In a "typical" split-type system in an area like Texas, with the furnace/condenser coils in the attic and compressor outside, what is the typical way condensation is drained? Seems it involves two lines - a primary that routes to a bathroom pee trap and secondary to the outside. Also an overflow pan is placed under the condenser/furnace.

Considering the above: (1) If I have a slow drip from under the unit into the overflow pan, is this a sign of something wrong? (2) If the primary line is completely free, should the secondary have any flow? The way the unit is aligned/tilted, the secondary drain entrance will come first, then the primary, this seems backwards. This secondary line drains directly into the overflow pan, hence water will constantly be in the pan, leading to eventual rust.

  • You should consider adding an image of your attic equipment. Water in the drain pain beneath the furnace is a sign of a problem.
    – MonkeyZeus
    Jun 2, 2021 at 19:44

2 Answers 2


It really depends on how it is arranged. I don’t like attic units because of the high occurrence of problems thus the double system. I would expect the primary to get the water out clear is not everything with these drains if the end has a loop or is submerged that can stop the grain from working properly.

It may sound funny that a drain with 8-10’ of drop could be plugged up by the hose end being submerged or a loop that has an inch of water but I have seen this and even spiders stop a drain line that worked when I poured water in it but created an air bubble that stopped the natural flow of condensate.

Verify the end of line is clear that may be the issue and although hard to believe I have found all 3 cases on more than one system and the loop or submerged line an closer to a dozen systems causing overflowing drip trays.


If your furnace has an AFUE of 90% or higher then the furnace's inducer motor should be producing condensate when the unit is providing heat. This would have its own drain line.

The evaporator coil which is connected to the furnace would produce condensate when in cooling mode. It should have 2 drain holes: a primary hole with no obstructions and a secondary hole which is slightly higher than the primary so it would drain condensate if the primary hole get's clogged. Both of these would ideally have a line which leads to a drain.

Sometimes HVAC techs like to install an overflow switch on the backup hole which basically detects water and shuts down the furnace.

In a negative pressure configuration (evap coil behind furnace) a p-trap would be installed for the evaporator coil drain lines. If the p-trap is dry then the line cannot drain properly. A positive pressure system (evap coil in front of furnace) does not require a p-trap unless arbitrarily enforced by local code.

The overflow pan is a last ditch backup to keep your house from incurring water damage; it should not be used as a primary drain source! The fact that you have water dripping into the drain pan indicates a problem.

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