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  1. #1
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    May 2018
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    Question Do I need a WH dehumidifier?

    I live near Washington DC, kinda hot & humid in the warmer part of the year.

    My house has about 4000ft^2 above grade with a full basement. It was built in 2012, and has EnergyStar certification. There are two completely separate HVAC zones (basement location covers unfinished basement and first level, attic location covers second level).

    When it's notably hot outside (near 90), when the thermostat is set to 77 it seems to run most of the time (I understand from reading other posts that this is about correct -- it can get to 100 occasionally, so it seems like when it's 90 it should run "most" of the time).

    But the humidity on the first level seems to be ~52% (@ 77F) ... and I've read everything from "35-45% is what you want" to "<55% is fine". During heating season, with a WH humidifier, the house seems (from what I recall) to be around 35% @ 73F.

    And an additional complication: I don't really have a good baseline, we had a minor flood in the basement (clogged egress drain). There was water on the floor of the basement for a couple of days before we noticed, when we started up the restoration dehumidifier (a Dri-Eaz LGR 3500i) it read 70% at 70F (upstairs was ~50%), and after running for three days it was able to get the basement down to 40% at 70F (and upstairs was ~45%). I don't know if my current humidity is partially an artifact of things in the basement still reaching equilibrium or not.


    So, questions...

    Can you tell from the above anything useful? I have plans to monitor temp & humidity in various places around the house (a bathroom, 2nd level living area, 1st level living area, basement unifinished area, return immediately before furnace, supply immediately after furnace) using data logging IOT things... is that necessary?

    Am I a worry-wart? Is ~50-52% at 77F just fine, don't worry about it?

    Is it likely a temporary condition, an artifact of the moisture which has been dealt with, but just needs a few weeks to reach equilibrium?

    Do I need a WH dehumidifier? Even if it's not "needed", would it be helpful?

    If I need one, what size? I've seen charts that would seem to indicate ~40pint cap would be fine, but I've also seen specs that say a 95 pint cap unit is for "areas up to 4000ft^2".

    If I need one, do I need two? One for each ~2000ft^2 zone?

  2. #2
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    You have all the good questions.
    Being able to maintain 50-52%RH during peak cooling is great. <55%RH should control mold and dust mites in all areas that do not have wet spots.
    As you start monitoring the your home, you will find that during evenings and rainy weather that has low/no sensible cooling loads and outdoor dew points of +60^F plus moisture from the occupants will raise your indoor %RH. If your home has adequate fresh air ventilation, it will happen within a day or two. Cool damp weather lasting for a couple weeks is risky. Thus, if you live in a green grass climate sooner or later, you will have a moisture problem that may grow mold. Also dust mites will be present because they survive at low 60%RH. Both go dormant during dry winter months and come back during damp months.
    My guess on your home is that you may not be getting good fresh air change during the mild months of the year but plenty during the cold windy times of the year. I would suggest a whole house dehumidifier with a small amount of fresh air for the 3 mild seasons and not for the winter. A fresh air change in 4-5 hours during mild seasons when occupied would be ideal to purge indoor pollutants and renew oxygen. 80-100 cfm of fresh filtered air mixed into the home air with 4-5 lbs. of dehumidification capacity would make your home ideal.
    Most are not aware of the finer points of indoor air quality. The AM Medical Ass, Am Lung Ass, and ASHRAE suggest this.
    Ultra-Aire 98H or XT105H are the top units for this application. Your a/c contractor is the best person to connect this to your a/c.
    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"

  3. Likes adamgoldberg liked this post
  4. #3
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    May 2018
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    Thread Starter
    Quote Originally Posted by teddy bear View Post
    My guess on your home is that you may not be getting good fresh air change during the mild months of the year but plenty during the cold windy times of the year. I would suggest a whole house dehumidifier with a small amount of fresh air for the 3 mild seasons and not for the winter. A fresh air change in 4-5 hours during mild seasons when occupied would be ideal to purge indoor pollutants and renew oxygen. 80-100 cfm of fresh filtered air mixed into the home air with 4-5 lbs. of dehumidification capacity would make your home ideal.
    Part of the EnergyStar 3.0 requirements is a quiet fan (e.g., bathroom fan) to allow fresh air change? (I have the fan, on a wall switch, but I've never had anyone explain how/when I should turn that fan on). Should I be turning this on? On a schedule? Certain seasons?

    Thanks!!!

  5. #4
    Join Date
    Jun 2003
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    Madison, WI/Cape Coral, FL
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    If this is only source of air change, operate this fan when the home is occupied. As the air exhausted from the bathroom, fresh air will infiltrate the imperfections of the home, move to the exhaust fan and exit.
    When the wind blows and stack effect is working, probably do not need the fan for fresh air change. A 10 mph wind on most homes will change the air enough to purge indoor pollutants and renew oxygen in a typical home.
    Your conditioning system will heat and cool this natural and mechanical air change as the loads are small.
    The exception is moisture. Occupants add moisture plus the moisture in the fresh air during the mild seasons of the year. When outdoor dew points are +60^F, dehumidification is require to avoid mold and dust mites. The a/c setup correctly will remove excess moisture during peak cooling loads. Evenings and rainy day will require supplemental dehumidification to maintain ideal 50%RH.
    4-6 lbs.(pints) per hour is typically adequate to maintain <60%RH.
    Ideally sucking in <100 cfm of fresh air when occpied is best. The make-up air will help kitchen hoods and clothes drier function plus purge the indoor pollutants and renew oxygen. Ventilate when occupied during the mild seasons of the year. This may cost <200$ per year in energy cost.
    In green grass climates, providing the fresh filtered air via a whole house dehumidifier is simple efeective method of providing top quality and comfort to concerned home owners.
    Keep us postedl
    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. #5
    Join Date
    May 2018
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    Thread Starter
    Quote Originally Posted by teddy bear View Post
    If this is only source of air change, operate this fan when the home is occupied.
    Does "occupied" mean "people are home," or does it mean "people are living there" (not on an extended vacation or etc).

    I work at home and my wife doesn't work, so "people are home" is always.

    Let's see if I understand (stand back, software engineer at work):


    if(occupied && wind speed <= 10mph) {
    fan = on
    } else {
    fan = off
    }

    if(outdoor dew point >= 60F && indoor humidity >50%) {
    dehumidifier = on
    } else {
    dehumidifier = off
    }



  7. #6
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    Jun 2003
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    Quote Originally Posted by adamgoldberg View Post
    Does "occupied" mean "people are home," or does it mean "people are living there" (not on an extended vacation or etc).

    I work at home and my wife doesn't work, so "people are home" is always.

    Let's see if I understand (stand back, software engineer at work):


    if(occupied && wind speed <= 10mph) {
    fan = on
    } else {
    fan = off
    }

    if(outdoor dew point >= 60F && indoor humidity >50%) {
    dehumidifier = on
    } else {
    dehumidifier = off
    }


    Even a sense of humor!
    Someone in the home, wind under 10 mph, fan off. Wind under 10, someone in the home, fan on.
    Home +55%RH, dehumidifier on. <50%RH fan off.
    Of course, I will assume that you will the world's greatest whole house dehumidifier, Ultra-Aire XT105H.
    Keep us posted.
    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"

  8. #7
    Join Date
    Apr 2003
    Location
    St. Louis
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    Try changing blower speed from hi to the next lower speed and check discharge air temp, also get a blower door test this will tell you how tight the house is.

  9. #8
    Join Date
    May 2018
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    Thread Starter

    Confused What happened???

    So, I've had two HVAC contractors come out and give me quotes. In addition, I installed some temp/humidity dataloggers.... and bear with me, this might get long.

    We have an EnergyStar 3.0 home which was completed 5y3mo ago (March '13), it's about ~4200ft^2 above grade plus about 2000ft^2 unfinished basement. There are two HVAC systems, one installed in the basement which covers the basement & main level, and one installed in the attic which covers the upstairs (bedrooms mostly).

    The downstairs (main) unit has a large air return near the thermostat in the main level, plus a louvered grate in the return (to return basement air), plus a fresh air supply via an insulated duct with a electric damper. I haven't gone up in the attic to survey that setup...

    You'll recall (from above) that I was concerned that perhaps I needed a dehumidifier. But there was so much unknown about what was actually happening that I'm trying to instrument my house to figure it out before I spend thousands of dollars on a WH dehumidifier.

    But something weird is happening. I'm looking for ideas..... I have a sensor on the main level near the thermostat, one in a bedroom near the upstairs thermostat, and one inside the cold air return in the HVAC unit in the basement. I've gathered data for about a week from those three locations, plus downloaded meteorological data from DarkSky for my location (to compare against outside conditions).

    Consider figure 1. (note that I'm using dewpoint vs. RH in order to have a constant-water-amount discussion vs. one that takes into account temperature...yell at me if you disagree)

    (cant post links.... go to imgur dot com slash a slash x6Kio1S )

    Please disregard the first handful of datapoints, the sensors went from the hot-as-hell mailbox directly into operation ...

    The top solid mostly straight lines are the temperature of the family room (top, green), the MBR (middle, red) and the furnace air return (bottom, blue). Mostly the temperatures stay where they are, while the outside temperature (brown) varies from a low of ~55 on 6/12 to a high of 83 on 6/4, and the outside dewpoint (dotted pink) kind of follows the outside temperature around.

    The lower three lines (below the "dewpoint goal" line) are the dewpoints at the furnace return, MBR and family room. From 6/11 through about 4pm on 6/13, the furnace and family room dewpoints were creeping up (while the MBR dewpoint was steadier). That makes me say "interesting..."

    But look at what happened around 4:10 PM yesterday, zoom in figure 2:

    (Cant post links, go to imgur dot com slash a slash nxxF6o8 )

    You can see a precipitous drop of the furnace return and family room dewpoints at 4:10pm, and a corresponding jump in furnace return /air temp/.... and at the same time NO CHANGE to the MBR (either temperature or dewpoint).

    Anyone want to guess what happened that made things change? No changes were made to any thermostat. No showers were taken, no pools drained, no cooking, no manual fans, nothing.

    My original guess was that the damper on the fresh air intake was not working properly, and at 4:10 PM it opened when it hadn't been open in a while... this might be true, but I don't know how I could verify that. I do know that the fresh air intake is NOT always open (just checked, when the AC running, there was no air being drawn in from the air intake).

    Is this normal? Weird? HElp? I should post this elsewhere?

  10. #9
    Join Date
    Oct 2016
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    I wouldn’t worry about it in answer to your original post. If you did need a wh dehumidifier it would only be on less hot days. If your unit is running most of the time it’s dehumidifying most of the time.

    As servicefitter mentioned slowing the fan speed could help dehumidify. I would only recommend a pro changing the fan speed, as you would need to verify you were moving enough air to support the ac unit.

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