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Yesterday's high was 106.0F, low of 79.2F, full sun all day. AC set at 78F, indoor temperature reached about 85F. Today's high was 105.4F, low of 81.1F, full sun all day, now about 102 at 7pm. AC set at 78F all day, temperature climbed steadily through the day, and is now 89F.

Is it reasonable to expect an AC unit to maintain 78F in this weather?

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Is your air conditioning running all day and not able to get the house temperature down to 78F? Or is it on and off (like normal expected operation) all day long? –  Jeff Widmer Aug 3 '10 at 0:45
@Jeff If it shuts off, it doesn't stay off very long. For this question, my focus is just on "are my expectations out of line?"; I'll deal with troubleshooting separately. –  retracile Aug 3 '10 at 1:58
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3 Answers

up vote 8 down vote accepted

A properly functioning and properly sized central air conditioning unit should be able to maintain the temperature of your house at any reasonable temperature you select. The amount of cooling available is related to the size of the system, insulation in the house, and finally outside air temperature.

From your description I would guess either you don't have a big enough compressor, or you don't have enough airflow through the house. It's also possible the cooling element in the system has become dirty and no longer transfers enough heat from the air into the element.

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What is a "reasonable" temperature? In Phoenix it could be 117 outside and if your AC gets it down to 80 you're lucky. I thought the rule of thumb was that if the inside temp is more than 30 degrees different from the outside the unit was working well. –  tooshel Aug 3 '10 at 16:18
I meant "reasonable" in the sense that you can't expect miracles from you system (don't try to build an ice rink in your living room and complain that your central air isn't up to the task). The system described was getting something on the order of 15 degrees Fahrenheit separation, and it's reasonable to expect more. –  acrosman Aug 3 '10 at 17:47
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The first thing to check is the return air filters. If they're dirty, your system won't be able to keep up. Also, if the coils are dirty or the refrigerant is low, you will not get efficient cooling.

You can clean your condenser coils (the outside unit) yourself using a jet of water from a hose. Contact a service company to clean your evaporator coils, do a system check, replace refrigerant and do repairs.

The amount of in and out traffic you have, shade trees, insulation, type of windows, etc., all play a part in how well your system works.

It's been about 100 degrees F here the last couple of days and the thermostat is set at 78 and has had no problem maintaining that.

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As others have said, if the unit is properly sized for the house, and the insulation is good enough to keep the heat out, and there is enough airflow in the house, the A/C should be able to handle almost any temperature you set it to.

There is a point where it gets so hot outside that air conditioners simply can't do anything to get rid of the heat (the outdoor coil temp equals the outdoor air temp). This is extreme though, >110 deg F. You're getting pretty hot, but shouldn't be at that point yet. (Most A/C units will have specs on this. You can try looking up some specs for it online.)

You indicated the system is turning on and off, so I don't think it's your thermostat.

How cold is the air coming out of your vents? If the system is working properly, it should be as cold as your fridge. If it isn't cold (not cool, cold), you have a problem.

Look at the lines on the outdoor unit. Are they frosting up at all? Is there excessive dripping from your indoor air handler? If your lines are freezing up, turn the system off and call a service company. That means you are probably low on coolant, and running the system without will burn up the compressor.

Other problems that would prevent cold air would be crud on the coil, or the fan on the outdoor unit not working properly.

If the airflow is weak, then you have an airflow problem. Check the air filters in the system. Make sure your ducts aren't plugged somehow. Check for closed dampers or closed vents. Make sure a duct didn't pop loose somewhere (blowing air into the attic instead of out the vents). Make sure your air handler is blowing air.

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