Pressure Testing Water and Drain Piping

A pressure test is the only time a system has to prove itself before the walls close. As a lead, you are the one who signs for that proof. You also own the safety of the test, because a test puts stored energy into pipe that has not been trusted yet. A good test finds every bad joint. A bad test passes a system that leaks, fails a system that is fine, or sends a fitting across the room. Use the test your code calls for, read the gauge the right way, and keep people out of harm's way.

What You Will Be Able to Do

• State what the model codes require for water and drain tests
• Explain why air and water tests carry very different risks
• Set up a test with the right gauge, isolation, and venting
• Tell a true leak from a temperature effect
• Trace a failed test by sections
• Flush and disinfect a new potable water system

What the Codes Ask For

Both model codes, the International Plumbing Code (IPC) and the Uniform Plumbing Code (UPC), set a minimum test. This table shows the common model code values. Editions differ, and a state, city, engineer, or project spec may ask for more. Many commercial specs call for test pressures well above working pressure. The adopted code and the contract documents always win.

California's edition of the UPC holds water supply at test pressure for at least 15 minutes. It also allows PEX, PP, and PE-RT tube to be tested with air where the maker's instructions permit it. Your state's edition may or may not do the same. Ask your inspector.

Drain Tests in More Detail

A 10-foot head means the pipe is filled until water stands 10 feet above the part being tested. For a tall building, test in stages. Under the IPC, each section gets the 10-foot head, and each new section must also retest the upper 10 feet of the section below, so that no joint is left out.

Water is heavy, so check supports before you fill.

Worked example. A 10-foot head is 10 x 0.433, or 4.33 psi at the bottom. A 4-inch test plug has a face of about 12.6 square inches. 4.33 x 12.6 is about 55 pounds pushing on that plug. Tie your plugs in, and do not stand in line with a plug when you pull it. A sudden rush of water can knock a person off a ladder.

Water or Air

Use water wherever you can.

Water barely compresses. If a joint lets go in a water test, the pressure falls at once, and the water leaks out. Air compresses, so a pipe under air pressure is a loaded spring. When something fails, all that stored energy comes out in one instant.

Worked example. Take 100 feet of 2-inch pipe at 100 psi. Worked out from basic physics, the air in that pipe stores thousands of times more energy than water would at the same pressure. The force on a cap does not change with the medium, though. A 4-inch cap at 50 psi takes 12.6 x 50, or about 630 pounds. Under air, that force keeps pushing after something starts to move.

Warning: Do not test PVC, CPVC, or ABS with compressed air or gas. The Plastic Pipe and Fittings Association says not to, makers of those pipes say not to, and both model codes exclude plastic from the air test. Rigid plastic can shatter and throw sharp pieces. OSHA has published a hazard bulletin about exploding PVC air lines. Use water.

Air is not always wrong. On metal pipe in freezing weather, it may be the only practical test. On PEX, it may be allowed. When you use air, keep to the lowest pressure that meets the rule, keep people clear, bring the pressure up in stages, and never go looking for a leak up close while the pipe is at full pressure.

Setting Up the Test

1. Plan the section. Test what will be covered. Leave nothing out, and do not test against something that cannot take the pressure.
2. Protect equipment. Isolate or remove anything not rated for the test. This includes water heaters, expansion tanks, pressure-reducing valves, pumps, and many fixture valves.
3. Wait out the joint times. Follow the cement cure chart for the pipe size, temperature, and test pressure. Follow the cold-weather times for expansion PEX.
4. Check supports and restraints. Every capped end needs to be held.
5. Pick the gauge. Use a gauge that reads the test pressure near the middle of its dial. A 300 psi gauge cannot show a 2 psi drop at 50 psi. Use a gauge known to be accurate, and fit it where you can read it.
6. Fill from the low point, slowly. Open a high point to vent the air. Air trapped in a water test makes the gauge drift, and it stores energy.
7. Bring it up in stages. Stop at a lower pressure, look for leaks, then go on.
8. Isolate the source. Close the fill valve and disconnect the pump or hose. A test with the supply still connected proves nothing, because the supply covers a leak.
9. Record it. Note the start time, pressure, and temperature, and take a photo of the gauge.
10. Hold, walk, and read. Walk every joint, and read the gauge at the end.

Most press fittings are built to leak if left unpressed. To find one, use the test pressure and method the maker gives. Some makers ask for a low-pressure test step for this reason.

Reading the Gauge

A gauge that moves does not always mean a leak. The temperature of the pipe and what is in it can change the reading.

Air Tests and Temperature

The pressure of trapped air follows its temperature. Use absolute values for the math: add 14.7 to the gauge pressure, and add 460 to the temperature in F.

Worked example. You set a 50 psi air test at 70 F at 3 PM. Overnight, the building drops to 50 F. Absolute pressure is 64.7 psi, and absolute temperature falls from 530 to 510. 64.7 x 510 divided by 530 is 62.3, and that gives a gauge reading of about 47.6 psi. A drop of 2.4 psi with no leak at all. If you read 47.5 psi the next morning, you may be fine. Warm the space back up, or retest. A drop far larger than the math shows is a leak.

Water Tests and Temperature

Water expands when it warms, and it has nowhere to go in a closed test. A rigid pipe filled with cold water and left in the sun can show a large rise in pressure. Keep this in mind on roofs and in hot attics. Do not bleed off a rise and call it good. Test early in the day, or when temperatures are steady.

PEX Under Test

PEX tube stretches a little under pressure, so the gauge can fall at the start with no leak. Some makers give a procedure for this, such as bringing the system back up after a set time before the hold begins. Follow the instructions for the tube you are using. A dropping gauge after that needs to be investigated.

Finding a Leak

Work the same way you would trace any other fault: split the system.

1. Close a valve near the middle of the section, or cap the system into two parts.
2. See which side loses pressure.
3. Keep splitting that side.
4. On water, look for wet spots and listen. On air, use a leak detection solution made for the pipe material, and let the pressure come down before you go close to a suspect joint.

Some soaps and detergents are not safe on CPVC, so check before you use them. Fix a leak with the system depressurized and drained as needed. Then retest the whole section. A repair is not proven until it holds the full test.

Freezing Weather

Water left in a test can freeze, and it will split pipe and fittings. Cold-weather choices include heating the space, draining right after the test, or using air where the rules allow it. Never leave a water test standing unattended if the temperature might drop below freezing.

Flushing and Disinfection

New potable water lines need to be flushed and disinfected before use. The IPC and UPC set out the steps, and the local health authority may add more.

1. Flush until the water runs clear, and clear the aerators and strainers.
2. Fill the system with a chlorine solution of at least 50 parts per million and hold it for 24 hours, or at least 200 parts per million for 3 hours.
3. Flush until the chlorine is gone.
4. Where required, take a water sample for bacteria. If it fails, do it again.

Warning: Chlorine solutions can burn skin and eyes, and they give off harmful gas if mixed with acids. Wear eye protection and gloves, and follow the Safety Data Sheet. Keep that water away from occupied fixtures, and post signs that the system is being disinfected.

Checking a Test Someone Else Ran

When an apprentice tells you a test held, look before you sign. Ask:

• Is the gauge on the section being tested, or on a section that is valved off?
• Are all the valves in the section open?
• Is the supply disconnected?
• Was the pipe cured or set before the test?
• Was the test held for the required time?
• Does the photo show the start and end?
• Did anyone walk the joints?

A closed valve between the gauge and the new work is a common way to pass a test that never happened.

Common Mistakes

Check Your Understanding

1. You are working under a model code, and your job has CPVC water lines. The contractor wants to save time with an air test at 50 psi. What do you say?

No. CPVC is plastic and is excluded from the air test, and makers warn against it. Test with water at or above working pressure, or as your code and specs require.

2. A 50 psi air test on a copper system was set at 80 F and reads 47 psi in the morning, when the building is at 60 F. Does that prove a leak?

Not by itself. 64.7 x 520 divided by 540 is 62.3, which is a gauge reading of about 47.6 psi. Most of that drop is temperature. Retest when the building warms, or at a steady temperature.

3. You test a tall drain stack in stages under the IPC. The first stage passed. A helper wants to plug the stack at the top of that stage and fill only the new pipe above it. What is wrong with this?

The joints near the top of the first stage never saw a full 10-foot head. Each new section has to include the upper 10 feet of the section below, so that no joint is left out.

4. An apprentice says a section held for an hour. You see the fill hose is still attached with its valve open. What is wrong?

The supply can feed a leak, so the gauge proves nothing. Disconnect it and run the test again.

Key Takeaways

• Model codes set a minimum. Local rules and specs can ask for more.
• Water first. Do not air test PVC, CPVC, or ABS.
• Use a gauge with the right range, vent from high points, and isolate the supply.
• Temperature changes the gauge, so do the math before you tear anything out.
• Split the section to find a leak, then retest it all.
• Flush and disinfect new potable lines before use.