The largest of my three central air units in the attic has two drip condensation pipes coming out of the evaporator coil box (not one that had an overflow opening that drips into the overflow pan). It has two drip lines coming out of the unit. One goes to the standard pvc drain that goes to a bathroom drain. The second drip pipe goes directly to the overflow drip pan and drains out to the eave of my house.

This year (after 10 years of use), the 2nd drip pipe (the one that goes directly to the overflow pan) is getting used and there is a constant drip out the front of the house from that emergency overflow pan.

Why is that 2nd drip line from the A/C getting used this year? Thanks.

  • 2
    I would say that the other line is clogged, partially or completely. Do you have access to the end of the primary line? I think there is supposed to be an air gap where it delivers condensate into the sanitary sewer. You could open the access ports to the evaporator coils and add chlorine bleach. Or you could cut into the horizontal run of the drain line just as it comes out of the evaporator chamber and add a Tee (right angle arm oriented upward). This would allow adding bleach without having to open the access port. You'd put a stopper into the opening when not using it. Aug 19, 2018 at 13:29
  • 2
    If you have access to the end of the primary condensate drain, you could suck on it with a shop vac to clear a blockage. This should stop the dripping out of the secondary drain under the eave, but then you might want to introduce bleach into the line to kill mold and algae. Aug 19, 2018 at 13:31
  • 2
    Adding to @JimStewart, The reason the backup drain drips from the eve (instead of being ported to the drain) is so that you are alerted that the primary line is plugged up and that maintenance is required.
    – Tyson
    Aug 19, 2018 at 13:45
  • My evaporator has fittings for two drain lines, but only one has piping on it. The one with no connection is beside and slightly higher than the one which is being used. It could be that the 2nd one would be used for a backup, My simple installation has the evaporator, furnace and air handler in a closet on the living floor in the middle of the house. If I had a 2nd condensate line, it would drain into the same pipe that the current line does. I suppose I could rig up a water sensor and an alarm to the 2nd one. Our system has been in place since 1991 and no blockage. Aug 19, 2018 at 19:15
  • Another possibility is that the 2nd drain fitting is for a horizontal placement of the the furnace, evaporator coil, and air handler. My installation is an upflow through a plenum under the filter and air handler. This same system may also be mountable horizontal which seems to be how it is done in attic installations. Aug 19, 2018 at 19:43

2 Answers 2


My FIL is an HVAC guy and he explained it to me when we installed a unit in a house I built.

The first drip line is the main one. Mine had a float switch that would turn off the unit in case it was clogged. The 2nd one is a backup. In the case that the float switch fails, a clog in the drain line won't flood your attic (where the unit was installed). On mine we ran it so that it would drip in front of a window, increasing the likelihood I would notice it. Sounds like you have a clog in the drain line and yours either doesn't have a switch, or it does and it failed, sending the condensation through the 2nd line.


I have the same problem about once a summer. I have access to the end of the drain pipe so it's easy to clear the blockage with the air compressor blower attachment.
Upvote to Mr. Stewart for the bleach idea. Maybe I can skip a year.

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