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How to Flush an Air Conditioning System's Drain Line

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Written by Bert Holopaw   

Flushing an air conditioning system's drain line is one of the few projects a homeowner can legally complete. When the air conditioner's evaporator coil removes humidity from the air, condensate forms on the coil's fins. The condensate then drops into a drain pan, where it collects until its level has reached the drain pipes. Condensate does not create enough current to flush the lines. Because of this, tiny bits of sludge forms in the low spots and in the eddies behind any fittings. Over time these pieces of sludge break free and gather at a restriction. It is this gathering that clogs the air conditioning system's drain line. Flush the air conditioning system's drain line every spring.

1) Turn the air conditioning system off at the thermostat.

2) Look for the end of the condensate drain line outside. Often air conditioning installers run the condensate line with the copper refrigeration lines. If so, then the drain line should be next to the outside unit where the copper lines exit the house. The drain line usually uses either a white or a black plastic pipe. Often a 90-degree elbow will be connected to the end of the pipe. Remove this elbow. Many times an air conditioning drain line's backup will be at this elbow. 

3) Suck the sludge from the drain line with a wet/dry vacuum. Hold the vacuum's nossle on the end of the drain pipe and turn the vacuum on. The vacuum's motor will change its tone once the drain line is clear.

4) Replace the elbow on the end of the drain line. Do not glue this fitting.


5) Look for an emergency float switch next to the inside AC unit. Not all units have this feature. This float switch shuts the system off if the drain line backs-up. The float switch connects to the unit's drain line next to the evaporator coil. It has a low-voltage wire set that runs from the top of the switch to the AC unit's electrical box. Remove the float switch from its housing and clean out the bowl. Replace the float switch.

6) Pour one gallon of warm water into the drain system. Pour the water into the drain pipe either through the clean-out tube or by dumping the water into the drain pan. The clean-out tube, if equipped, connects top a Tee-fitting where the drain enters the platform. If you must flush the drain using the drain pan, remove the screws holding the evaporator coil's cover then remove the cover and slowly pour the water into the pan.

7) Have a helper watch the drain line outside. Pour one more gallon of warm water into the drain line. The helper should watch the water exiting the drain line. Continue to add water until the water leaving the drain line is clear and does not contain any sludge particles.

8) Replace the coil's cover or the clean-out tube's cover.


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