How to Pressure Test a Sewer Line: Plug Selection and Safety Guide

The inspector's coming at 8 AM. You've got 200 feet of new 6-inch PVC to test before final backfill. The crew's waiting. Whether your spec calls for a low-pressure air test or a hydrostatic test, you need the right plugs seated correctly and a procedure that passes the first time. Here's how to set up both test types and which Cherne plugs to use for the job.

Why Every New Sewer Line Gets Tested

Testing isn't optional. Every municipality and most plumbing codes require an installation acceptance test before a new gravity sewer line goes into service. The reason is straightforward: a leaking sewer line means infiltration, exfiltration, or both—and that means groundwater contamination, overloaded treatment plants, and sinkholes down the road.

The numbers back it up. The American Society of Civil Engineers gave U.S. wastewater infrastructure a D+ grade in 2025, and the EPA estimates the country needs up to $690 billion in wastewater infrastructure investment over the next 20 years. Collection system failures have increased from 2 to 3.3 per 100 miles of pipe over the past decade. Between 23,000 and 75,000 sanitary sewer overflows happen annually in the U.S. alone. Proper testing at installation is the first line of defense against adding to those numbers.

Two Ways to Pressure Test a Sewer Line

There are two accepted methods to pressure test a sewer line for installation acceptance: the low-pressure air test and the hydrostatic (water) test. Your project spec or local code will dictate which one to use. Air testing is more common for new PVC gravity sewer because it's faster and doesn't require a water source, but hydrostatic testing is often required for force mains and in areas with high groundwater.

Low-Pressure Air Test

The low-pressure air test is governed by ASTM F1417 for plastic pipe. It works by pressurizing an isolated section of sewer with air and measuring the rate of pressure drop over a set time. If the line holds, joints are tight and the pipe is intact.

Procedure overview:

1. Flush and wet the line. Run water through the pipe segment before testing. A wetted interior surface produces more consistent air test results because it reduces air permeation through the pipe wall.
2. Plug both ends. Install pneumatic test plugs (like the Cherne Test-Ball) at the upstream and downstream ends of the section. Each plug's sealing length must equal or exceed the pipe diameter. Brace all plugs against the thrust force—more on that in the safety section below.
3. Pressurize to 4.0 psi. Add air slowly until the gauge reads approximately 4.0 psi above any groundwater back pressure. Never exceed 9 psi internal pressure per ASTM F1417. Allow at least 2 minutes for the air pressure to stabilize before starting the timed test.
4. Start the test at 3.5 psi. Once pressure stabilizes at or above 3.5 psi, begin timing. The test measures how long it takes for pressure to drop from 3.5 psi to 2.5 psi (a 1.0 psi drop).
5. Compare against the acceptance table. ASTM F1417 provides minimum time values based on pipe diameter and segment length. If the measured time exceeds the minimum, the section passes. If pressure drops faster than the table allows, you've got a leak.

Note: ASTM F1417 covers two methods—the time-pressure drop method described above (most common in the field) and a constant-pressure method. Check your project spec for which applies.

Hydrostatic (Water) Test

The hydrostatic test fills the pipe with water and checks for leakage under a sustained head of pressure. It's the go-to method when air testing isn't appropriate—typically for force mains, large-diameter lines, or when groundwater is more than 2 feet above the pipe crown.

Procedure overview:

1. Plug the downstream end. Install an inflatable test plug at the low end of the section. For larger lines (12 inches and up), use a Cherne Underground Test-Ball or I-Series blocking plug rated for the required back pressure.
2. Fill the line with water. Slowly fill the pipe section from the low point to minimize air pockets. Vent air at high points. The water level must reach at least 2 feet above the crown of the pipe at its highest point, or to the height specified in the project plans.
3. Let the pipe absorb. PVC and concrete absorb water at different rates. Allow time (typically 24 hours for concrete pipe) for the pipe wall to saturate before running the test.
4. Hold and measure. Maintain the test head for the required duration—commonly 2 hours for a standard acceptance test. Measure any water added to maintain the head level.
5. Evaluate leakage. Allowable exfiltration is typically 50 gallons per inch of pipe diameter per mile per 24 hours. If measured leakage stays under that threshold, the line passes.

Choosing the Right Plug: Mechanical vs. Pneumatic

Plug selection depends on three things: pipe diameter, test type, and how long the plug needs to stay in place. Getting this wrong isn't just an inconvenience—it's a safety hazard.

Pneumatic Test Plugs (Cherne Test-Ball)

Cherne Test-Ball plugs are inflatable rubber plugs that expand to seal against the pipe wall when aired up. They're the standard choice for pressure testing sewer lines.

• Best for: Air tests and hydrostatic tests on lines from 1 inch up to 96 inches
• How they work: Insert through a manhole, cleanout, or test tee. Inflate with a standard tire fitting to the pressure stamped on the plug (typically rated up to 13 psi for plumbing Test-Balls, 15 psi for underground models)
• Plumbing Test-Balls (3/4" to 6") are sized for DWV testing in residential and commercial plumbing. Molded-in sealing ribs grip out-of-round pipe and hard-to-access openings
• Underground Test-Balls (6" to 18") handle sanitary and storm sewer mains. Built from heavy-duty, tear-resistant natural rubber for municipal work
• I-Series Test-Balls (12" to 48") are built for large-diameter blocking and testing with a 15 psi back-pressure rating
• Key limitation: Pneumatic plugs need monitoring. Inflation pressure should be checked and adjusted every four hours during extended use

Mechanical Test Plugs

Mechanical test plugs use a wing nut and compression plates to expand a rubber gasket against the pipe wall. No air source needed.

• Best for: Blocking individual stubs, long-term holds, DWV rough-in testing on smaller lines
• Size range: Approximately 3/8 inch to 18 inches
• Advantages: Smaller, lighter, won't lose seal over time since there's no air to leak. Set it and leave it
• Limitation: Not designed for line acceptance testing on mains. Mechanical plugs are blockers, not test plugs—they don't have the sealing length or back-pressure rating for air or hydro testing on long runs

Quick Selection Guide

What Makes Tests Fail

Nobody wants to run a test twice. Here are the most common reasons a sewer line pressure test fails and how to avoid them:

1. Joint leaks. The number one cause. A gasket that rolled during installation, a pipe not fully seated in the bell, or debris in the joint. Fix: camera the line to find the bad joint, pull it apart, clean, re-seat.
2. Plug not seated properly. If the plug is sitting on debris, in an out-of-round section, or not inflated to its rated pressure, it'll leak around the seal. Fix: clean the pipe at the plug location, verify inflation pressure, confirm the plug is the right size.
3. Wrong size plug. An undersized plug won't seal. An oversized plug won't fit or won't inflate to its proper shape. Fix: match the plug to the actual inside diameter of the pipe, not the nominal size.
4. Temperature changes during air tests. Air pressure is temperature-sensitive. A test started in early morning cool air that runs into midday heat will show rising pressure; a test started in warm conditions that runs into evening will show a false pressure drop. Fix: allow adequate stabilization time and test during stable temperature conditions when possible.
5. Cracked or damaged pipe. Impact damage during backfill, a stone point-loaded against the barrel, or a pipe dropped during handling. Fix: camera inspection, cut out and replace the damaged section.
6. Service connections not plugged. Every lateral connection and stub-out in the test section must be plugged. One open connection means instant failure. Fix: walk the line and verify every opening is sealed before pressurizing.

Safety: Plug Blow-Out Prevention

This is the section that matters most. A pneumatic plug under test pressure stores serious energy, and a blow-out can be fatal.

Understand the thrust forces. Even at low pressures, the force on a plug is substantial. At just 5 psi, an 8-inch plug has 250 pounds of thrust pushing against it. At 4 psi on a 60-inch line, that force exceeds 10,000 pounds. At 9 psi on the same line, it's over 24,000 pounds. These numbers are why bracing and proper installation aren't suggestions—they're survival requirements.

Mandatory safety practices when you pressure test a sewer line:

1. Never exceed the plug's rated pressure. The maximum back pressure is printed on every Cherne plug. Exceeding it risks catastrophic failure of the rubber.
2. Brace every plug. Plugs must be secured against the thrust force. Use mechanical bracing against the manhole wall, pipe structure, or purpose-built bracing equipment. Never rely on friction alone.
3. Never stand in the possible path of a plug. ASTM F1417 explicitly states: no personnel shall be in manholes or in the potential trajectory of a plug during pressurization or testing. Stay out of manholes. Stay away from pipe ends.
4. Never inflate a plug outside the pipe. An unconfined inflatable plug can rupture unpredictably. Only inflate after the plug is fully inserted and positioned.
5. Vent pressure completely before removing plugs. Release all test pressure and confirm the gauge reads zero before loosening or removing any plug or bracing.
6. Monitor inflation pressure. For extended tests, check the plug inflation pressure every four hours. Temperature changes and slow air loss can reduce the seal over time.
7. Use plugs in good condition. Inspect rubber for cuts, cracks, and abrasion before every use. A damaged plug is an unpredictable plug. Replace it.

Get the Right Plug for the Test

EDP stocks the full line of Cherne Test-Ball plugs, from 3/4-inch plumbing sizes up through I-Series plugs for large-diameter municipal work. We also carry mechanical test plugs for blocking and DWV rough-in, and Muni-Ball bypass plugs for flow control during live sewer work.

Need help matching a plug to your pipe size, material, and test type? Contact us—we'll get you the right tool for the job and ship it fast.

Browse the full selection:

• Pneumatic Pipe Plugs (Test-Balls)
• Mechanical Test Plugs
• Blocking Plugs
• All Cherne Products