me, as I am in the hot-humid S. Texas. Where I am the house RH (relative humidity) rarely gets below 30%. But as Joe Lstiburek likes to say, the laws of physics are the same whether you are in Canada or Mexico.

    It's been a real eye-opener to me to understand that the measure I should look at is not really RH in outside air, but grains/cubic foot or its equivalent. Cold air has so little moisture capability that when it is warmed up, its RH is usually lower than my goal for the house. The fact your house has little air infiltration should be good for your winter purposes. In the summer I have to wonder if different rules might apply.

    I'm not sure whether you clarified whether your ductwork is inside the building envelope, or outside it. Where I live, ductwork is typically located in an attic ventilated to the outside, and any duct leakage results in some degree of house pressurization or de-pressurization (depending on whether supply or return). As I understand it, 15% leakage is typical unless the installer took pains to reduce it. If your ductwork is inside the building envelope, then you have less to worry about with this matter.

    There is a microprocessor gadget named "Smartvent" (a too-common name) which samples air from outside and based on its temperature and humidity measurements, makes a decision whether to begin a longer ventilation session. The maker is Arkansas based and I believe his full attention is toward *reducing* humidity especially in crawl spaces. But the principle of a little computer using rules to "harvest" lower humidity when it is ambient, could be turned upside down possibly for your needs. Wish I could point to a well developed product, but the concept is all I can offer.

    Best of luck -- P.Student

    [Edited by perpetual_student on 01-28-2005 at 07:57 PM]

  6. #19
    Going back to the root of the thread...if a return air plenum can be too big, what are the reasons?

  7. #20
    Join Date
    Jan 2003
    Location
    Durango, Colorado
    Posts
    101
    Thanks for the info, Perpetual. I will check that product out.

    Coologic, I'm guessing that the relevant goal is to keep the velocity of the return air above some minimum value. This may be in part to ensure good flow through the filter, particularly if it has a high static pressure drop (like our electrostatic filter does). Please correct me where I’m wrong, those who know.

    I was surprised that Dash recommended a maximum of a 21” x 13” plenum size. This equates to 273 sq. in. of cross sectional area. The return ducts feeding into it consist of 3 12” @ 113 sq. in each, plus one 8” @ 50 sq. in. The ducts total 389 sq. in. while the return plenum reduces to 273. Maximum system airflow should be in the 1100-1200 cfm range.

  8. #21
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,271
    I'm just curious, and this is probably barn door after horse is missing talk, but did you consider evaporative (swamp) coolers for the home? We have family who live in the hills above Loveland, CO and they use a swamp cooler on those hot, dry days.

    But I'm guessing it's year round humidification you're after, whether the system is heating or cooling. One system that does everything has great appeal.
    • 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. #22
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    Originally posted by jjnet
    Thanks for the info, Perpetual. I will check that product out.

    Coologic, I'm guessing that the relevant goal is to keep the velocity of the return air above some minimum value. This may be in part to ensure good flow through the filter, particularly if it has a high static pressure drop (like our electrostatic filter does). Please correct me where I’m wrong, those who know.

    I was surprised that Dash recommended a maximum of a 21” x 13” plenum size. This equates to 273 sq. in. of cross sectional area. The return ducts feeding into it consist of 3 12” @ 113 sq. in each, plus one 8” @ 50 sq. in. The ducts total 389 sq. in. while the return plenum reduces to 273. Maximum system airflow should be in the 1100-1200 cfm range.
    Didn't realize how many and oversized(not a problem,just larger than needed)your return branch duct were.

    Since they at around 400fpm(velocity),the plenum could be 20X21.Of course we don't know how many Total Equivalent Feet each run is,nor the size of each grille,or how many supply cfms are delivered to the area each return serves.Plus a whole bunch of other things.

    When the velocity changes ,for X to more thean X and then back to X ,ESP increases,not a "big " factor.Most good designs will keep the fpm < 800 to 900,so the 21X13 was based on keeping it at >600 fpm.

    As always the best way ,is a Manual D,from ACCA to design the entire duct system.

  10. #23
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
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    3,304

    That 'SmartVent' product

    It wouldn't hurt to give the website of the "SmartVent" product:
    http://www.smartvent.net/

    They have a couple of papers on that site which look to me to be professional quality. I will state again their focus seems to be on avoiding the harms of too *much* humidity either in a crawlspace or inside the house. I might actually buy this thing in the future, but I understand your problem is exactly the opposite of what it does. Still, it seems that a similar device with the same sensors but different microprocessor programming, could in concept be helpful to you. Possibly a phone call to the guy who makes it, would result in some new insight or ideas.

    Best wishes -- P.Student

  11. #24
    I know of no technical reason to limit the size of a return plenum, although there are many practical reasons.
    The filter will have a maximum velocity but no minimum velocity requirement.

  12. #25
    Join Date
    Apr 2008
    Location
    Rochelle Il / Chicago
    Posts
    169
    Simple, Yes

    Have seen returns too big.

    by the time you are able to start pulling from House tstat is satisfied.

    In this case you have caused a house to become over pressured.

    Sorry, no time to go into depth.

  13. #26
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,595
    Even though they were proceding slowly.
    In the 3 years since this thread was made, they should be done with their return system.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

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