For the past couple weeks, there’s been a trail of water leaking from my furnace. I believe it is condensation from the air conditioner. After doing a little research online, I cleaned out the U trap, which was absolutely packed with what I assume is some type of algae. I then stuck a garden hose down the drain pipe and flushed out any other goop that could have been blocking the way further down the line. The drain line now flows freely, but I’m still getting a leak from somewhere inside the furnace. Is my condensation pump bad? Is there something I’m missing? I’m really trying to avoid an expensive repair bill.

  • The A/C evaporator inside the furnace's air handler generates condensation normally. Where does this drain to? Water wants to go down because of gravity. Does the water simply go down into a drain? Or does it go somewhere? Jul 25, 2023 at 20:42

2 Answers 2


If the trap and the tube were packed with debris then the port going into the condensate pan is likely plugged up too. This won't have anything to do with a condensate pump -- if you have one at all, it would be sitting on the floor at the downstream end of that drain pipe that you flushed out. Most homes have a floor drain and so do not have a condensate pump.

If you pulled the cover off the front of the evaporator coil, which is what's sitting above the furnace and is the source of the condensate water, it might look similar to the one pictured here (photo: acwholesalers.com):

uncased evaporator coil

The black plastic pan at the bottom is where the condensate collects. This coil has two 3/4" pipe thread fittings at the front left corner for drainage. You probably need to clean out whatever piping is between your U-trap and these ports. Some evaporators are installed such that there's a door that can very easily be removed for inspection or cleaning. If yours is not one of those, you may need to cut and replace some PVC fittings in the drain line.

I often do condensate drains with clear vinyl tubing. It turns out that a 3/4" irrigation "funny pipe" barbed adapter fits just right into 1/2" ID vinyl tube. One might argue that, because of the smaller tube diameter, this is more prone to clogging than the traditional 3/4" PVC solution would be. However, it can't be beat for ease of diagnosis and repair if a clog ever does occur!


You'll need to get to the pump and observe that it is, in fact, running. There are "float" switches that can shut off pumps and airhandlers when the water line gets plugged up. Make sure the switch is free to operate and not jammed with goop.

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