So we have been in our slab house how for 3 years and just noticed that we are getting water underneath out dining room floor. Come to find out it is coming from our AC unit.

While running, condensation is escaping from over the rear wall of the drain pan, being blown out through the gap between the coil box and air handler and finally running down the exterior of the air handler.

This is would be considered a blow-through or positive pressure system, since the A-coil is above the air handler. So the drain line for the condensate pan is just gravity fed (no traps, vents or clean outs) with a vinyl hose to a sewer drain.

I am just absolutely baffled as to how to keep the condensation from being blown over the walls of the drain pan. It is almost like there is a negative pressure inside the A-coil box.

Someone please help!

Post a picture, something sounds wrong with the whole setup.

the drain pan should have somekind of pipe or plastic tube for the water to drain. most likely the pan and these pipes are pvc and guncked up with dirt and slime.this has to be cleared with a solution of water and some drops of dawn dishwashing liquet and a brush. once the water drains mix some bleach with water and pour down the drain pan.do not rinse.this will kill the slime...we do this every 2 years to keep it flowing.

If the drain is running to a sewer drain and there's no trap then sewer gas can come out into the room though the pan.

we had a piece of garden hose on our drip pan, and came down to the basement-in-progress last year to find the old carpet squishy wet. cut an access panel to the A-coil in the duct, knowing I've had to blow out a lot of drain tubes and clear some drip pans in my multiple phases of life.

turns out the pan and drain were clear, but the solid dark plastic hose was plugged well, proper, and totally. got a piece of reinforced clear vinyl and a hose mender, and we're not going to get caught again.

if the system has ever worked right, it is probably mounted at a good mild angle to drain through the hose bibb. pull the hose off and see if you get water. if there is goo in there, you are going to have to use a poking brush and some vinegar to clean out the pan and drain line.

if the drainage comes through the furnace around the blower motor and not down the hose, big indicator you have a plugged drain from the A coil.

Well, knowing that a clogged drain tube is the most common of problems I immediately replaced mine. It used to be a PVC pipe with a running trap in the line. Running traps are useless. Especially when the drain line is not part I'd a sealed system. It was an open pipe that hovered over a sewer drain in the floor. So now I have 3/4" braid reinforced vinyl so that I could see that it is in fact draining. Which it is. If I remove the line water trickles out of the barbed fitting.

What I have been able to see in my little utility closet is that the seam where the A-coil box meets the blower to allow air (and water) to escape and run down the outside case of the unit. Unfortunately since my system sits in the corner of a finished closet I can not see all sides. I can see the front and right side clearly. The rear is only partially visible by using the camera on my iPhone. The right side is not able to be seen at all. From the rear I have seen some water spitting and trickling out of the seam. This would be the back of the drain pan.

As far as I can tell the pan is completely clear of any blockages. Is what I can't seem to figure out or explain is how water is able to escape OVER the rear wall of the pan. I've watched it with having the service door off of the A-coil and it seems as if it is being blown up and over the pan.