So I live in austin tx and the nights are not getting above 80 degrees and the days are in the 90s outside, but at night the ac is not able to bring it down to 73 deg from about 78 deg. For exmple last night at 8:00 the thermostat said it was 80 deg(after showers and making dinner) when I got up the next moring the thermostat said 77 deg. A little info about the house the ac program is mon-sun 8am-8pm 78, and from 8pm to 8am 73. The house is a two story, three bedroom, 2.5 bath with a attic with blown insualtion. The windows are the engery efficient along with two glass doors. The ac is only a couple years old. I have lived here for about 1.5 years and last summer it did not do this. So here is my question why is the ac taking 12 hours to cool the house three degrees?
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As you say, this doesn't make sense--the performance of any AC system is dependent (among other factors) on the temperature difference between the outside and inside, or "delta-T" as the pros say. The nightime delta-T is maybe about 10 degrees (73 inside vs. 83 outside) and the daytime delta-T is likely about 17 degrees or so (78 vs 95).
The outdoor relative humidity at night will be somewhat higher than during the day, meaning that the latent load is a little higher, but that should not affect the performance in the way you're seeing it, because, wetter air passing over the condenser coil also has a greater capacity to carry off the rejected heat.
It could be problem with the thermostat and controls. If it's working OK during the day, that suggests that the refrigerant charge and airflows are OK, so the only thing that changes between night and day operation is the controls.
Update on ac
so I checked the temp of the air going in to the ac and I got 78 and the temp coming out of the ac is 70, during this time period the thermostat was set to cool at 72. So does this mean that it is working ok(I am going to geuss not)?
With a leaving air temperature of only 70, it is not likely a clogged filter, because the LAT ought to be near 55 if the refrigerant charge and pressures are normal.
A clogged filter (low airflow) would result in an excessively low LAT, along with icing of the indoor cooling coil.
As DanH suggests, it sounds like you need to call in a reliabele AC company to check it out.
From the sound of it, your AC needs service. Could be something as simple as a clogged filter (have you checked?), could be low on freon, or the compressor could be shot.
ac not cooling
If you mean air filter yes I have checked it, if you are not talking about the air filter, where can I find this filter. what other signs of low freon or a shot compressor can you tell me about.
Ice on the evaporator coil is a sign of low freon (or a clogged air filter). Sometimes (but not always) low air flow from the AC ducts is a sign of ice. A shot compressor is generally determined with pressure measurements.
Also, of course, check that the fan in the outside condenser unit is operating at full speed, and that nothing is blocking airflow outside. (Check for leaves, etc, in the condenser coil.)
ac not cooling
thank all it ended up being low on freon
Keep in mind that if it's low on freon then there's a leak somewhere. An AC should not leak down that much in two years.