Filling plumbing pipes with pressurized air is a quick and effective way to inspect for leaks in new installation of water pipes and drain/waste/vent systems. You can also air-pressure-test existing pipes, but pumping the drain/waste/vent system full of pressurized air becomes impractical after installation of the plumbing fixtures and connection to the sewer. It's easier and often just as effective to do a water-pressure test instead.
Testing New Plumbing
Cap all the stub-outs connected to the system you're testing. For a DWV system, this usually means gluing a cap to each stub-out with plastic pipe cement. If you're testing a water system, you may have to glue or solder the caps, depending on the type of pipes in your system. You'll cut off the caps when the test is complete.
Install or find a fitting that allows you to connect an air compressor hose and pressure gauge to the system. In waste systems, you usually do this by screwing an adapter to a clean-out fitting and installing a tee. For water systems, it usually works to leave one stub-out uncapped and install a tee and adapter for the hose and gauge on that stub-out.
Turn on the compressor and fill the pipes with air until the gauge reaches the test pressure reading. For water systems, this reading is usually around 80 psi. For drain systems, it's much lower -- about 5 psi.
Turn off the compressor and leave the system pressurized for 15 minutes. Any reduction in pressure during that time signifies the presence of a leak.
Air-Testing Existing Plumbing
Air-test a system of water pipes by turning off the main shutoff valve and draining the water from the system. After all the water has drained, turn off all the faucets.
Screw a pressure gauge to one of the faucets -- usually a laundry-room faucet or outdoor faucet with a threaded spout. Screw an adapter to another faucet that allows you to connect a compressor hose.
Charge the system to about 60 psi and leave it charged for 15 minutes. The system has no leaks if the pressure gauge remains stationary.
Water-Testing Existing Plumbing
Test a water system without draining it by locating an outdoor or laundry faucet to which you can attach a pressure gauge.
Screw the gauge to the faucet spout, open the faucet all the way and turn off the main shutoff valve for the house.
Watch the gauge for 15 minutes. If it moves, there's a leak in a pipe or in a faucet. Check the faucets, and if none of them are dripping, the leak is in a pipe.