Once the clog is determined to NOT be in the water closet or in a sink trap, I check to see if it’s in the main drain line. Usually, that’s where it’s going to be.

I plunge and snake until I determine it to be futile. At that point, I do the best to determine where in the line the clog likely is. Then I start plugging branches. First, I unhook any sinks or water closets, and insert polymer ‘test caps’ into the pipe. If they’re not nice and tight, I use a few wraps of Vinyl electrical tape to make them a snug press-fit into the pipe openings and/or toilet flanges. In fact, I usually work from a toilet flange. (Yes, this means removing a toilet!) I have a test plug with a small hole drilled through it which is a nice snug fit for a barbed nosepiece for a handheld blow gun.

While I have the air compressor building pressure in the tank, I fill the drain with water. If it doesn’t hold water, it’s not plugged! The more water it takes, you can determine two things. If it drains, but slowly, it’s a partial blockage. If it hardly goes anywhere, it’s really plugged. How much water it takes before you start to see the drain ‘fill’ tells you how far down the line the blockage is. The more water you’ve got to pour in before you see your drain line approach full, the farther away it is.

Make sure all other test caps are in very tightly, save for maybe a floor drain if you’ve got one nearby you can see. This is something you could watch to monitor your progress.

The trick with this is that air will compress to high pressure. Liquids do not compress. This is how hydraulics can be used to lift a car in a mechanic’s garage. You’re ‘dumping’ high pressure air into the small air space between the water you’ve poured in to fill the drain line and the bottom of the cap with the hole through which you insert to blowgun nosepiece. The pressure of the air is fairly high, and it pushes the water. The pressure of the water isn’t so high, as it does not compress, but it’s going to exert pressure all the same. It does not take much to open most blockages.

If you have a floor drain, it’ll probably back up. Let off the air and withdraw the nozzle when the floor drain is about to overflow. Let it recede, and try again. I have best luck plugging the floor drain with a tight fitting plug with a small hole in it. It may take half a dozen tries, but I have yet for this technique to fail. If it ever does, then the next step would be renting a power auger, and barring that, breaking the concrete to replace the drain line…

Fred

I also read Becca’s comment about having your pipes blown free and wanted to offer a word of caution. VERY often homes that are around a hundred yers old still have their original clay or cast iron pipes. And very often these pipes have accumulated hair line crackes in them over the years. Not big enough to leak per say, but there non the less. And while the combination of vinegar and baking soda working it’s way through them MIGHT affect these cracks, it seldom does. However, the pressure that air blowing puts through pipes is far more likely too!
just for what it’s worth.