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  1. #14
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,338
    Get a temp/%RH meter to measure the inside conditions. <50%RH is important for comfort and health. Set the fan in the "auto" mode to max the dehumidification of the a/c. What is the run cycle time like at 10:00 PM verses 5:00 PM? Nice to have a little extra capacity to be able to cool the home down in a couple when you arrive home after the a/c has been off.
    Of course a/cs are grossly oversized during moderate weather or overnite. Consider when extra occupants and during the hottest weather of the year, nice to beable to to maintain your set point.
    Short cycling during these peaks cooling loads is considered oversized. The downside is that the a/c does not remove as much moisture when short cycling. If you are unable to maintain <50%RH during the hotter parts of the day, have your a/c tuned to get the coil temperature <25^F below the room temp to max the moisture removed by the a/c. Next is reducing duct air leakage by duct tightening. After all of that, you have no choice but to make the a/c run cycle as longer. As previously stated, the cooling 3-4 degrees will lengthen the cooling cycle.
    Reducing the size of the a/c is costly and limits your ability to handle peak cooling loads.
    Any a/c that handles the peaks will short cycle during lesser cooling loads. Humidity control will be impossible during low/no cooling loads and high outdoor dew points, which green grass climates have.
    If you want perfect temp/%RH control regardless the cooling loads, consider adding a small whole house dehumidifier to slightly oversized a/c as the ultimate control of temp/%RH.
    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"

  2. #15
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    What are you overnight temepratures running when there is dew on the grass in the morning? This time of year the ground, oceans and lakes are still relatively cool, so they tend to run lower than in July-September. If it's been into the 70's for dewpints, then your aren't far from design conditions.

    IF you go with a 2.5 ton without changing the indoor coil, you will gain a lot of efficency, but might struggle with humidity control. Just going ot a 3 ton probably won;t make a big enough difference.

  3. #16
    Join Date
    Jun 2012
    Location
    Folkston, GA
    Posts
    6
    The run cycle time at 10:00 PM is about 10 mins and the off time is about 30 mins or more as compared to 5:00 PM when its on for about 12 mins and off for about 20 mins.

    When we were on vacation before in the middle of summer with the old unit I made the mistake of turning the AC off. When we got home at 10-11 that night it was 95 degrees in the house....I thought we would never get it cooled....anyway I don't think the house is very efficient at all.

    I'll get a meter and check the temp/%RH.

  4. #17
    Join Date
    Jun 2012
    Location
    Folkston, GA
    Posts
    6
    Overnight, it maintains the house at 78 and runs for about 10 mins and seems to be off longer like 30 mins or more but i've not actually timed it in the middle of the night. There has been dew on the ground the past couple of mornings though.

  5. #18
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Careful leaving a AC system completely off for more than a couple days in a southern hot humid climate... especially in a brick house. You can create major moisture and mold problems. I would never set it above 80-85 so the dewpoint remain below 60% indoors.

    Last summer a bunch of schools tried to save money leaving their systems off and keepign the building closed up. They spent 2-3 weeks cleaning the mold that was everywhere and probably spend 3-4 times what they save on electricity.

    With a properly sized system, even at 10PM, if it's in the 80's and the attic is still hot, it will take the first hour just to drop the humidity and maybe cool it even 2F. Then anther 4-6 hours to get it down to around 80F. It probably wouldn't fully recover until the morning when it has more capacity. It might even take longer. A brick structure holds a lot of heat. You might have 30k lbs of brick you have to cool by at least 10F to be the same as the outdoor temp. That takes a lot of energy. Not to mention that all the wood and flooring in the home holds moisture that will have to be removed over a 2-3 day period.

  6. #19
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Quote Originally Posted by bigfoot04 View Post
    Overnight, it maintains the house at 78 and runs for about 10 mins and seems to be off longer like 30 mins or more but i've not actually timed it in the middle of the night. There has been dew on the ground the past couple of mornings though.

    What was the outdoor tmeprature when there was dew on the ground? 70F? 75F?

  7. #20
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,963
    Bigfoot04: The run cycle time at 10:00 PM is about 10 mins and the off time is about 30 mins or more as compared to 5:00 PM when its on for about 12 mins and off for about 20 mins.
    If your room thermostat had a cooling anticipator & a half degree or degree off temperature spread, then 20 to 30 minutes off time 'would' indicate (IMO)that your home is decent efficiency-wise.

    If you use a RM TH with a 2 or three degree temp-spread setting, then the runtime will increase to satisfy the 3-degree spread, it will also stay off longer before it gets back to the 3-F on spread.

    Let's say its 'temp-swing' is set for ON at 78-F OFF at 75-F, or, 77-F ON & 74-F OFF, etc.

    If you keep air infiltration low & run the blower at 350-cfm per ton of cooling & use the above RM TH settings, that will help 'some' to lower humidity even with an oversized A/C...
    Last edited by udarrell; 06-05-2012 at 10:03 AM. Reason: and use the above RM TH settings...

  8. #21
    Join Date
    Jun 2012
    Location
    Folkston, GA
    Posts
    6
    Yeah, returning home after that week vacation was a lesson learned. I've not shut it off like that since.

    The temperature was in the low 70's when the dew was on the ground.

    I'll see if I can adjust the temperature spread when I get home and play with the 3-F spread.

  9. #22
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,338
    http://www.weather.com/maps/maptype/...nts_large.html

    This shows that the current dew point is 70^F.
    Regarding using the a/c to control humidity to avoid mold, this only works during significant cooling loads. Also a/c humidity control is energy intensive and marginal during wet cool weather. I have monitored several homes that use a dehumidifier to maintian <50%RH when unoccupied, <50%RH with continuous +70^F outdoor dew points for <$20 per month.
    I do agree that using t-stat setup when unoccupied is not good with a marginally sized a/c.
    On the positive side, t-stat setup is great tool to improve the moisture removal of conventional a/c.
    A ceiling fan with 78^F, 45%RH, 55^F dew point is comfortable for most. During the evening, most a/cs are not running enough to remove much moisture. A small whole house dehu fits well with an a/c sized to handle peak loads. Set the temp on the t-stat and the <50%RH on the dehu get what you want. When out of the home for any extended time, turn off the a/c. the home will be <50%RH when you return. The home will be less than the outside temp. Seldom will a well sized a/c operate more than an hour to recover. That hour of continuous a/c will remove enough water to keep the a/c off until the a/c starts to short cycle. Your total cost to condition will be less and the space will be more comfortable.
    With an undersized a/c, you must leave the a/c on all the time. Yet during wet cool weather, the a/c will short cycle and the home will require supplemental dehumidification to maintain <50%RH.
    Ideal to have slightly oversized a/c and small whole dehu.
    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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