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  1. #1

    2 story house and 2 AC units

    I have a 2-story home with 2 A/C units. The master bedroom is down stairs and the other bedrooms are upstairs. During the summer, we are all sleeping upstairs so that we do not have to cut the A/C on (Energy Savings?) I would like to be able to sleep in the bedroom down stairs but my spouse wants to conserve energy (Bless her) and wants to stay upstairs. I.E. will keeping both A/C units on during the night utilize more energy relative to having 1 A/C running upstairs?

  2. #2
    Join Date
    May 2002
    Location
    Houston Texas
    Posts
    6,316
    Quote Originally Posted by Lroes1 View Post
    I have a 2-story home with 2 A/C units. The master bedroom is down stairs and the other bedrooms are upstairs. During the summer, we are all sleeping upstairs so that we do not have to cut the A/C on (Energy Savings?) I would like to be able to sleep in the bedroom down stairs but my spouse wants to conserve energy (Bless her) and wants to stay upstairs. I.E. will keeping both A/C units on during the night utilize more energy relative to having 1 A/C running upstairs?
    You should start your own thread instead of redirecting an old thread.

    Short version it is less expensive at night to just run the upstairs unit; however there are always variable that will affect the validity of that statement.

  3. #3
    Join Date
    Jul 2007
    Location
    Mount Airy, MD
    Posts
    7,281

  4. #4

    Variables

    So, what if I set the thermostat at a higher temperature down stairs? Would the AC unit cut on less because it would be cooler upstairs?

  5. #5
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,829
    Here's the ammunition you need to shatter your wife's opinion.

    It's common in our industry for us to refer to cooling systems in "Tons" of cooling capacity, which is 12,000 Btu's. But what is almost universally omitted from that description is that the correct definition is 12,000 Btu's PER HOUR. So here's what happens.

    When the AC is allowed to operate and maintain a thermostat set-point, it is doing so by removing heat over an extended period of time. If properly sized, it's taking the heat out at mid-afternoon on a design temperature day, at about the same rate as the heat is coming into the space. So let's use a 3-ton, 36,000 Btu capacity (nominal) system as an example. On a design temp day, there are 36,000 Btu's coming into the house each hour. The unit is taking out 36,000 Btu's per hour so the unit maintains indoor set-point. Now the unit is turned off or stops working. That doesn't stop the 36,000 Btu's per hour from coming into the treated space. So let's further assume the unit is 'off' for 3-hours. The heat debt in the building is now 3 x 36,000 Btu's 108,000 Btu's and the room temp is well above set-point. Now the AC is restarted and begins to remove 36,000 Btu's per hour. It would now take 3-hours to remove the 108,000 Btu debt and return the room to set-point IF THERE WAS ZERO HEAT COMING INTO THE SPACE. Unfortunately, in such a situation, the heat from outdoors continues to come into the space at near 36,000 Btu's per hour until the sun goes down and the incoming heat begins to diminish to something under 36,000 Btu/hour, which is when that poor 36,000 Btu/hr. AC unit can begin to chip away at the 108,000 Btu debt built up.

    So no matter when the electricity is purchased to remove the 36,000 Btu/hr coming into the space, it's going to cost just as much, except in a case where the house reaches its maximum indoor temp and the AC is still left inop and the house cools by it's own leak rate. The only difference in turning the unit off for a few hours and not turning it off is that when it's turned off, the house becomes uncomfortable with both temperature and in all likelihood, higher humidity too.

    Good luck with your marital problems too! I can only guess who's the captain of that boat!
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  6. #6
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,190

    Want to try an experiment?

    How about setting the second floor unit at 78 and the first floor unit at 80
    That way the second floor unit will get the extra heat that migrates up the stairs and will have longer run times that we know will remove moisture.
    If your first floor is 80 and 50% RH then you will feel pretty comfortable compared to the roller coaster ride you are taking now.

    Just a thought. How about trying that and reporting back
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

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