Humidity rises after AC cools?


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Old 08-04-14, 07:45 AM
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Humidity rises after AC cools?

I posted before about my window AC in a small bedroom and humidity being too high. I'm trying to understand why the humidity goes up so quickly.

I have a 5000 btu in a 120 square foot room. When I first turn on the AC it runs with the compressor on for awhile and brings the temp of the room from say 80-85 degrees down to what I have it set for say 72. When it gets down to 72 the room is perfect, maybe a little too cold. And the humidity is between 45-52. But within 20 mins the humidity jumps to 65%. The room temperature stays 72ish so the AC doesn't go on again for awhile.

I wondering how does so much humidity get back into the air, why doesn't the conditioned air stay the same. I mean why does the temp stay constant but the humidity jumps so much higher from the 45-50% it got down to

I have a really hard time sleeping in the higher humidity, I feel clammy, almost like the sheets are a little damp. If the humidity was 45% I bet I could sleep with the temp at 75 degrees or higher. Its humidity that gets me.

Any ideas or advice on how to keep my humidity at 45-50% that the AC gets it down to originally?

Thanks
Jase
 
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Old 08-04-14, 07:59 AM
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Make sure you have no air leaks into the room and the A/C is set to not vent outside air in.

You are the biggest cause of humidity, your breath is full of water. Also, if you have plants in the room that is another source.
 
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Old 08-04-14, 08:03 AM
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All houses exchange inside air with outside at an alarming rate. Unless extreme measures have been taken, rarely, 1/3 of the inside air will be replaced every hour. That's one source. Another is the moisture held within the contents of the room. The ac has reduced the moisture content of the air, but the wood, drywall, and other items are still holding moisture.

With a smaller ac unit it would run longer and remove more moisture during the longer run times and effectively help dry out the structure/contents. If you add a dehumidifier it will address the moisture only, all-be-it at an energy cost, they add heat. But using the two will slowly reduce the content moisture and help stabilize the humidity.

What is the outside temp and humidity when you notice this rapid increase in RH?

Bud
 
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Old 08-04-14, 08:26 AM
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Thanks for the quick replies. As far as air leak I have no clue, I only have one window and don't feel leaks during cold winters.
No plants
AC is 5000 btu, I wish they sold 3000 btu ACs so the compressor would run all night and keep the humidity down. The switch to let in fresh air in is closed.
If 1/3 of the air is being replaced around every hour why is the temp staying down?
This issues has been happening all summer and I live in NYC. This affects me only at night so I'm guessing temps are around high 70's at that time. Humidity changes from day to day. Today outside its 65% humidity.

So I dont have an option of a smaller AC. I'm not positive about leaks but I don't think I have any major leaks. There is an inch gap under the door (used to be carpet).

I follow the logic of the AC lowering the temp and the humidity when running, but why does the humidity jump up and not the temp. Is it that at night outside the temp isn't that much higher but the humidity is alot more. So the humidity goes up but the temp doesn't?
 
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Old 08-04-14, 08:35 AM
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I'm more concerned at night but right now the room is showing 74 degrees and 70% humidity. I have 2 hygrometer from different manufactures and they are showing the same numbers. Outside temp is 80 degrees and humidity is 60%.
 
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Old 08-04-14, 09:40 AM
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Try leaving the A/C on all day or from say 5PM on. This will help dry out the room and it's contents much more and may help keep the RH levels down when the unit is cycling much less during the cooler evening hours.
 
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Old 08-04-14, 09:40 AM
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@jasesun: "Is it that at night outside the temp isn't that much higher but the humidity is alot more. So the humidity goes up but the temp doesn't?"
I suspect that is playing a part.
The 80 @ 60% RH is slightly higher in moisture content than 74 @ 70% RH. Play with this calculator.
Temperature, Dewpoint, and Relative Humidity Calculator

As for the leaks, they are everywhere, electrical, plumbing, even where the drywall is attached to the studs, air leaks past.

But I suspect it is a case of moisture in the walls and contents of the room, especially if you do not keep it cooled all of the time. Since you can't reduce the size of the ac unit, I'd go with a dehumidifier. In combination with the ac it shouldn't run a lot.

Bud
 
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Old 08-04-14, 10:13 AM
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Bud and Tom, thank you for replying. Bud as a test I put a humidifier in the room. Its a big model 70 pints a day but in less than 20 mins the room humidity was below 50%. It did raise the temp but even at 77 degrees I was so much more comfortable at 48% humidity than at 71 degrees and 60% humidity. Looks like I'm affected by humidity more than temperature. Might have to find a 30 pint, low amp, quiet humidifier. This humidifier and an AC running in that room has tripped the breaker in the middle of the night.

Tom I have accidently left the AC on some days and the problem seems to still persist at night. As soon as the compressor stops removing moisture level climbs until it is probably equal with the air outside, usually over 60%.

Anyone have recommendations on quiet small dehumidifiers?

Thanks for the help guys.
 
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Old 08-04-14, 11:00 AM
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A dehumidifier will pretty much act the same as your A/C with the exception that you will have to drain the water from the tank every so often. Your A/C should be dripping the condensation outside, you should confirm this.
 
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Old 08-04-14, 12:15 PM
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Tolyn- Water is dripping out the back so AC is working fine. Its just that the humidity climb up after the AC reaches the set temp. The AC would have to run much longer for it to keep the humidity down.
Looks like I'll have to bite the bullet and run both to be comfortable. Dehumidifier will lower the humidity and raise temp, the ac will run longer due to the increased temp and will also dehumidify the air even more. Hopefully I can find the magic middle ground to run each one the least amount of time and I can set the AC temp higher with the lower humidity. The sound of both running might pose a problem but I'll try it for a couple nights.
 
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Old 08-04-14, 12:19 PM
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Ear plugs are a wonderful thing.
 
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Old 08-04-14, 06:37 PM
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U could also leave the door open to the hallway then use a floor type fan to move air into the hall & other rooms resulting in longer runtimes plus drying out the air in the hall & other rooms; perhaps that is part of the reason the humidity climbs so fast.

Do U have the A/C set so he blower goes off when the compressor goes off?
 
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Old 08-04-14, 07:09 PM
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I am confident that HVAC Retired has caught the issue. The fan is not cycling with the compressor.
If I am not mistaken the OP posted earlier that the model he has does not offer this function..
 
 

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