Home cooling: leave the A/C on or turn it on/off?

Discussion in 'Off Topic [BG]' started by elgecko, Mar 10, 2014.

  1. elgecko


    Apr 30, 2007
    Anasleim, CA
    It's been a warm winter and it's gearing up to be a hot summer. I've got a question about home cooling and was hoping OT might have the answer...my search fu has turned up nothing.

    Is it more energy efficient to:

    a) leave the thermostat set to say...83F while no one is home then adjust as necessary when I get home?


    b) turn the A/C completely off while no one is home then crank it up when I get home?

    I've always gone with "b" and there were times when I'd get home and it'd be 94F INSIDE! "b" would result in the A/C running continuously for hours to get to the desired temperature. A friend told me he leaves his on because it takes more energy to get those kinds of inside temps down to a comfortable level than leaving it set to a moderate level. It makes sense that "a" would "save" the energy needed to cool down the house but how does that compare to energy needed to maintain a moderate temp?

    Depending on how you define moderate, I imagine there's a point where the energy needed to maintain a temp is equal to that necessary to cool a house down to that temp. I wonder what that point is.


    Anyone know? Anyone have any links to actual data?
  2. Reprise

    Reprise Supporting Member

    Aug 3, 2012
    I can't link to data but I've seen more than once that a) is the way to go, for same reasons your friend gave. In addition, when you get the really hot spells, set the fan to run continuously (or install a whole house attic fan). This last one I got from Lou Manfredini's show.
    bnolsen likes this.
  3. yodedude2

    yodedude2 Supporting Member

    option 'a' is better unless you are leaving it off for more than 48 hours or so. no hard data, sorry; only decades of personal experience.
  4. is there a way you can put a timer on it so it will turn on an hour before you get home?
  5. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Yes, there are programmable thermostats, and even a unit called NEST that lets you control your thermostat remotely via your iPhone.
  6. I use to work in heating and cooling. The furnace/AC uses more energy if you leave it turned off all day then turn it on when you come home to get to the desired temp. It could take hours running non-stop to get the house down to the desired temp. Your better to set the thermostat a little higher and let the AC cool the house during the course of the day then turn it down to the desired temp when you come home. This way the AC cycles off and on and uses less energy. Trust me.
  7. MothBox


    Oct 25, 2010
    I've been looking into this lately as usually I leave windows open on both sides of my apartment during the day to get a breeze through. I live in a humid area during the summer and alongside the heat its not pleasant to walk into a flat which has had no air moving through it. I tend to create a breeze and leave a fan on all day, turning the AC on when I get back from work.

    Looking into it I found this thread which is a bit more of an incite:

    I've got friends who leave the AC on 24/7 on a thermostat, however I've noticed increases in my bill when I've left AC on constantly - this may be related to the inefficiency of my AC units or just that I don't use that much AC when I am home. I never cool the apartment to an office temperature, its always t-shirt and shorts sort of temperature.

    I'd run both scenarios for a week and check your meter for consumption.
  8. This is what I do for a living. 25 years HVAC tech, and I will tell you what I think and what I recommend to my customers.

    Leave the A/C on a moderate temperature that you are comfortable with. To understand why, first you must understand how an A/C system works. It does not manufacture cold air. Actually, there is no such thing as "making cold". Cold is the natural state of things, and heat is an added thing....that can be removed. And essentially, that's what your A/C system does. The refrigerants in the evaporator coil actually absorb heat from the air traveling across it, and that heat is relocated outside the building when the refrigerants return to the condenser. That's part of the reason why when you hold your hand above the outside unit, the fan seems to be moving warm air out of it.

    Every cooling system is rated in "tons", which essentially is the measurement of how much heat it can absorb to melt a certain amount of ice in a 24 hour period. A 2 ton system can transfer enough heat to melt 2 tons of ice in a 24 hour period. So, in reality, it's actually proper to say that if you turn your A/C off a lot, there is a much higher load on it to cool a building down versus maintaining a set temperature, where smaller amounts of heat are removed more frequently.

    Then there is the comfort factor. Generally speaking, your A/C system will dehumidify the air as it cools the area. Not technically a dehumidifier, but it does remove significant moisture. In most cases, the real comfort zone cannot be achieved until significant amounts of humidity are removed. Ever have that clammy feeling in an air conditioned space? That's due to the system removing heat faster than it can remove humidity. Not really a good thing. So, again, shutting your system down may allow the humidity to build up in your home, and the A/C will fight to pull this out of the air and cool things off. And if the A/C is off for a long period of time, even a properly sized A/C can take several hours to bring a home down to a comfortable level. We see homes that have mildew and mold problems due to oversized A/C systems, or customers who don't control their humidity well during the cooling season.

    IMO, once you start it up and feel you need it, find a good level you are comfortable with and leave it there. If you live in an area where you need to, or plan to, run it even when outside temps are somewhat low, talk to your local A/C guy or gal about the benefit of low ambient controls.

    No data, just my experience.
  9. elgecko


    Apr 30, 2007
    Anasleim, CA
    I've perused all the links and all your comments and guess what? There's no consensus! I'm sure it's because it depends on where you set the thermostat if you elect to leave it on.

    I think that's what I'm going do.

    1) Leave the thermostat set at 83F

    2) Leave the A/C then crank it where I get home

    3) Set it so the A/C fires up an hour before I get home

    Hopefully, the three weeks are similarly hot to make the findings somewhat meaningful.
  10. Bingo. Running your A/C unit 24/7 will not save energy compared to only using it when you get home (and maybe an hour beforehand). It's simply less work for the A/C unit.

    But using less electricity may not always save money in the bill. If you're leaving that A/C unit off much of the day, then cranking it just in time for peak electrical rates to kick in, that can be expensive. Might be cheaper just to let it use the cheaper electricity rates as much as you can.
  11. pedroims


    Dec 19, 2007
    I live in Michigan, thermostat is 70 degrees all year round
  12. MJ5150

    MJ5150 Terrific Twister

    Apr 12, 2001
    Lacey, WA
    70? Wow, you like to keep your house warm.

  13. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    ... or cold.
  14. 6jase5

    6jase5 Mammogram is down but I'm working manually Supporting Member

    Dec 17, 2007
    San Diego/LA
    I keep heat at 73, A/C at 76. We like a pretty consistent temp. Basically the heat kicks on at 3-4am and runs on and off until 8am. By 1pm the A/C kicks on and runs until about 8 or 9pm. Last few days have hit 80+ so the A/C has been feeling great.

    I also use a whole house fan to extend the cool or heat time in the house. Working from home I like to be in my uniform..... tshirt, shorts and flip flops all year.

    Nest is overrated. A bunch of friends got them for fun. No ROI and not good if you work from home.
  15. jmattbassplaya

    jmattbassplaya Supporting Member

    Jan 13, 2008
    Tampa, FL.
    I envy your working life :D
  16. MJ5150

    MJ5150 Terrific Twister

    Apr 12, 2001
    Lacey, WA
    Good to know, thanks for the tip. I was a little freaked out by the Google acquisition, but still thought about buying one.

    We thought about buying an AC unit earlier this year, but opted for a heat pump instead. It was a better decision for our climate.

  17. jmverdugo


    Oct 11, 2012
    Katy TX
    I would say that it depends and there isn't a real answer here but here is my take on it. I would not completely turn off the ac, I would just adjust the temperature to a higher one when I'm not at home and turn it back down when I'm at home.

    Unless you set it up to an unreasonably indoor temp in comparison with the outside temp, the room temp should not change that much if no people is inside the room since stuff like furniture, floor and walls are usually helping to maintain the temp, they don't generate heat. Curtains would also help a great deal in helping maintain the temp, normally is not the hot air what raises the temp is the sun coming through the glass, of course there is no substitute for good isolation, I think this is way more important than anything else.

    Most heat in regular homes is generated by people, the more people you have the more heat the ac needs to take out.

    Other thing already mentioned is humidity, you need to control it or it would eat you clothes and furniture, if there is no AC working the humidity keeps building up.

    Finally what is important here is not really the temp as a number but the feeling and comfort, numbers are just a reference, fans help a lot maintaining temp uniformity in a room and increasing the comfort feeling so I think it's a good idea to use them at a low speed just to keep the air moving, this doesn't mean the AC will cool down faster and work less, it means that you can set it up to a higher temp because the fan will make that temp more comfortable, the temp you feel is not the same the thermostat is feeling and fan moving air will even out this difference moving the warm air around you. Jm2c
  18. pedroims


    Dec 19, 2007

    Warm in winter and cold on summer, that what the AC unit is for right. Otherwise there is not need for it :D
  19. viper4000


    Aug 17, 2010
    Another thing to consider is how well your house or apartment is insulated. You can have the most efficient A/C or heat pump on the market, but if the insulation or windows are not up to par, then that efficiency literally goes out the window.