A/C myths or truths?
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  1. #1
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    A/C myths or truths?

    It's summer time, and here in Florida it gets HOT and HUMID. So there are some differing opinions about Air conditioning useage.


    1.) It saves money to leave your thermostat at a consistant temp all day even when you're not a home rather than to turn it off when you leave only to turn it back on when you get home.

    2.) It is detremental to your vehicle's A/C system to turn it on when you are going say above 50 mph.

    3.) Turning your vehicle's A/C to MAX is more efficient than using Norm because the MAX setting recools the same air over again.

    I have my own opinions on all three, but what do y'all think?
    Resistance to tyranny is obedience to God. - Thomas Jefferson

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    Re: A/C myths or truths?

    It is cheaper to keep your A/C at a constant temp all day or just a tad lower while your not home,it keeps the system from having to work overtime later on to cool the house down whenever you get home and turn it on.

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    Re: A/C myths or truths?

    kold war kid...cold war...cold..

    A/C thread

    my face

    "There really is something broken between your ears. You know this, yes?" - Redbear

    Closed captioned for the drinking impaired. Wait...

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    Re: A/C myths or truths?

    I always use max. Recirculating the same air allows the a/c to be more efficient at keeping temps
    lower. Unless someone farts the it a quick hand move to normal.
    The shortest distance between 2 "points" is a straight line.

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    Re: A/C myths or truths?

    Quote Originally Posted by Marco
    I always use max. Recirculating the same air allows the a/c to be more efficient at keeping temps
    lower. Unless someone farts the it a quick hand move to normal.
    I have heard that MAX AC actually uses more power. I believe I read that in my 06 Mustang manual. It has a setting to recycle inside air and a MAX setting. I prefer to leave it on regular

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    Re: A/C myths or truths?

    Quote Originally Posted by koldwarkid
    It's summer time, and here in Florida it gets HOT and HUMID. So there are some differing opinions about Air conditioning useage.


    1.) It saves money to leave your thermostat at a consistant temp all day even when you're not a home rather than to turn it off when you leave only to turn it back on when you get home.

    2.) It is detrental to your vehicle's A/C system to turn it on when you are going say above 50 mph.

    3.) Turning your vehicle's A/C to MAX is more efficient than using Norm because the MAX setting recools the same air over again.

    I have my oun opinions on all three, but what do y'all think?
    imo,#1 is false for the most part but can depend. most central A/C units,are matched in efficiency to the size of the house/rooms and are very efficient.they can cool down all rooms in just a few minutes after being turned off all day. wall units tend to be much less efficient in their electrical use to begin with but if the BTUs is overwhelmed by the space involved,#1 can be true,..but this can also depend on how long your away and how hot it gets inside.insulation is a huge factor.

    to keep it short,#2 is false because there are safety mechanisms involved.

    with #3 it depends.many vehicles have a separate temperature control(thermostat)that controls the compressor and a separate vent control system,...some are interlinked.there are many cars of different years and makes out there.believe it or not,even on max setting and all vents closed,they are designed to allow at least a small amount of outside air to come in. i prefer to have the recirculate button on,set the fan speed at it's highest at first for a short time,and then set the fan speed to low while leaving the temperature at my prefered setting.

    now,a different true or false question. is it true that having the A/C on gives the same mpg,at highway speed,as having the A/C off but with the windows open? what say you?

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    Re: A/C myths or truths?

    I have been told that turning the A/C off saves gas. However, if you have the windows down, that creates more drag and there for your gas milage suffers.

    So therefore, if you are running out of gas turn your air off and keep your windows up and roast.
    Resistance to tyranny is obedience to God. - Thomas Jefferson

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    Re: A/C myths or truths?

    On #1 it would be a case by case basis if true or not. I have been in houses that were either not effectively insulated and/or the A/C unit was underrated for the house it was installed in. These house could take almost a day or more to stabilize.

    I have also been in houses were it has cooled down from outside ambient temp to set temp in less than 20 minutes.

    #2 is total bullshit.

    #3 is somewhat valid from the vehicles I have owned of MPG taking a hit when in max.
    Veritas vos liberabit


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    Re: A/C myths or truths?

    Well, while I don't do much automotiove AC or light residential for that matter ( I do big building HVAC). Here is the low down on you questions.

    1) Depends on two things, thermal effiency of the structure and thermal effiency of the AC system. If the structure isn't so ineffienct that it leaks heat as fast as you can reject it and the system is sized proporly it will cool the space in a short period of time. So no, it's not cheaper to let it run.

    2) Automotive AC is pretty simple stuff, most systems are controled only by a low pressure switch in series with a magnetic clutch. They don't really care how fast or slow you spin the compressor (with in reason). They just don't work at low speed becouse R-134a doesn't work well in ambient temps over 90 deg F, doesn't reject heat in the condensor and remains a saturated vapor.

    3) I doubt that running your AC on max does more than make it stale in your car. Although, my Impala gets a bit less milage with the AC on max.
    Chris

    "Ride until we find them, Kill them all!!" Herger in 13th Warrior

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    Re: A/C myths or truths?

    All three false.

    But on #1 you do not turn off the AC ., just let the temp rise to a reasonable ceiling of 76 degrees etc. If you are gone for 8 hours you can set the automatic program thermostat to cycle to lower temp an hour before you get home. Same with heat in the great north.

    As noted there is a recirculate settuing and a max which are separate. Recycling inside air is mor efficient.

    My .02 but I'm not a HVAC engineer.

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    Re: A/C myths or truths?

    1. Things that will truly save money on your home A/C is a digital thermostat. Set it for higher temps while you're gone, and lower when you're home. Another thing many people never think about is an attic fan. Obviously you have a layer of insulation between you and the attic, but that air up there is hot and quite often hotter than outside air. It just makes it harder for the A/C. A simple attic fan will blow out the super-hot air and make it easier on your system. Ceiling fans are winners if used correctly. They don't do squat for temps, but they can even the temp out in a room by moving the cool air around. The wind they make also makes people feel cooler. So run them in the rooms you are in and you'll feel cooler. There's no point in running them in rooms you aren't in, because they don't do anything for temps, just make people feel cooler.

    2. I've never heard it bad to run the A/C at highway speeds. There are some cars and systems that should not be started with the A/C on. RTFM.

    3. Depends on the system. On some cars the A/C may actually freeze-up and stop all cold air from coming out if you run it on max for longer than a certain amount of time. I've had a couple cars and several trucks that would do that. The recirculating feature is separate on the cars I own today, and most new ones I've looked at are similar nowdays. Window tint can make a huge difference, as can theose cardboard window screens (while parked of course ).

    As for MPG windows, etc. Aerodynamics make more and more of a difference the faster you go. The A/C is going to rob pretty much the same amount of power no matter what. On the highway, windows up and A/C off is the best. Windows up and A/C on next, then windows down and A/C off, and last windows down and A/C on. If you normally don't use cruise control, using it when you put the A/C on may off-set the MPG loss on the highway.

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    Re: A/C myths or truths?

    Quote Originally Posted by priler
    wall units tend to be much less efficient in their electrical use to begin with
    That is no longer strictly true. I'm remodeling a 1942 house and adding AC. I was astonished to find out that the SEER of modern window units exceeds anything except the special high-efficiency central air systems.

    I expect the central AC market will catch up rapidly, but if you're shopping anytime in the next year or so, you might want to make sure you look at the ratings while checking out your options.

    I chose to go with three through-the-wall units; it's a small house, and it was more convenient than running ductwork in the attic.

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    Re: A/C myths or truths?

    It's Florida. Run the AC! At low speeds you're hurting MPG with the AC--high speeds the drag with the windows down hurts MPG more than the AC--but what are you going to do? Sit in traffic in August with the windows down?

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    Re: A/C myths or truths?

    i've heard from numerous sources that windows up + AC is more efficient at highway speeds. i suspect AC itself is more efficient at high speeds too because you have better airflow through the condensor.
    i've personally never noticed any real massive change in MPG with AC on vs off. i don't like driving with it on around town because the constant compressor on/off cycle is annoying. i always notice it... i like my vehicle to run smooth <OCD>
    join the JPFO.

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    Re: A/C myths or truths?

    Number 3 was done by Mythbusters, they found that windows up, A/C on was more efficient than windows down.

    Number 2 just seems stupid. They can design cars to keep occupants alive in 70 MPH head-ons, but they didn't bother to build an A/C that can be run on the Interstate.... Besides, I don't see the mechanism by which the A/C would be harmed. If anything it should work more efficiently because of the increased cooling flow over the condenser.

 

 
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