Thread: HVAC condensate drain not draining

I have an HVAC unit where the condensate drain does not drain while the unit is running. It has a primary (trapped) drain leading outside, an auxiliary drain which drains into the drip pan. Out of the drip pan there is another drain going outside again. All the drains are clear of debris but still the AHU doesn't drain. Once the thermostat setpoint is reached and the indoor fan time's off. Both the primary and auxiliary condensate lines flow with water, some going outside through the primary drain line, some going into the drip pan via the auxiliary drain line. The key appears to be the indoor fan shutting off. I have some idea's why this may be happening but rather than experiment I thought I'd get the experts word first. The house is only 3 years old and I assume it has been this way since installation. There is another part to the issue since the ceiling below has water stains which is what alerted me to the problem. This I believe is a seperate issue with a leak in the drip pan somewhere.I haven't been able to find how the water gets to the ceiling with no signs of water anywhere around the drip pan

Thanks for your input

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#1. Is it a heat pump?

#2. If it is a heat pump, does the trap clean-out have a cap on it?

It sounds like you have a primary drain without the appropriate trap and vent.
Get the brand and model number from the evaporator coil / FAU in order to find the manufacturer's installation instructions.

If you have a unit under negative pressure, there must be a trap deep enough to prevent air movement into the unit through the drain line, the internal pan must be sloped properly, there must be a trap on both the primary and secondary drains at the coil box. The drain line must be vented on the downhill side of the trap(s).

By now the inside of the coil box is likely molded and rusted.

As you likely know, there should never be water in the overflow pan, it is only there for emergencies.

The Unit is a heat pump, the primary drain in trapped and vented but the auxiliary drain is not but that is only 2 feet long down to the drain pan. The internal drain pan is rusted but the rest of the coil boxes appears fine, no mold etc. Is it possible that the fan speed is too high creating either positive or negative pressure ? Preventing the water from draining. The filter is clean and all vents are open.

Again, find what the manufacturer says about the installation. just because it has a trap does not mean it is deep enough or designed properly.
Got a picture? Model #?
Don't worry about fan speed, a proper trap and drain will overcome the pressures... Must find the manufacturer specifications!

Originally Posted by Jim Holgate

The Unit is a heat pump, the primary drain in trapped and vented but the auxiliary drain is not but that is only 2 feet long down to the drain pan. The internal drain pan is rusted but the rest of the coil boxes appears fine, no mold etc. Is it possible that the fan speed is too high creating either positive or negative pressure ? Preventing the water from draining. The filter is clean and all vents are open.

Be sure you are looking at a vent and not a clean-out. I have had this argument with numerous HVAC techs that don't know the difference. The clean-out is between the unit and the trap. The vent is after the trap.

The trap may be either insufficient (water seal not deep enough), or to much (water seal too deep), or configured incorrectly, or not properly vented (see Vern's post), or the condensate line may be reverse sloped, or there could be a blockage in the condensate line, or ... (many more possibilities are out there when you start thinking of them)

The ceiling may be water stained because the condensate line may not be insulated properly and/or may be reverse sloped, those would be the two most obvious things to check, then there could be a leak in the condensate line or at a fitting, or the fitting may not be glued together (yes, I have seen that too).