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Drain plumbing – A beginners Resource

Understanding drain plumbing in new home construction

Now, it’s time to move on to drain plumbing. This is usually referred to as Drain, Waste, and Vent systems. Most plumbers now use plastic pipes for these systems because of the ease of use.



Plastic is much lighter, cheaper, and more flexible than cast iron. It really is fun to work with. If your local codes prevent the use of plastic, then you’ll have to use cast iron. By and large though, plastic is an acceptable product to use.

Two types of plastic pipe used for drain plumbing are PVC and ABS. Both are readily available and inexpensive, but the better of the two is probably the PVC.

It is more durable than ABS. There are a lot of plumbers that still use ABS because it was the first plastic product for DWV systems and they have grown attached to it. I think that both are fine to use.


drain pipe



Plastic pipe is extremely easy to cut. You can use a small-toothed saw, or a pipe cutter. One thing that is crucial to remember is that when you cut plastic pipe, it leaves behind burrs that need to be taken off with a utility knife on both the inside and outside.

If you forget to do this, things will snag on the inside of the pipes and you will have a lot of plugged lines after awhile.

When you do the layout for your drain, you’ll use many different fittings to help with corners, branches, traps, and clean outs. It is always good to do a dry run with all the pipes and fittings before gluing them together.

After you cut and burr the pipes, put them all together to make sure they are the right length. After you know for sure that everything fits, you can use the joining compound and cement them in place. ABS and PVC cement is a one-time try.

It welds the pipes and fittings together almost instantly. If you do need to get them apart after cementing, you’re better off cutting the pipe apart with a saber saw and putting in a coupler.

Remember, even if you can get them apart by pulling hard, the cement will leave a residue on the pipe and it will be a leaker. We’ve all done that before.

When you put the pipes and fittings together, make sure they are in all the way. This gets a little difficult when the pipe has to run through joists and studs and is often times hard to reach.

A good seal is understandably important though. Let’s go through the most common fittings that are used in the DWV system.


pvc pipes



There are many other kinds of plumbing fittings besides these, but we’ll just focus on these right now. First off, you don’t want to mix and match PVC and ABS. They aren’t the same material and they each use a different kind of solvent cement.

There are several different variations of fittings because they do different thing. An elbow is either a
vent elbow or a wastewater elbow.

The difference is that the vent has a short turn radius and the water has a long turn radius. Air goes around corners easier than water. Vent pipes can be 1 1/2 inches, but pipes carrying wastewater (washing machine, sinks, dishwashers, and baths/showers), need to be 2-inch lines.


pvc plumbing



That is the difference in the sizes and the turn radius of pipes and fittings. Now let’s put it all together to see what they do.


plumbing diagram



You will need to do a pressure test on the water lines before connecting any fixtures. It is a good idea to do pressure tests on both the water and septic lines at the same time so the plumbing inspector doesn’t have to show up twice for the same inspection.

Most waterline inspections are just a simple matter of turning on the water and checking for leaks, but you may have to do an air pressure test.

If so, you can cap off all the water lines with test caps and then integrate an air compressor and gauge in the line. The line will need to hold constant air pressure for a specified amount of time.

The septic lines will definitely have to have an air pressure test, but it’s not too bad. You just cap off all the waste and vent lines with plastic caps or test balloons made especially for testing waste systems and integrate an air compressor and gauge in the line.

The usual pressure test for plumbing systems is only 5 pounds for 15 minutes. Once the inspection has passed, you can knock out the caps and move on to other things.


testing plumbing



Make sure that the inspector has approved your work before you put the sheetrock on the walls. They get cranky if they can’t see your work.


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