The evaporator coil unit in an air handler dehumidifies the air because of condensation that forms on its chilled fins. The water drips down into a collecting trough that has a drain line on the side and funnels it down to the ground outside. But there is a potential problem always lurking in this system: if the drain piping gets clogged with debris or a fungal growth, water overflows the bottom of the air handler, puddling on the floor below. That puddle will continue to grow if not observed soon enough, and soak through framing to the ceiling below, if the air handler is in an attic or upper floor of a home. Ceiling collapse and mold growth are both possible if the clog is not fixed promptly.

   Leaking condensate does the worst damage when the air handler is in an attic, because the leak is usually not observed until the ceiling is saturated and water begins to drip from it. The International Residential Code (IRC) addresses this problem with a requirement that “a secondary drain or auxiliary drain pan shall be required for each cooling or evaporator coil where damage to any building components will occur as a result of overflow from the equipment drain or stoppage in the condensate drain piping.”

    So the basic options for compliance are either a secondary drain or an auxiliary drain pan sitting under the air handler, but the code further lays out four recipes for doing it:

1. An auxiliary drain pan with a separate drain to a conspicuous location.
   “An auxiliary drain pan with a separate drain shall be installed under the coils on which condensation will occur. The auxiliary pan drain shall discharge to a conspicuous point of disposal to alert occupants in the event of stoppage of the primary drain. The pan shall have a minimum depth of 1.5 inches (38 mm), shall not be less than 3 inched (76 mm) larger than the unit or the coil dimensions in width and depth and shall be constructed of corrosion-resistant material. Galvanized steel steel pans shall have a minimum thickness of not less than 0.0236-inch (0.6010 mm) (No. 24 gauge). Nonmetallic pans shall have a minimum thickness of not less than 0.0625 inch (1.6 mm).”
2. An auxiliary drain pan with a water-level detection device instead of a separate drain.
   “An auxiliary drain pan without a separate drain line shall be installed under the coils on which condensation will occur. The pan shall be equipped with a water-level detection device conforming to UL 508 that will shut off the equipment served prior to overflow of the pan. The pan shall be equipped with a fitting to allow for drainage. The auxiliary drain shall be constructed with (materials described above in) Item 1 of this section.”
   
3. A secondary drain to conspicuous location.
   “A separate overflow drain line shall be connected to the drain pan installed with the equipment. This overflow drain shall discharge to a  conspicuous point of disposal to alert occupants in the event of stoppage of the primary drain. The overflow drain line shall connect to the drain pan at a higher level than the primary drain connection.” 
   
4. A water-level detection device in primary drain line that will shut off the system if it backs up.“As an alternative to a separate drain line, a water-level detection device that will shut off the equipment served prior to overflow of the pan shall be provided. The water-level detection device shall connect to the drain pan at a higher level than the primary drain connection.”

    Although the code appears to allow for a secondary drain (choice #3) or a water-level detection device in the primary drain (choice #4) in an attic, we have never seen those options fitted for an attic installation and suspect that many local jurisdictions do not allow them for attics.

    Extra-careful HVAC contractors combine #1 and #2, so that the auxiliary drain pan has both a separate drain and a water level detection device. The photo at the head of this article was taken looking down from the top of a horizontally installed air handler in an attic. Some blown insulation next to it had gotten pushed into the auxiliary drain pan—which is the dark crud in the pan—and had floated over to clog the separate drain after the main drain clogged. Luckily, the water-level detection device backup (not visible in the photo) shut off the system to alert the homeowner before an overflow onto the ceiling below.

    There is also an exception for downflow units and other coils that do not have secondary drain or provisions to install a secondary or auxiliary drain pan: a water-level monitoring device must be installed inside the primary drain pan. The device shuts off the system in the event that the primary drain becomes restricted. It cannot be installed in the drain line.

    By the way, the odds are against a clog backing up your condensate drain line and a water-level detection device shutting off the a/c system on a pleasant autumn day. It will likely happen on one of the hottest afternoons of summer.

    Also see Should I move my air conditioner into the attic?

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

• What is the difference between an upflow, downflow, and horizontal air handler or furnace?

• 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.