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Rick Mather
Rick Mather, HVAC Technician
Category: HVAC
Satisfied Customers: 3634
Experience:  35 years experience, HVAC
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Furnace water drain wont drain water. How to fix it?

Resolved Question:

The water drain line won't drain the water. When I take it off, the water comes pouring out. I reattach it.....nothing. I checked the line with pressurized water and it's not clogged. What's going on and how do I fix it?

Submitted: 8 years ago.
Category: HVAC
Expert:  Kenton Guidry replied 8 years ago.

Does the drain line slope upward, or is it installed at a downward slope?

Customer: replied 8 years ago.

It is installed on the side of the furnace and goes downward for about a foot then sideways across the unit and then back down the other side of the unit. It then is basically horizontal on the floor for 12 feet and empties into the sump.

Expert:  Kenton Guidry replied 8 years ago.

Is there anyway you would be willing to take a picture of your system and post it on here?
Have you always had this problem with it not draining?

Customer: replied 8 years ago.

As far as I know it has drained. The furnace was new last year. They kept the upper part of the furnace as is though which includes the drain.

I could post. graphic

Expert:  Kenton Guidry replied 8 years ago.

What efficiency is your furnace?
Is the drain that you are referring to the clear tubing?

Customer: replied 8 years ago.

That is the clear tubing. Correct.

It is high efficiency. 95%

Expert:  Kenton Guidry replied 8 years ago.

The clear tubing is connected to your evaporator. The condensation that builds up will collect into your drain pan and then to drain.
It should drip very slowly into the drain.
Does it drip slowly?

Customer: replied 8 years ago.

Right now the air is on pretty high since it was 90 most of the day and will be tomorrow and still humid. Feels decent inside though but I know it's working hard.

With that in mind, the water is draining pretty heavy right now. You can actually see the drip in this picture.graphic

Expert:  Kenton Guidry replied 8 years ago.

The more humid the air the more condensation you will have.
Looks like your drain is working.

Customer: replied 8 years ago.

I agree, but only when the hose is not attached. When I attach the hose, the water doesn't drain. It backs up into the system. I heard the water splashing on the ground because it was coming out of the bottom of the nozzle after backing up inside the system instead of going down the hose. When the hose comes off, the water comes gushing out. Hose back on.....no drainage.

Expert:  Kenton Guidry replied 8 years ago.

Does this hose at anytime go higher than where the drain is in the last picture you showed me.

Customer: replied 8 years ago.

Nope. That's the high point.

Expert:  Rick Mather replied 8 years ago.

Make a P-Trap right after it exits the coil so it won't suck back in with vacuum effect. The trap should be 3" deep or so then it will drain properly.

You can use 3/4" pvc fittings to make it up. Its cheap and works well.

Customer: replied 8 years ago.

I asked the hardware store guy and he had no clue what a P trap for a central air system was. He was thinking a T, but that doesn't sound like what you want me to do. He thought this would allow the air in so the vacuum effect would cease.

Expert:  Rick Mather replied 8 years ago.

You just have to come out of the drain pan, attach a 90 degree elbow, face it down; then another elbow, face horizontal, then back up then back horizontal and to your drain. Just like whats on a sink drain but made with elbows.

Customer: replied 8 years ago.

Ok I got it now. It already has the elbow coming out of the drain pan. I can do the rest. As far as I know this never happened before, but we did get a new high efficiency furnace last year. 95% would that have caused this vacuum?

Expert:  Rick Mather replied 8 years ago.

The air going by the drain fitting makes a slight vacuum not allowing the water to drain properly. A P-trap overcomes this vacuum effect.

Rick Mather, HVAC Technician
Category: HVAC
Satisfied Customers: 3634
Experience: 35 years experience, HVAC
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