A/C Thermostat operation

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On Thursday, July 16, 2015 at 4:05:13 PM UTC-4, J Burns wrote:

I'd say if it's sized that small, you're not going to be very happy. If it's running constantly on the hottest day just to maintain, what happens on days when it's say 10F below the hottest, but the AC has been off or turned way back? It's going to take a very long time to bring the temp down. You need some reasonable amount of reserve.
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On 7/16/15 4:35 PM, trader_4 wrote:

That happened when the system was new. The house had been empty a couple of weeks when my aunt brought my 90-year-old grandmother home. It was in the mid-80s and humid inside, and a lot hotter outside. For hours, my aunt thought the AC was broken.
However, you're most comfortable while the AC is running. While it's off, humidity increases. At a given thermostat setting, a little AC that keeps running will keep you more comfortable than one with twice the capacity running half the time.
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On 7/16/2015 4:05 PM, J Burns wrote:

So far, with the span set to 1, it's not running as long. I can't yet comment of whether the a/c cycles on and off too often.
(With my thermostat I have to press the temperature Up and Down buttons for about three seconds to get to the span adjust mode.)
The indoor filter was thoroughly cleaned just a few weeks ago, after what may have been 15 years without a cleaning. (After clogging to a certain point, the resulting force of the incoming air causes the filter to be released from its retainer, which in turn allows the air to bypass the filter around its edges. Clever design.)
A hour ago, I cleaned the fins in the outdoor unit. We'll see what happens over the next day or so.
One thing I'm missing is the humidity connection. The thermostat responds only to temperature, so why does humidity affect run time?
A common problem happens most nights here in central Florida, 20 miles from the Gulf. During the evening, the outside temperature drops so the a/c doesn't run as often. At the same time, outside humidity increases and the house becomes mildly uncomfortable. At 6:30 am, it not unusual for the outside temp to be 75 and the humidity to be 93%.
Thanks, everyone, for your responses.
R1
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On 7/16/15 7:32 PM, Rebel1 wrote:

The output temperature can't be below the output dew point. The evaporator has to soak up a lot more heat to bring the dew point down a degree (condensing moisture) than to bring dry air down a degree.
I monitor the output with a thermometer probe in the vent nearest the evaporator. It won't cool the air as many degrees when the air in the house is humid. It is making me more comfortable by removing humidity, but it will take longer to cool the thermostat to the shutoff point.

Some mornings, the indoor temperature is great, but it's very humid. That's uncomfortable and can cause caked salt, caked sugar, and mildew. I may turn down the thermostat to run the AC a little.
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On Thursday, July 16, 2015 at 9:23:47 PM UTC-4, J Burns wrote:

I find that just running the system for 20 mins or so will take enough humidity out so that it feels a lot more comfortable. During that time, the temp reading may not even move down one degree.
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On 7/17/2015 8:17 AM, trader_4 wrote:

Very good point. I have a Honeywell that has a humidity setting. It allows the AC to run, actually overrun in order to try to achieve better humidity. And because of what you just said, it can do it with only minor temperature change. That's very important where I live because the temp is usually cooler but, because of the rich forestation and lots of streams, the humidity is high. I have a 2 stage compressor and it usually only runs on stage one. In fact, it just turned on now at 8:30 AM; the set point is 77 it currently reads 74 inside. The humidity is 51%. Outside it is 68 degrees but humidity is 91%. That thermostat is a great product, at least for my area.
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On Friday, July 17, 2015 at 8:35:57 AM UTC-4, Art Todesco wrote:

And having a two stage is helpful in those cases, ie where you don't need or want a drop in temp, but want the humidity taken out. It can run longer, take more water out, circulate more air around, in low stage.
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On 7/17/15 8:55 AM, trader_4 wrote:

Yeah, that's what I want! (Now I feel guilty for coveting. Thou shalt not covet they neighbor's wife, nor his ass, nor his two-stage AC.)
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On 7/17/15 8:35 AM, Art Todesco wrote:

If the RH is high enough, my AC may use 75% of its energy to wring out moisture and 25% to cool the air. It would condense moisture better if I could bring the blower down to the recommended 400 cfm per ton, but it's the furnace air handler and can't be set that low. (I've read of variable-speed air handlers. That sounds dandy for humidity control.)

I didn't know it was available. You've shattered my dream of inventing it, someday when I got smarter!
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On 7/17/15 8:17 AM, trader_4 wrote:

It will feel more comfortable, but according to my readings, humidity will immediately start to climb.
Most of the humidity in my house is in solid materials. At a given temperature, that water will have a certain vapor pressure. It will try to maintain a certain relative humidity in the air. When the AC dries the air a little, evaporation from wood, fabric, paper, and other materials will start bringing the relative humidity back up.
Yesterday the temperature reached 89 and the dew point was in the high 60s. In the morning, the house was cool, but I set the AC to run a little to control the humidity. Altogether, it ran under 3 hours from midnight to midnight.
That's an average of 7.5 minutes per hour. If the unit had been twice as big, it would have averaged under 4 minutes per hour at the same temperature settings. Humidity would have felt worse.
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On 7/17/2015 2:21 PM, J Burns wrote:

I had visions of sugar crystals and chunks of salt in the air, as the humidity was solid materials.
That sure was a bit strange.
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On 7/17/15 5:05 PM, Stormin Mormon wrote:

I think nylon is one synthetic fiber that will absorb lots of moisture at high humidity. I have lots of carpet. I don't know what it's made of. If I could reduce the stuff in the house that absorbs moisture, the AC could give me a quicker drop in humidity.
Moisture-absorbing content can be an advantage. If I can get a good day to dry by venting, the absorbent fibers will help reduce indoor humidity on subsequent humid days.
Cotton will absorb an enormous amount of water at high humidity. Before AC, I wonder if southerners took advantage of that. Hang a lot of quilts in the sun to dry, then bring them in to hold the humidity down.
Venting before dawn, I used to let the indoor humidity go to 80% and above sometimes. (I didn't know it was that high because my mechanical gauge was faulty.) In terms of temperature, that wasn't a disadvantage; in pulling moisture out of the air, the AC was causing interior cooling by evaporation from humid material. Letting it get that high even temporarily caused trouble with sugar, salt, and the digital timer on the side of my refrigerator.
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