Why does my air conditioner keep shutting itself off?

Thursday, October 28, 2021

If your air conditioner doesn’t seem to be cooling adequately, and you notice that the system has shut off even though it didn’t reach your temperature setting, then there are four things that can cause this problem:

Clogged condensate line - Your air conditioner can produce as much as 20 gallons of water on a hot summer day. That’s a lot and all of it has to drain away through a small pipe to the outdoors. Bacteria can breed in the water in the drain line and, combined with a little dirt and debris that gets in over time, will gradually create a gooey blockage in the line. Also, most a/c indoor units cause suction in the drain line when operating. The combination of the suction and blockage makes the water back up, and a float switch in the drain line then automatically shuts off the system. 

    But, when the system shuts off, the suction stops and the water slowly drains through the gooey globs over a couple of minutes. Then the system has a timer that then allows about 10 minutes for the refrigerant to settle before it will start back up. All of this means gaps of 15 minutes or longer of no air conditioning, repeating again and again throughout the day.    

    The cure is easy: flush the drain line with distilled vinegar, followed about 30 minutes later with hot water. One local a/c tech we talked to recently said that half of his service calls during the summer were fixed by flushing the condensate drain line. So do it again every couple of months during the cooling season, by uncapping a port in the piping in front of the indoor unit and using a small funnel, to keep the problem from recurring.

Bad thermostat - Like all devices, a thermostat can start to malfunction as it ages, and a service tech can check this for you.

Dirty air filter - As a dirty air filter slides toward totally clogged and seriously restricting air flow, two things can happen: it can cause the blower to struggle and overheat or the evaporator coils to frost-over because of lack of air flow over their surface. Both of these things will cause your system to shut down. Replacing the filter fixes it.

Refrigerant leak - As a pinhole leak in the refrigerant line slowly drains the coolant, it will cause a pressure change in the line that will eventually make a system sensor shut it down. Then the pressure will gradually return to an acceptable level and, after that timer delay, will start back up. Fixing this problem requires a service tech to find and repair the leak, then recharge the refrigerant. 

    Also see My air conditioner won't turn on. What's wrong? and Why is my air conditioner not cooling enough?

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  To learn more about heating and air conditioning systems, see these other blog posts:

Frequently Asked Questions (FAQ) about Air Conditioning Troubleshooting

How can I find out the SEER of my air conditioner? 

How can I find out the size of my air conditioner? 

How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

Where is the air filter for my central air conditioner and furnace? I can’t find it? 

Does an old air conditioner use more electricity as it ages? 

How did homes stay cool in Florida before air conditioning? 

What is wrong with an air conditioner when the air flow out of the vents is low?

Why has the thermostat screen gone blank? 

Why does it take so long to cool a house when an air conditioner has been off for a while?  

What are the most common problems with wall/window air conditioners?  

Will closing doors reduce my heating and cooling costs? 

   Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles. 

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