Gurgling Sink Drain?

[Bird]

The vertical pipe in the photo on the left has a cap on it in the photo. That cap is not glued on and I was always removing it once a month to pour bleach in there. But now I never put that cap on. Leaving it off all the time solved the gurgling in the sink. And that condensation drain goes down in the wall and come out under the sink and is plumbed into the sewer drain just inches below the sink drain (above the P trap).

 

[ametcalf]

Plastic wrap over the drains in both sinks are definitely pushed (or sucked) down into the drain when the heat comes on.

Before I explain what we mean by air gap in Kentucky lets see if we agree with what is happening when you did the above test. The way I see it is that if the water in the trap on the condensate drain has evaporated to the point where it was allowing air to be sucked through making your sink gurgle and the plastic wrap was being sucked down that means that air is being sucked out of your plumbing waste and vent system and being distributed through your house by your heating system. I'm actually surprised that there wasn't some odor. Even without the odor the germs are still moving from your plumbing waste and vent system through your a/c system into your living area. Now the fact that the a/c system was making your sink gurgle means that you have two problems one is that the a/c system is tied directly to your waste and vent system and two is that the vent system isn't working properly or the a/c system would be drawing air from the vent system rather than trying to pull air through your sink trap.
Now as to what an air gap is. The way it was in Kentucky when I was plumbing and I assume it still is we generally ran condensate drains to the outside of the house or to a floor drain. In some rare cases we would install a p trap in the plumbing system just to handle the condensate drain but usually there was a better way to do it. Some times they would be run into a laundry sink. When ever the condensate was piped so it would end up in the sewer lines the end of the condensate drain would be above the receptacle that was receiving it. ie: a couple of inches above the floor drain or a couple of inches above the rim of the laundry sink. This way the trap of the floor drain or the sink would keep the sewer gas from coming back into your a/c system. The reason that you have the gap above the floor drain or the sink is so that if the drain or the sink gets backed up you are not introducing contaminated water into your a/c system. I don't know if they are required in Texas or not but in Kentucky all dishwasher drain hoses are supposed to be tied into the plumbing system through an air gap. It is a device usually chrome colored about an inch and a half in diameter and two and a half inches high and sits on the top of the sink. The purpose of this air gap is so that if your sink drain gets plugged the contaminated water can't get backed up into your dishwasher. The air gap inside the device prevents the suction from taking place that would draw the waste water back into the dishwasher discharge hose.
I know Kentucky is backwards in the way we do a lot of things but we do have one of the strictest plumbing codes in the nation. I know that codes and the idea of licensing plumbers is appalling to many people but let me share a situation that we ran into. Our small town in Central Kentucky installed a city sewer system in the mid 1970s. All businesses and residences had to abandon their septic tanks and hook onto the city system much to the chagrin of many an independent minded soul. The local pizza restaurant was located in an old house that was built before the plumbing code came to our area. When we went to hook them up to the new city sewer there wasn't room on the small lot to run the new drain line around the septic tank. We got approval to pump the old septic tank, take the top off tank fill it with crushed limestone and run the new drain line through the tank. When we got the top off the tank we found that the old galvanized water line that was feeding fresh water supply into the restaurant was run through the septic tank and it was leaking!! As long as there was positive pressure on the water lines they were OK but whenever there was a water main break or some other reason for there to be a negative pressure on the line, raw sewage was being drawn into the city's water supply. It's no wonder it wasn't unusual to hear of someone getting hepatitis in those days.
Enough of my soap box. If I haven't been clear enough on what an air gap is and why we need them let me know and I will try and shoot you some pictures.

 

[ametcalf]

The vertical pipe in the photo on the left has a cap on it in the photo. That cap is not glued on and I was always removing it once a month to pour bleach in there. But now I never put that cap on. Leaving it off all the time solved the gurgling in the sink. And that condensation drain goes down in the wall and come out under the sink and is plumbed into the sewer drain just inches below the sink drain (above the P trap).

I didn't pay enough attention as to who was who when I wrote my previous post. I thought it was Bird who asked what an air gap was so I wrote my post with that in my head. Now I realize my mistake so I am addressing this to Bird. As long as the condensate drain goes into the drain under the sink above the trap you do not have the dangerous type of cross connection that I was talking about. Leaving the top off the stand pipe is a good idea. I am really puzzled that the heating system has enough suction to make your sink drain gurgle. In fact if the sink does not have the stoppers in it and you don't have the plastic over the drains the air that is being sucked should go from the room down the sink drain and up the condensate drain to the heating unit and never go through a trap until it gets to the trap on the condensate line. Is it possible that it is the trap on the a/c system that is gurgling and the noise is being transmitted down the pipe so that it sounds like it is coming from the sink drain?

 

[Bird]

I don't know much about about our plumbing codes, but I do know that two licensed professional plumbers have seen and approved this.

 

[chopped]

The vertical pipe in the photo on the left has a cap on it in the photo. That cap is not glued on and I was always removing it once a month to pour bleach in there. But now I never put that cap on. Leaving it off all the time solved the gurgling in the sink. And that condensation drain goes down in the wall and come out under the sink and is plumbed into the sewer drain just inches below the sink drain (above the P trap).

Maybe this is a good add on

Oatey Sure-Vent 1-1/2 in. x 2 in. PVC Air Admittance Valve-39016 - The Home Depot

I had no vent on my sink drains.(they empty to the lawn) But when the washer dumped it would pull the water from the second floor trap. I dont have this vent installed yet. But I drilled a small 1'2 inch hole in the elbow and solved the problem.
Air behind the water makes a huge difference.