Short version: once the water in the bucket rises past the end of the hose, the drain stops working the way it should, and the water ends up coming out of the indoor unit instead.
Don't try this at home
Dave says it in the first five seconds, and we'll say it again here. You never want your condensate hose coming back inside the house. In the video the hose runs down the wall to a bucket on the floor so you can see what's going on. That's a demonstration, not a suggestion.
The setup
We took apart an indoor unit for this. When the unit is running, water condenses on the coil, drops into the drain pan along the front of the unit, and runs out through the drain hose. In the video, Todd stands in for the coil and pours water straight into the pan.
The hose runs down into a blue bucket with some water already in it. At the start, the end of the hose sits just above the water line. That matters, because it's the one thing that changes during the experiment.
Left: the end of the hose starts just above the water line. Right: once it's under water, air comes out of the hose instead of water.
What happens as the bucket fills
The first pour behaves the way you'd expect. Water goes into the pan, runs down the hose, and comes out into the bucket. The bucket fills, and the end of the hose is now a little below the surface.
Keep pouring and things change. The flow slows down. Then you see air bubbling up from the end of the hose. The hose is pushing air out instead of water. Dave's take, from the floor of the shop: the water in the pan still has enough weight behind it to push past the water in the bucket. For now.
Water keeps coming out for a while because there's still a column of water in the hose pushing down. But a real unit doesn't stop making condensate. As Dave puts it, condensate never quits. So we kept pouring.
Then it backs up
A couple of bottles later, Dave looks at the drain pan and the water level is rising. The pan is backing up. Then water starts dripping out of the end of the unit. Not the end with the hose, either. Both ends.
Left: the water level in the drain pan climbing. Right: water coming out of the end of the unit. In your house, that's the wall.
Dave's explanation is the one you learned at McDonald's as a kid. Put your finger over the top of a straw, lift it out of your drink, and the liquid stays in the straw. Nothing can flow out because no air can get in. A condensate hose with its end under water is the same thing. Without air, there's no flow.
And it gets worse as the bucket fills. The higher the water in the bucket, the more pressure there is on the end of the hose, and the more the unit leaks.
So, no bucket
That's why we don't put the end of a condensate hose into a bucket. It looks harmless when the bucket is empty. Once the water covers the end of the hose, the drain is blocked, the pan fills, and the water comes out of the indoor unit and down your wall.
Let the hose do what it's designed to do: run through the wall and drip outside, with the end open to the air.
If you have a drain question about your own installation, call or text us at (725) 699-1200. We build these. We're glad to help.
More maintenance videos: cleaning the indoor coil with foam and cleaning the outdoor unit.
