Current IRC and Florida code (M1411.3) does not require a p-trap in the condensate drain line. It only specifies a minimum 1/8” per foot slope for the line, and that the condensate "shall not discharge into a street, alley or other areas where it would cause a nuisance.” 

    However, all building codes require following the manufacturer’s installation instructions and here’s where it gets interesting. Most manufacturers specify a trap in the line to avoid sucking or blowing air through the line between the evaporator coils and exterior termination of the drain. Some don’t require it, based on the design of their system. And several mini-split manufacturers specifically mandate no trap in the line.

    There is, of course, an exception. A trap is listed in the Florida code in their “Standard for Mitigation of Radon in Existing Buildings” at the Appendix (E406.1.1): “If a portion of the condensate pipe does not drop below the height of the condensate drain outlet, then a trap shall be installed to prevent suction of outdoor air into the air handler.” But this is a standard for radon mitigation only.

    A drain line p-trap is usually the right thing to do, and many HVAC contractors always include one, although they can clog and cause condensate backups into the air handler and plenum if the line is not regularly flushed. A newer air-type flow control device can be installed as an alternative that avoids this problem. 

• • • • • • • • • • • • • • • • • • • • • • • • • • • • 

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

• 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.