Letting the condensate water created by your air conditioning system drain under the house is convenient for a mobile home with a split-system air conditioner and an older home with elevated floors over a crawl space. But it causes two problems:

1. The puddle of water raises the moisture level of the air under the home—exactly where you do not want high humidity. It can lead to mold growth and wood rot on floor joists. The photo above shows a typical situation at a drain under a 1960s house in a corner of the crawl space, with mold and efflorescence on the block wall. 
2. As the saying goes, “out of sight is out of mind.” A termination under the house means that it is not easily checked for clogs or broken pipes. The first sign of a problem is when the drain line backs up into the bottom of the air handler and then onto the floor, or the float switch shuts off the system.

    We always recommend extending an under-the-house drain termination to the exterior of the home, and preferably at least one foot away from the exterior wall. If the location of the air handler and construction of the home makes it difficult to have a simple drain that empties using gravity and a downward slope, then a good alternative is to have an air conditioning contractor install a condensate pump. It would sit on the floor under the air handler, and has a small reservoir to collect the condensate water. When the reservoir is filled, the pump turns on and expels the water through flexible tubing to an exterior location. One option is to run the hose up to the attic, then over to an outside wall and down for drainage.

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

• Does code require a p-trap in an a/c condensate drain?

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

• My air conditioner won't turn on. What's wrong? 

• 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? 

• Why is my air conditioner not cooling enough? 

• 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.