An oxygen system normally operates at 50 psig. What are the test pressure and the maximum allowable leakage for the installer's 24-hour standing pressure test?

60 psig, with 0.3 psi allowable. The 24-hour standing test pressure is 20% above normal line pressure: 50 times 1.2 equals 60 psig. Leakage may not exceed 0.5% of the starting pressure, which is 0.3 psi at 60 psig, excluding temperature effects. 150 psig belongs to the initial pressure test, and 75 psig would be 1.5 times operating pressure.

What vacuum level does NFPA 99 require for the installer's 24-hour standing vacuum test?

Between 12 in. HgV and full vacuum, with the vacuum source disconnected. NFPA 99 requires a 24-hour standing vacuum test between 12 in. HgV and full vacuum, with the source of test vacuum disconnected. Leakage is limited to 0.5% of the starting vacuum, excluding temperature effects. Running pumps would hide leaks, and a 10-minute hold is the verifier's standing test, not the installer's.

Who may witness the installer's 24-hour standing pressure test of a positive-pressure medical gas system under NFPA 99?

An ASSE 6020 inspector, ASSE 6030 verifier, or the AHJ or designee. NFPA 99 requires the 24-hour positive-pressure standing test to be witnessed by an ASSE 6020 inspector, an ASSE 6030 verifier, or the AHJ or its designee, with a form provided to the verifier. The installing contractor's own personnel, including its 6010 installers, cannot provide that independent witness.

Installer Tests II: 24-Hour Standing Pressur | Free Guide 2026

Key Takeaways

Positive-pressure systems get a 24-hour standing pressure test with nitrogen NF at 20% above normal operating line pressure, after outlet valve bodies, faceplates, and all other distribution components are installed.
Leakage during the 24-hour positive-pressure test may not exceed 0.5% of the starting pressure (for example, 0.3 psi starting at 60 psig), excluding effects of ambient temperature changes.
Vacuum systems get a 24-hour standing vacuum test between 12 in. HgV and full vacuum, with the test vacuum source disconnected, and leakage limited to 0.5% of the starting vacuum.
The 24-hour positive-pressure test must be witnessed by an ASSE 6020 inspector, an ASSE 6030 verifier, or the AHJ or its designee, and a form documenting it goes to the verifier.
System inspections, including labeling and valve tagging of concealed components, happen before piping is concealed and must be done by a party other than the installing contractor.

The 24-Hour Standing Pressure Test (Positive-Pressure Gases)

Item	Requirement
Prerequisite	The initial pressure tests passed
Timing	After final installation of station outlet valve bodies, faceplates, and all other distribution system components
Source valve	Closed
Test gas	Oil-free, dry nitrogen NF
Duration	24 hours
Test pressure	20% above the normal system operating line pressure
Acceptance	Leakage over 24 hours may not exceed 0.5% of the starting pressure (for example, 0.3 psi starting at 60 psig), except changes caused by ambient temperature
Leaks	Locate them, repair (where permitted) or replace, and retest
Witness	An ASSE 6020 inspector, an ASSE 6030 verifier, or the AHJ or its designee; a form documenting the witnessed test goes to the verifier at the start of verification

Worked examples

System	Normal line pressure	Test pressure (normal x 1.2)	Maximum leakage (0.5% of start)
Oxygen	50 psig	60 psig	0.30 psi
Medical air	55 psig	66 psig	0.33 psi
Nitrogen	160 psig	192 psig	0.96 psi
Instrument air	175 psig	210 psig	1.05 psi

Accounting for temperature

Gas pressure follows absolute temperature. Suppose an oxygen line starts at 60 psig (about 74.7 psia) at 70°F (529.67°R), and the space cools to 65°F (524.67°R) overnight. With no leakage at all, the absolute pressure falls in proportion:

P_2 = 74.7 \times \frac{524.67}{529.67} \approx 74.0 \text{ psia}

That is a drop of about 0.7 psi, more than twice the 0.3 psi limit, with no leak. This is why NFPA 99 excludes changes attributed to specific changes in ambient temperature. Record the temperature at the start and the end of the test. Then you can show whether a drop is temperature or a real leak, and a real leak must be found and fixed.

The 24-Hour Standing Vacuum Test

Item	Requirement
Prerequisite	The initial pressure tests passed (vacuum piping is also pressure tested at 150 psi minimum)
Timing	After installation of all components of the vacuum system
Duration	24 hours
Test level	Between 12 in. HgV and full vacuum
Test source	Disconnected from the piping during the test
Acceptance	Leakage may not exceed 0.5% of the starting vacuum (for example, 0.125 in. HgV starting at 25 in. HgV), except changes caused by ambient temperature
Leaks	Locate them, repair (where permitted) or replace, and retest
Witness and form	The test is witnessed, and a form documenting it goes to the verifier at the start of verification

Example: a WAGD and vacuum branch is pulled to 24 in. HgV and isolated from the pump. The maximum allowable loss is 0.5% of 24, or 0.12 in. HgV. A reading of 23.7 in. HgV the next morning (a 0.3 in. loss) fails unless the change can be attributed to temperature.

System Inspection Before Concealment

NFPA 99 also requires inspections separate from the installer's own tests:

When: before piping is concealed in walls, ceilings, chases, trenches, underground, or otherwise hidden from view
Test gas: nitrogen NF, where gas is needed for the inspection
Who: a qualified party experienced in medical gas and vacuum pipeline inspection and testing, certified to ASSE 6020 (inspector) or ASSE 6030 (verifier)
Independence: performed by a party other than the installing contractor. Where the system was not installed by the facility's own personnel, the facility's qualified personnel may inspect it.
What: the presence and correctness of labeling and valve tagging for all concealed components and piping
Witnessing: the initial pressure tests performed by the installer are witnessed by an ASSE 6020 inspector, an ASSE 6030 verifier, or the AHJ or its designee, and documented on a form for the verifier

Practical Test-Day Checklist

Confirm the system is complete for the test stage: rough-ins for the initial test; valve bodies, faceplates, and components for the 24-hour test.
Close the source valve. Disconnect or isolate anything that test pressure could damage during initial tests.
Use calibrated gauges with a range and resolution suited to the acceptance criterion. A 0.3 psi limit cannot be read on a gauge with 5 psi divisions.
Record the start time, pressure, and temperature, and the end time, pressure, and temperature.
Have the witness sign the form, and file it for the verifier.
If a leak is found, repair it as the code permits. A joint may be reheated only once. Then retest from the start.

Tip

Do not mix up the two "20%" rules. 20% above normal pressure is the installer's 24-hour standing test pressure. Plus or minus 20% is the alarm trigger for line pressure. Also keep the durations straight: the installer's standing test lasts 24 hours, and the verifier's standing test lasts 10 minutes (Section 7.3).

ASSE 6010 Medical Gas Systems Installer

7.2 Installer Tests II: 24-Hour Standing Pressure & Vacuum Tests, Inspection & Witnessing