Why is AC running in the morning when it's cooler outdoors?

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We have moved into a house that has central AC. We've never had or used central AC before but it's very humid here in the South so we've set the thermostat at 78 degrees to dehumidify the house.
I'm baffled as to why the AC is running in the morning even when the outdoor temperature is significantly cooler. This morning, for example, it was 58 degrees outdoors and the indoor temperature was 78 degrees with the AC running. The only source of heat that I can think of is from the refrigerator. The hot water heater is in the basement.
I understand why a house requires cooling during the day even when it cooler outdoors but I'm totally clueless as to why the house would need AC in the early morning after the house has been cooling down for over 10 hours. I'm especially baffled because yesterday it was mostly cloudy and cool.
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Every running appliance gives off heat. Your body gives off heat (800 btu/hr IIRC), light bulbs, the toaster, etc.
If your house is well insulated, it is not giving off the heat to the outdoors rapidly. The unit is controlled by the thermostat indoors. Open the windows and let in that beautiful cool air.
If the air is dry, open the windows at night and turn on a window fan to draw the air through. Ed
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Yes, this is a fresh message (July 12, 2003). It was 58 degrees (what a relief!) right before the sun rose. The ground was covered in condensation as if there was a rainstorm. By noon, it was in the mid-80's.
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If the homeowner has a combination thermostat/humidistat that will turn on the A/C to lower humidity, you make an excellent point. In other words, most current Tstats have no way to measure the %RH in the house.
Otherwise, the Tstat is just reading the heat level in the house.
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In alt.home.repair on Sat, 12 Jul 2003 18:47:57 -0400 "HeatMan"

Yes, but I think it is harder, that is, takes longer, takes the removal of more heat, to cool humid air than to cool dry air. I can't explain this well -- the boundary between water and ice might be more explainable -- but somewhere in the cooling process one has to cool the water vapor until it condenses into water itself again. Obviously it's not 212 degrees farhrenheit in the house, but there is still some temperature where the humidity condenses into water, and it takes a lot of cooling to do that.
One would think one could just cool the air and leave the humidity high, but the laws of physics and the AC won't let that happen. The maximum amount of water in the air is lower when the air is cooler. So when the air gets cooler, some water has to condense out. But in the process of cooling that water vapor from X degrees as a gas to the same temperature, X degrees as a liquid, the gas gives up a lot of heat, heat that has to be removed by the AC.
The one big question this leaves is: Is the relative humidity inside a a room with AC always 100% (or near there). I don't understand why it wouldn't be, but by personal experience I don't think it is that high. It doesn't seem humid but maybe that is just because it is cool.
Does anyone know?

I think the humidistat means that the AC will run even when it is below the set temperature, until the humidity is at the desired level. But iiuc, it does this by cooling the air some more and driving out the water. Maybe there is some arrangement of the cooling coils that reduces humidity more efficiently than it lowers temp. Does anyone know?
Meirman
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HvacTech2 wrote:

That's about as twisted an explanation as you can get. It's much simpler to forget humidity and relative humidity and to focus on the actual amount of water in air. The key is how much water the air will hold, and hot air will hold a lot more water than cold air. So as air is cooled, water condenses out at some temperature. Relative humidity only means how much water the air actually holds compared to how much it is capable of holding at a specific temperature. As air is cooled the relative humidity increases although the actual water content remains unchanged until the temperature is reached where the humidity is 100 percent and then the actual water content of the air decreases as the excess water condenses into a liquid.
The answer to meirman's question is that no, the relative humidity in an air conditioned room is not always 100 percent. If you live in a arid climate the humidity in an air conditioned room is seldom, if ever, 100 percent. In our house, the relative humidity is usually about 60 percent. The relative humidity of the air outside is often close to 30 percent. The reason we don't have 100 percent humidity is simple, we aren't taking hot air with lot of water content and cooling it. When we close our house and start the air conditioning, the temperature is usually 72-76 degrees and we set the temp for 74-75 degrees, therefore the great mass of air in the house doesn't really change in temperature or water content, the AC just maintains the temperature. Only the replacement air throughout the day starts out hot and ends up cool and that hot air often has a low humidity so very little water is condensed out of the air as it is cooled. In contrast, in the east the outside hot air may have lots of water and cooling the replacement air from 90 degrees to 72 degrees may result in lot of water condensing out of the air.
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meirman wrote:

space read pages. - Darrell
--
Air Conditioning - Latent Heat Removal -
http://www.udarrell.com/air-conditioning-latent-heat.html
  Click to see the full signature.
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LuckyDucky spaketh...

...and the indoor temperature was 78 ...we've set the thermostat at 78 degrees
...and the indoor temperature was 78 ...we've set the thermostat at 78 degrees
...and the indoor temperature was 78 ...we've set the thermostat at 78 degrees
...and the indoor temperature was 78 ...we've set the thermostat at 78 degrees
...and the indoor temperature was 78 ...we've set the thermostat at 78 degrees
get it
--
McQualude

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wrote:

When i moved into my house recently (we have a heat pump) I used a digital meat thermometer to check the indoor temp. I steadied the temp at 72.2 degrees and left the thermostat alone. Hey, I used whatever was on hand! And it worked quite well.

Mine isn't anywhere near what it says it is. The "desired temp" lever is way off, and the actual temp reading is higher than what it should be as well.

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wrote: [snip...snip...]

Insulation works both ways. Or, rather, a 20 degree delta T is the same when it's 58 outside, 78 inside as when it's 48/68 or 38/58 (to a first approximation). In addition to appliances, *you* give off heat as well (cf: The Matrix) so there's still heat energy that needs to be removed.
Another effect is stratification. At night, with the occupants quiet and minimal differential heating, the air will tend to settle into layers. As people move about, or as the sun hits one side of the house, you'll see mixing of the layers and at some point the thermostat kicks on.
--
Rich Webb Norfolk, VA

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Yes, we have unshaded windows on all sides of the house.
I'm aware of the greenhouse effect -- ultraviolet light converted to infrared light (heat) as the light passes through glass.
However, late yesterday afternoon was wet and cool. At 12am, the outdoor temperature was 60 degrees. This morning, the indoor temperature was 78 degrees. The thermostat was set at 80 so the AC didn't kick in. Last night, we slept with the bedroom windows open with the door closed. That would exclude us as being the cause of generating enough BTU's to heat the house by 20 degrees.
Responding to an earlier post; we don't have an attic; we have rooms upstairs that are also air-conditioned. We do have a finished basement which is cooler than the rest of the house.
I hope that this situation continues into winter -- we won't have to use the furnace.
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So far, I see lots of ideas being tossed, but NO ONE has stated the simple facts..
Your home..its a heat bank...its being heated all day long, and that heat is making its way to the inside. When you wake up in the morning, the heat that is indoors, other than the heat created by your electrical that might be on all night, such as lights, power supplys, etc, and the heat given off by you , and all living things in the home that is added to the load. The home was heated all day, and that heat is finding its way into the home at night..its been stored in every inch of the home...come morning, its still radiating into the home.. Not a big deal.
wrote

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Am I the only one who thinks this is not normal, other than LickyDucky? His house has no attic - just A/C rooms. The entire house is cooled to 78 and it is 58 outside? Any heat they can create should be going to the big heat sink outside. How does the house heat back up to 79 to make the A/C kick back on? Where is the big heat source? A fridge and couple of people can't create that much heat. Of course, eventually those sources and all the other heat sources will cause the A/C to kick back on sometime depending on insulation. But it should not be very frequent. Is the A/C running long, or just a little. Could it be off 90% of the night and is just coincidence it is running when you wake?
In my experience with A/C in South Louisiana and Florida, all houses I've lived in, (all with non A/C attics), did not have the air running when the thermostat was set at 78 and the outside temp was below 65 - aproximately.
As has been mentioned, check for a humidity control.
If the A/C is coming on and off, then it is probably working. To check, just take the temp of the air coming out of a register as another poster suggested. It should be at least 15-20 F cooler than the return air. Also, see if water is dripping from the coils? It is usually routed outside or to a drain.
Another problem which is not common is the placement of the thermostat. It could be near a heat source, or have a vent blowing right on it, causing the A/C to shut down too early.

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The A/C kicks in every now and then. The entire system had its bi-yearly maintenance done by a HVAC company a few weeks ago.

According to my meter, the indoor RH is about 33-35%.

The A/C is cycling on and off. Seems normal. The cooler it is outdoors, the less it runs.

I haven't measured the temperature coming out of the register yet, but it feels mighty cool. Water is dripping out of the drain. When it's hot out, then there's a gusher coming out of the drain pipe to the outdoors.

The thermostat is not near any heat sources (lights, appliances, fans etc.). At night, nothing except for the refrigerator (which is about 15 feet away from the thermostat) is left on that would generate any heat. Heck, we don't even use the oven very much so that it won't heat up the house. In addition, the TV and the computer is downstairs. We don't spend much time upstairs.
I understand why some people believe that the heat could be "leaking" in through the walls from the daytime heat but there have been those days when it has been cool and cloudy.
Mind you, I'm not concerned, worried or upset about the A/C coming on from time to time when it's 20 degrees cooler outdoors. I'm simply curious as to why/how this is happening when there's no *apparent* heat source.
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Personally, I'd go looking for the heat source... Or just turn the AC off some day, and see what the temp in the house does.
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Learn what a manual J and T are, and latent heat is included...and yes..it happens...
Hell..it was 65 this morning and my air was on at 75 and the air kicked on.....it happens...but then, you learn that after years in the biz, and a few dozen courses in thermal dynamics..storage....and such...

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I've had a few courses in Thermodynamics when I got my aerospace engineering degree, and use thermo often. However, you don't need more than common sense to know that if the entire volume of the house is 78 and the outside air temp is 58, then something is heating the inside of the house. There is no attic or other volume with high enough temp to raise the inside of the house temp. Show some calulations and explain your assumptions please.

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http://pages.cthome.net/edhome
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On Wed, 16 Jul 2003 01:59:18 -0700, "CBHvac"

This thread is really interesting, CB. I would think all folks would understand what yer tryin' to say.
Have a nice week...
Trent
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I understand about thermal mass and I agree with you on that. My question is - where is the thermal mass? Is it the bricks? What heat sources are there - water heaters inside, etc.
Obviously, if the system is working correctly and it is coming on all night, then something is adding heat to the house. This I can't dispute. And possibly the nights are not 58F long enough to deplete the latent heat inside the walls, bricks, etc.
If it were 58 for a long enough time, the heat would disipate outside faster than on the inside where it is 78, only because of the higher gradient. This depends on where the mass is in comparison to the insulation, of course. But one would think that over an 8 hour period of outside temps being 58, and inside temp stable at 78, the thermal mass of the house would be at or below 78 by morning.

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