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  1. #1

    New unit-trying to find balance

    My home is 8 years old, 1680 sq ft. Had to replace the 3 ton builders model this past week and discussed with my contractor a 3. Ton vs a 3.5 ton. Because we like to run it cold (70 degrees from April-September) the contractor suggested we go to a 3.5 ton, 15.25 seer. He did not think it would be overkill since we keep it lower than most people.

    The new unit has no trouble keeping 70 degrees and cycles 3 times an hour almost like clockwork. 10 minutes on, 10 minutes off. I am a little concerned about humidity. At 70 degrees, it stays anywhere between 50 and 53%. To get it into the 40's I have to knock the temp down to 69 or below. I know low 50's is acceptable but it does make me wonder what I will be in for when summer ends and we want to run the thermostat at 72 or 73

    Would you suggest I have the contractor reduce the blower speed? How much of a difference will that really make?? I think it's running full throttle right now...pretty strong. I know that would give longer cycles (and fewer) but how much of a real difference will it make in humidity? Or should I just leave good enough alone and be glad my system finally can keep up, and accept the low 50's as our RH range.

    Full disclosure...the 2 days I have been testing this, it has been cooler than normal, high 70's low 80's outside, and mostly stormy with cloud cover. When full Florida summer returns, I have no idea what to expect.

  2. #2
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    This is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise, commentary, or ask questions of the OP here.

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  3. #3
    Join Date
    Aug 2003
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    Wait for the hot weather to return before you decide on changing blower speed. 70 degrees at 50% RH yields a 50 degree dew point...well within comfort levels for many people...almost chilly for others.
    • 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.

  4. #4
    Any guesses on what will happen to my humidity level when I leave the house for 8 hours a day, and set the thermostat to 75? Will it push humidity to levels I should be concerned about?

  5. #5
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
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    34,158
    Lower 50s for humidity doesn't sound too bad. Yes, raising the temp will keep the machine off and the humidity will climb. Not much can be done about that. Mine in a steamy spell will hit the lower 60s, not idea but short of leaving it cold all day, not likely to hurt anything in my opinion.

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,067
    I think your seeing the effects of going from a 3 ton to a 3.5 ton.

    I'd ask then to lower the blower speed.
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  7. #7
    Join Date
    Aug 2010
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    West Monroe, LA
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    Quote Originally Posted by beenthere View Post
    I think your seeing the effects of going from a 3 ton to a 3.5 ton.

    I'd ask then to lower the blower speed.
    Agreed! I would also suggest having them lower the fan speed to say 350 cfm per ton. This will allow for longer run times, better humdity control and better comfort.

    You are describing what a oversized system does with 10min on and 10min off. In your case you wanted the home to maintain 70 degrees so the installing company either factored into load cal or went one size bigger then your old system to achive this.

    As for humdity what system did they install? Model numbers would be great along with t-stat installed. There maybe some other things we can suggest based off knowing what the equipment and t-stat are?

    Depending on system and t-stat you many have ablity to set the temp higher but still keep the humdity in check if your system and t-stat will allow for humdity settings to be set up.

    If this is the case you can set humdity levels on t-stat for 50-55% and set the t-stat for 75 when you leave. If the temp is meet but the humdity is not depending in the t-stat and system it can be made to come on to help control humidity.

    Of course I would not suggest this unless you have at min a variable speed air handler or furnace. If you just have a basic fixed speed air handler or furnace I would not suggest this. Might consider a lower away temp say 72-73 over 75.

  8. #8
    Thanks for the opinions. The system is a Trane Tam7 with the XR15 3.5 ton. I don't know what speed the blower is set on, but he did tell me they could dial it back if it was pushing too much air on us. Again, his primiary reason for moving us into a 3.5 was that for 7 years, our old Lennox 3 ton just could not keep up with the abusive Florida heat. We would try to maintain the low 70's on the weekends when we were at home, and at times it would get as high as 76 even though it had been running all day. It just could not handle it. 3.5 ton can definitely handle it, it's just on the verge of humidity being a slight problem, thankfully not severe.

    Doing my own amateur research, it looks like my home would call for about 39,000 btu and we are at 42,000 so not a tremendous difference off.

    PS- Thermostat is a basic Trane model...does not have separate humidity control. Asked if he would suggest it and he said not really, because when you dial down the humidity, the thermostat simply responds by lowering the temperature anyways. He said for his money, if it were his house, he'd pass on the fancier thermostats and just lower the temp himself if the humidity was getting higher.

  9. #9
    Join Date
    Jan 2004
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    The TAM7 is a VS blower air handler. A good thermostat like the IAQ would slow down the blower to help remove more humidity. And often wouldn't have to over cool, or over cool by as much to keep the humidity down.
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