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  1. #1
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    Question for those who know how air conditioning works

    Well, for those who know more than "I turn it on and it blows cold air out". I'm sure digitialdeath will love this thread.

    I know that outside air temp affects a/c performance as in how hot it is outside affects how well the a/c performs, but what about when it's colder outside than inside? When we go to bed it might be, say, 75 outside but the house temp is still rising, let's say it's 77. So we turn the window unit on. But by morning it might be 60 outside. Our room doesn't seem to get very cold, even if the a/c is set at 66, and even though the unit seemed to be working pretty well when we went to bed. Is it because we have a shitty small window unit, or does the outside temp dropping make it not work very well? Does something about the outside temp being colder than the temp I'm trying to get to make it not work well? I think humidity is part of the problem, the unit can't get rid of the moisture when it's colder outside?

    These things are a mystery to me so I don't have a clue. And the Google tells me stuff about outside air being hot, not cold.

    Also, I know that a whole house fan is a solution, and we have one of those (although it's underpowered). But in the above scenario, it's still 75 outside when we go to sleep, and the house is still getting warmer, so the whole house fan doesn't help. In any case, I'm not specifically looking for a solution to making my room or house cold per se, I'm really just wondering if there's an explanation for what I have experienced (beyond "you have a shitty underpowered a/c").
    "fuck off you asshat gaper shit for brains fucktard wanker." - Jesus Christ
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  2. #2
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    Most won't run below about 60.
    ACs work by manipulating the refrigerant between gas and liquid. When it gets too cold (less hot) outside the refrigerant behaves differently. You will fuck up your compressor if you do it enough.
    You should open the windows at night.

    Edit: think of it this way- in the scenario substitute water for heat and a sponge for the refrigerant. You are trying to transport water from inside to outside using a sponge. You absorb the water on one side of the wall and squish (compress) the sponge on the other to expel the water. As the temps on either side of the wall get closer you are getting equal amounts of water on each side. So you start just shuttling a wet sponge back and forth.
    Last edited by mcsquared; 06-19-2015 at 11:50 AM.

  3. #3
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    From my basic understanding of the refrigeration cycle, the condenser stage is where hot, high pressure gas is cooled down by the outside air into high pressure liquid refrigerant, so if it is cold outside it should still be performing better than if it was hot outside. Since mechanical work is being done to cool the inside, the temperature difference between inside and outside shouldn't really matter. Could be you just have a sucky window box.

    ^^^ ok, I was wrong. Better stick to electrical engineering
    Education must be the answer, we've tried ignorance and it doesn't work!

  4. #4
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    My rudimentary understanding of the hvac world is that if your outside temp falls into the low 60's or below then your coil may begin to ice up which will cause your system to stop cooling. Once it gets really cool at night then running your window unit on fan only mode with recirculation off (so that outside air is pushed into your room) might be a better solution.

    FWIW, we run the whole house fan on a timer so that it cools things down for a few hours after bedtime but doens't run all night. We'd run it all night but most of the time we get too cool in the middle of the night if we let it keep going. Usually we open the balcony door and one window in our bedroom and then one window in the kids bedrooms as well. That ensures the cool evening air comes in through the places where it'll have the most immediate effect.
    Brandine: Now Cletus, if I catch you with pig lipstick on your collar one more time you ain't gonna be allowed to sleep in the barn no more!
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  5. #5
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    77 is too warm? Holy shit, I have central air and the a/c is set at 80 and I only turn is on if the inside temp moves toward 85. Open all your windows and turn on the whole house fan, it will pull in the cooler air.

    And yes your window unit is shitty. A good a/c unit will work at cooler temps, my neighbors run theirs 24/7 in the summer and they run even when the temps are mid sixties at night.

    The neighbors with heap pumps never open their windows and run them 24/7, 365 days a year, it's annoying a fuck.

    Get a good wall unit, my neighbors have one and their whole house is like a meat locker.

  6. #6
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    An air conditioner is basically two heat exchangers. The first heat exchanger puts cold refrigerant, in liquid form, in the room. It absorbs heat and expands into a gas (a gas can hold more heat than a liquid). Then the gaseous refrigerant is pumped outside, where it is mechanically compressed. This forces it back into liquid form, which forces it to release heat into the ambient air. The now liquid, cold refrigerant goes back into the room.

    Colder outside air makes it easier for the refrigerant to release heat. Humid air makes it a little harder. Overall, it's mechanically being compressed to release heat, so it will work regardless of the outside air temp, until the coils ice up from condensation freezing on them.

    AC units don't actually move air from inside to outside, they cool the interior air and recirculate it. Removing humidity from your interior air happens because as air cools it cannot hold as much moisture, so water naturally condenses out of the air onto the interior cold coils. It's then drained outside, as water. It doesn't transfer the humidity (water vapor) from inside air to outside air, it changes the interior water vapor into liquid and drains it outside, unless it's too cold and the condensation freezes on the coils.

    tl:dr - outside air being colder than your inside set point is not hindering your AC. You just have shitty, underpowered AC, or your unit isn't working correctly, or it's getting cold enough outside that the coils are icing up.
    Last edited by adrenalated; 06-19-2015 at 11:52 AM. Reason: typos

  7. #7
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    Yeah an AC will run more efficiently as the outside temp drops, not worse.

  8. #8
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    Something else to consider - how is your insulation? You say your house continues to heat up which makes me wonder if perhaps your insulation is failing you. Proper insulation will go a long way to keeping you cool in the summer as will good blinds.

  9. #9
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    Adrenelated nailed it; get a new unit. Cooler ambient air temps will not kill a compressor driven AC unit, although humidity will impact it's efficiency to a limited degree. If you're getting icing up it has nothing to do with the ambient air temp - the most likely problems are pressure regulation with in the cooling circuit.

  10. #10
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    If you need it cooler than 60F in the house then a walk in freezer might be something to look at.
    "timberridge is terminally vapid" -- a fortune cookie in Yueyang

  11. #11
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    check for frozen coils first. Get a new window unit maybe? How "new" is the unit? Some have digital readouts that allow "temp" settings. Could the thermostat in the unit be bad?

    #swampcoolersforthewin (if you life in the right climate)
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  12. #12
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    well, FWIW, I have noticed a similar effect in my daughter's room with a different window unit, that in the morning both rooms are not as cold as one would expect and they do seem to maybe be a little humid. So maybe it is a question of freezing condensation on the coils.
    "fuck off you asshat gaper shit for brains fucktard wanker." - Jesus Christ
    "She was tossing her bean salad with the vigor of a Drunken Pop princess so I walked out of the corner and said.... "need a hand?"" - Odin

  13. #13
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    Quote Originally Posted by Danno View Post
    well, FWIW, I have noticed a similar effect in my daughter's room with a different window unit, that in the morning both rooms are not as cold as one would expect and they do seem to maybe be a little humid. So maybe it is a question of freezing condensation on the coils.
    This would be my first guess for sure...easy to see...fix for that is to slightly turn the temp setting down...not to crank it...less cooling cycles at cooler temps will work effectively. I've had this issue many times.

    My guess is the settings on the unit are high cool/low cool hig fan/low fan and a dial for temp...i.e. warmer to cooler. put that dial off the coldest setting.
    http://www.firsttracksonline.com

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  14. #14
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    Make sure the window unit is facing the right way. The hot air should exhaust outside and cool blows inside the house.

    If that doesn't work unplug it and then plug it back in, that usually resets the capacitor.

  15. #15
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    It probably needs a recharge. Low levels of refrigerant will make it freeze up especially if you aren't running the fan on high. It will probably cost you more to have it recharged than to just go buy a new unit unless you can do it yourself in which case you wouldn't have posted here. Aren't new units like $60-$100 now? Plus they are probably way more efficient than the piece of junk in your window.

    Edit to add: spend a little more for one of the energy star units. Seer 10.7 is pretty damned good for a window unit. If your old one is a few years old the new one will probably pay for itself in short order.

    http://www.amazon.com/gp/search/ref=...nid=3368021011

  16. #16
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    Quote Originally Posted by AK47bp View Post
    Make sure the window unit is facing the right way. The hot air should exhaust outside and cool blows inside the house.

    If that doesn't work unplug it and then plug it back in, that usually resets the capacitor.
    This, but make sure you check the oil level and the air pressure too.

  17. #17
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    Quote Originally Posted by flowing alpy View Post
    that sounds like my wife.
    Tell me about it. My wife grew up in Arizona and has no concept of how to cool a house using open windows and window ac units. She's used to 100% conditioned air year round with a thermostat.

  18. #18
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    How big is the space you are cooling ? Are all the doors closed ? I have seen folks trying to cool a bedroom with their door wide open with a window unit. Key concept about a/c is that it's essentially a dehumidifier for a enclosed space. If you have warm, humid air entering the space you are trying to cool at a rate faster than the unit can dry/cool you are left hot and sticky.

  19. #19
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    Quote Originally Posted by Skidog View Post
    This would be my first guess for sure...easy to see...fix for that is to slightly turn the temp setting down...not to crank it...less cooling cycles at cooler temps will work effectively. I've had this issue many times.

    My guess is the settings on the unit are high cool/low cool hig fan/low fan and a dial for temp...i.e. warmer to cooler. put that dial off the coldest setting.
    Will try this and see if it helps.

    Quote Originally Posted by uglymoney View Post
    It probably needs a recharge. Low levels of refrigerant will make it freeze up especially if you aren't running the fan on high. It will probably cost you more to have it recharged than to just go buy a new unit unless you can do it yourself in which case you wouldn't have posted here. Aren't new units like $60-$100 now? Plus they are probably way more efficient than the piece of junk in your window.

    Edit to add: spend a little more for one of the energy star units. Seer 10.7 is pretty damned good for a window unit. If your old one is a few years old the new one will probably pay for itself in short order.

    http://www.amazon.com/gp/search/ref=...nid=3368021011
    The unit is only 3 years old (a walmart special, but still). I doubt it needs a recharge; the one in the kid's room maybe, but I'm experiencing the same thing in both rooms so it seems unlikely to be a refrigerant refill issue. It is probably a bit underpowered, it's 5000 BTUs and the room probably needs 6000.

    Quote Originally Posted by adrenalated View Post
    This, but make sure you check the oil level and the air pressure too.
    I tried to check the blinker fluid, to no avail.
    "fuck off you asshat gaper shit for brains fucktard wanker." - Jesus Christ
    "She was tossing her bean salad with the vigor of a Drunken Pop princess so I walked out of the corner and said.... "need a hand?"" - Odin

  20. #20
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    It's all due to magnates.

    I agree it is a constitutional right for Americans to be assholes...its just too bad that so many take the opportunity...
    iscariot

  21. #21
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    My advice, next AC purchase your gonna want to spend a little extra for the TrueCoat

  22. #22
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    Quote Originally Posted by Danno View Post
    Will try this and see if it helps.



    The unit is only 3 years old (a walmart special, but still). I doubt it needs a recharge; the one in the kid's room maybe, but I'm experiencing the same thing in both rooms so it seems unlikely to be a refrigerant refill issue. It is probably a bit underpowered, it's 5000 BTUs and the room probably needs 6000.
    Fair enough. In that case you are freezing the unit up by having the thermostat set at 66. It gets cold in the room and outside but the compressor keeps running continuously and eventually there is not enough warm air flowing over the coils to keep them above 32 degrees and so the coils freeze and you get no more cold air.

    Solution: set the thermostat at 74 degrees instead of 66 and keep the fan cranked on high. The compressor will cycle off when it gets to 74 in the room which will allow the coils to thaw because you have the fan cranked pushing warm air over them and then when it cycles back on you will have cold air again keeping the room comfortable.

  23. #23
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    Window AC might have trouble when it draws humidity from the uncontrolled portion of the house.

    Also, opening the windows with the ac on can move air quicker but does not improve cooling of a house especially if you introduce more humidity.

  24. #24
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    I'm thinking that in the early evening, I can have it cranked up higher to cool the room down while it's still pretty warm outside, then turn it to a warmer setting when we go to bed (knowing that the outside air will get cooler possibly leading to freezing coils). Does that make sense?
    "fuck off you asshat gaper shit for brains fucktard wanker." - Jesus Christ
    "She was tossing her bean salad with the vigor of a Drunken Pop princess so I walked out of the corner and said.... "need a hand?"" - Odin

  25. #25
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    Quote Originally Posted by Danno View Post
    I'm thinking that in the early evening, I can have it cranked up higher to cool the room down while it's still pretty warm outside, then turn it to a warmer setting when we go to bed (knowing that the outside air will get cooler possibly leading to freezing coils). Does that make sense?
    Yep - except there is no point in cranking it during the day. The compressor is either on or off so cranking the thermostat down just tells the compressor not to cycle and when it is warm the unit likely doesn't freeze because the compressor can't cool the refrigerant enough to get the coils below freezing and you are also blowing warm air from the room over the coils keeping them thawed . At night the compressor can run more efficiently so it cools the refrigerant more and the air from inside blowing over the coils is cooler so it freezes up.

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