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  1. #1
    Join Date
    May 2009
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    System question: Trane, nighttime ventilation, IAQ...

    Hi,

    I moved into a house (Pittsburgh,pa) with an old rheem80 (natural gas) and a broken air conditioner. While adding an extension to my home I installed a Honeywell IAQ with an outdoor sensor and two indoor sensors (wanted to average the temperature between the 1st and 2nd floor). I'm now ready to get a new hvac system...

    I'm considering a Trane HP system:
    Condenser: XL16i 4ton (4TWX6048)
    Furnace: XV95 (TUH2B100A9V4V)
    Humidifier: (hvac guy says I can save money by fixing my old Aprilaire 550 unit)

    I have three questions:

    1. Is it better to get the XC95 instead because it supports communication? I'm assuming I can still use the dehumidifying features of the IAQ with either the XV95 or the XC95?

    2. I notice that the outside temperature at nighttime is almost 10deg cooler than the house. Wouldn't it make sense to bring fresh air in at night. I see the IAQ supports that, what would I use? FreshEffects?

    3. Does using my existing old Aprilaire 550 humidifier make sense?

    In general we're not the kind of family that needs cold nights, just RH around 50% would due and cheaper bills. (electric is high here)

    Thanks!

  2. #2
    Join Date
    Mar 2005
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    I will get 2 of these. Id say don't pull all that RH into your home. Bringing it in at night or any time will raise the RH. Some outside air is good for IAQ but you can have to much. 2nd don't know how old your 550 is but if more than a few id get the true steam.

  3. #3
    Join Date
    Jun 2004
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    4H: Hot, Humid Houston H.O.
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    What kind of overnight temperatures are you talking about? And more importantly what would be the humidity in that air? I am a Houston homeowner and night air would be too humid for us, looking for signs that there is less humidity in PA.

    The best way would be to measure by dewpoint, but %RH and temperature will do. According to the way we set our thermostats, the dewpoint should be below about 55F or else it will increase indoor humidity.

    Plugging in some numbers to a psychrometric calculator such as this, has helped guide my decisions in a valuable way:
    http://www.linric.com/webpsy.htm

    Hope this helps -- Pstu

  4. #4
    Join Date
    May 2007
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    DC Metro Area (MD)
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    Not sure about using the averaging sensors with the TCONT900 thermostat, but it may be possible. The TCONT900, as far as I know, can do most other tasks of the IAQ including dehumidification on demand. Double check using remote sensors with the TCONT900 with your dealer. If you want dehumidification, however, you must use the TCONT900 with the XL16i--just the way it's set up. Fresh air intake isn't a bad thing, but don't expect it to noticeably cool your home.

  5. #5
    Join Date
    May 2009
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    Thread Starter
    Thanks for the responses...

    DRK,
    Q2. (still not sure) Ok, bringing in too much RH is bad but can the Honeywell IAQ in conjunction with the XL16i (and a FreshEffects system?) automatically decide when to stop bringing in fresh air in? Or do I have to manually bring fresh air in? I was assuming the IAQ would engage the FreshEffects (or something like that) when it noticed the outside air was cooler.
    Q3. (decided) I'll probably stick with the Aprilaire 550 because it's cheaper to get it working than buying new..


    PSTU,
    Hmm... It can be humid at night. I was guessing the IAQ would put the breaks on the fresh air intake if it reached an RH value that was too high.

    RyanHughes,
    I have the Honeywell IAQ and I hooked the inside sensors up in series according to the instructions. From what I can tell it is averaging the two floors.
    -----

    Q1: (still not sure) I'm guessing the smarts of the XC95 over the XV95 isn't much value?
    .

  6. #6
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
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    Quote Originally Posted by mrvoidman View Post
    Hmm... It can be humid at night. I was guessing the IAQ would put the breaks on the fresh air intake if it reached an RH value that was too high.
    From the looks of historical average high and low temperatures, there are only a couple months of the year when you need to shut up the house because of the overnight average low.
    http://www.wunderground.com/NORMS/Di...&IATA=PIT&MR=1

    I am thinking along the lines that when overnight low is at or below 55-60F, then even at 100% RH the dewpoint will be 55-60F which is probably acceptable humidity when warmed to 75F indoor temp. On nights which are warmer than that, unless the air is dry you would prefer to keep that air outdoors. The key message is that absolute humidity is the more important measure, i.e. dewpoint.

    Where I live the summer overnight lows may be in the 75-80 range which easily allows the air to contain enough moisture for a 75F dewpoint. I envy those climates which cool off at night like yours apparently does.

    Your other questions are well outside what I know, I would like to hear the answers myself!

    Hope this helps -- Pstu

  7. #7
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    Quote Originally Posted by mrvoidman View Post
    Thanks for the responses...

    DRK,
    Q2. (still not sure) Ok, bringing in too much RH is bad but can the Honeywell IAQ in conjunction with the XL16i (and a FreshEffects system?) automatically decide when to stop bringing in fresh air in? Or do I have to manually bring fresh air in? I was assuming the IAQ would engage the FreshEffects (or something like that) when it noticed the outside air was cooler.
    Q3. (decided) I'll probably stick with the Aprilaire 550 because it's cheaper to get it working than buying new..


    PSTU,
    Hmm... It can be humid at night. I was guessing the IAQ would put the breaks on the fresh air intake if it reached an RH value that was too high.

    RyanHughes,
    I have the Honeywell IAQ and I hooked the inside sensors up in series according to the instructions. From what I can tell it is averaging the two floors.
    -----

    Q1: (still not sure) I'm guessing the smarts of the XC95 over the XV95 isn't much value?
    .
    Another way to look at fresh air and when.
    You need a small amount of fresh air when you are in your home. Letting the outside weather decide when you ventilate is not ideal. Bring enough fresh air to purge your indoor pollutants and renew oxygen. ASHRAE suggest an air change in 3-5 hours. Most of the year, there are minimal conditioning cost for that small amount. During the coldest weather, most do not need additional fresh air. So, no additional cost during the cold months. During the spring and fall, the cost are minor. What is left is the warm, humid months. During the hottest weather, your a/c will cool and dry the 100 cfm of fresh air for 12 hours for 6 kw per day. During the wet, cool weather, dehumidifying 30-50 lbs./day of moisture is all that is left. In the mid west, this may total to $200 to have 12 hours per day of ideal fresh air ventilation.
    I am attaching a graph of the effect of the current midwest weather on fresh air ventilation. High outdoor %RH with cool outdoor temps does not raise the indoor humidity levels. When the outdoor dew point is low, the Indoor dew point trends to decrease. The indoor dew point for these days was an average of 50^F, outdoor dew point was 45^F. 80 cfm of total fresh air removed:
    80 cfm/13.5lbs air/lb*60min*24hrs*-9gr./lb/7,000gr/lb=
    11 lbs. Average moisture per day removed by fresh air ventilation.
    Occasionally fresh air added moisture to the home. When the outdoor dew point is below the desire inside dew points (50^FDP@70^F,50%RH) ventilation removes moisture.
    Its the outdoor dew point that determines drying or wetting.

    Keeps the home ideal for comfort and health.
    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"

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