Bob is a service technician who is well trained and nationally certified. However, he sometimes suffers from the same confusion that all technicians occasionally do - the facts that he gathers may or may not point to the obvious cause of the problem or the best solution. But Bob has something that no one else has. He recalls his long-time HVACR mentor and imagines him accompanying him as "Btu Buddy," someone who reminds him to take time to stop and think before rushing to judgment, helping keep him on the right track, even with facts that are confusing.

The dispatcher calls and tells Bob that a customer has a real problem with a condensate leak. The unit in the attic has leaked so much that it overflowed into the insulation in the attic, which became heavy and caused the sheetrock in the ceiling to fall through into the upstairs den.

Bob arrives within 30 minutes and the customer meets him in the yard. The customer is really mad, as Bob's company had installed the unit about two years ago. Bob calms the customer down by listening to him and then says, "There must be some explanation to this. I will get to the bottom of it and give you a full report."

Bob then goes into the house to see the damage. A large piece of the sheetrock is laying across the furniture in the den. What a mess, he thinks; loose insulation is all over the place. Bob calls his company for some help cleaning up the den and removing all of the sheetrock and insulation. Meanwhile, he goes to the attic to see if he can get to the bottom of the problem. The owner has shut the unit off, so he cannot really tell what the source of the water was.

Figure 1. The auxiliary drain pan should extend out past the edge of the air handler for complete protection. (Figures are from Refrigeration & Air Conditioning Technology, 5th Edition, by William Whitman, William Johnson, and John Tomczyk, published by Delmar Publishers.)
As Bob approaches the unit, he finds that it is installed correctly. There is a secondary drain pan under the air handler that is supposed to catch any condensate that may run over and route it to an outside drain (Figure 1). The drain is terminated at the end of the house next to the driveway (Figure 2). There is an elbow turned down that would allow the secondary drain to drip water where it would be readily noticed.

As Bob gets closer to the unit, he finds out why the secondary drain was not draining. It was full of insulation; the drain pan still had water in it. Some of the insulation had plugged the drain hole. Someone had added extra insulation to the attic after the air conditioning installation.

First things first, Bob thinks. He removes all of the insulation from inside the drain pan. It begins to drain. That takes care of that problem. Now the question is why did the primary drain pan not handle the water to the outside. He turns off the power and removes the panel to the coil where he could observe the drain pan. It has a lot of debris in it. It looks like construction dust. Probably the people who sanded the hardwood floors or the sheetrock finishers ran the cooling system while sanding. This would generate a lot of dust that would accumulate on the coil. A lot of it would run down into the drain system and would eventually stop it up.

Bob is at his truck getting his air tank when Btu Buddy appears and says, "What is next, Bob?"

Figure 2. The auxiliary drain line should be terminated in a conspicuous location where the owner would notice it if it were to start to drip.
Bob says, "I am going to blow out the drain line to unplug it. It terminates over there next to the house. I will just stick this air hose in it and give it a blast of 100-psi air. That will clear it out."

Btu Buddy says, "I would suggest that if you clear the line by blowing the dirt towards the coil and drain pan that the plug of dirt will just move to the drain pan and will again float to the drain pan drain hole and plug it up again."

Bob responds, "I knew when you showed up that I must be doing something incorrectly. You are right, but what can I do to get the dirt to the outlet of the drain system? The coil end of the drain line is too close to get the air hose in and blow out."

Btu Buddy then says, "If you can't push it, pull it."

Bob asks, "What do you mean by that?"

Btu Buddy says, "Instead of compressed air, use a shop vacuum and pull the pipe clean. Just connect the suction to the drain termination point and tape it on. Then turn on the vacuum and pull the dirt out."

Bob then says, "That is so simple. Why didn't I think of that?"

Bob connects the vacuum and turns it on, and the vacuum pulls hard for just a second. Then he could hear the dirt plug come down the pipe into the vacuum with a lot of water.

"That worked great," Bob says.

Preventing A Plugged Condensate Line

"You have solved the problem as of now," says Btu Buddy. "There are also some things that you can do to prevent a reoccurrence:

1. The coil and pan need some cleaning, as there is more debris there.

2. The secondary drain pan installation meets the codes of this area, but you can still improve the reliability of the system by installing a drain pan switch that will shut off the condensing unit if the water level rises. This may seem like overprotection, but it would prevent the kind of accident that just occurred. It is a low voltage connection with all of the wires already at the air handler. All you have to do is run the yellow wire in the air handler (the wire that energizes the condensing unit) through the drain pan float switch. This is an ‘add on' device that the customer will have to pay for, but it is not expensive."

Bob uses an approved detergent and cleans the coil and drain pan. The detergent and water flowing through the drain system cleans the trap and the piping. The drain system is now working perfectly.

Bob then goes to the owner and reports what he had found out about the insulation. The owner says, "My brother-in-law added that insulation. He was out of work last winter and I thought anyone could do that job. He just went to a local hardware store and purchased bags of loose insulation and spread it. I still don't understand about the dirty coil and pan."

Bob explains, "Our installation crew should have left a note on the thermostat to not turn the unit on while sanding floors or sheetrock. Oftentimes the sanders get hot and turn the system on anyway and act like they didn't. This is not unusual. The secondary drain pan should have prevented the problem from happening."

Bob tells the owner about the secondary drain line float shut-off switch that could be added. The owner says, "Install it. It would be worth the extra protection."

By this time, Bob's crew from the shop has the den cleaned up and the owner is on the phone to a contractor getting an estimate on the ceiling repairs.

As they are driving away from the job, Btu Buddy says, "Service technicians often do not pay enough attention to drain line problems. A stopped up drain can really cause a lot of damage. A system will normally drain about three pounds of water per hour per ton of air conditioning. A pound of water is about one pint. This was a 3-ton system, so it would drain about nine pints per hour times 24 hours of operation, which would be 216 pints or 216 divided by 8 pints per gallon which equals 27 gallons. That will make a huge puddle. It would weigh 8.33 pounds per gallon times 27 gallons which equals 225 pounds. That is what brought the ceiling down."

Bob answers, "From now on, I will take particular care to check condensate drain systems. It never occurred to me that they were that important or that they handled that much water."

Bill Johnson has been active in the HVACR industry since the 1950s. He graduated in gas fuel technology and refrigeration from the Southern Technical Institute, a branch of Georgia Tech (now known as Southern Polytechnic Institute). He taught HVAC classes at Coosa Valley Vocational & Technical Institute for four years. He moved on to become service manager for Layne Trane, Charlotte, N.C. He taught for 15 years at Central Piedmont Community College, part of this time as program director. He had his own business for five years doing installation and service work. Now retired, he is the author of Practical Heating Technology and Practical Cooling Technology, and continues as a co-author of Refrigeration & Air Conditioning Technology, 5th Edition, all published by Delmar Publishers. For more information, he can be reached at 704-553-0087, 704-643-3928 (fax), or

Publication date: 07/25/2005