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?
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.
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.
We had the same problem in our house, and we've simply lived with it - having had HVAC companies come out and inspect or evaluate it. They all said the system was in good working order, and was appropriately sized for our house.
This year we replaced 3 windows and a doorwall in the first floor of the house, and suddenly we can not only maintain a temperature, but go lower if we want. The air conditioning is doing fine, even though we've still got three old aluminum frame windows upstairs. It was about 80 square feet of window surface that was cheap double glazed aluminum frame, and is now cheap composite framed, double glazed with low-e coating. (anderson 100 series).
So if you've checked the interior coils, the exterior coils, and the system performance and it is still not cooling your house to your satisfaction, it is likely that you need a bigger air conditioner, or you need to better insulate your house - windows might be a big part of that loss.
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.
To get a measure of your air condition performance , try to get a temperature reading, just in front of the inside unit. If it is below 10 Celsius - 50 Fahrenheit then it's probably working good and you have to consider the sizing issues of the unit (btu , size of room vs size of compressor etc).
All this from my personal experience. Hope to offer some help.
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