11.3 Purging Procedures, Safety Protocols & Leak Detection (Clauses 6.23–6.24)
Key Takeaways
CSA B149.1 Clause 6.23.1 requires purging to the outdoors when system pressure exceeds 2 psig or any section matches Table 6.4—for example NPS 3 to under 4 longer than 30 ft, or NPS 8 and larger at any length.
An outdoor purge (Clause 6.23.4) is continuously attended, controlled by a quarter-turn valve within 5 ft (1.5 m) of the open end, monitored by a combustible gas indicator, and discharged at least 10 ft (3 m) from ignition sources and buildings and 25 ft (7.5 m) from mechanical air intakes.
Indoor purging (Clause 6.23.7) burns the gas off at an appliance of up to 400,000 Btu/h with a readily accessible burner and continuous ignition, or follows Annex H; any open discharge must be attended and monitored with a combustible gas indicator.
Leak checks use a liquid solution or leak-detection device (Clause 6.22.3), never a flame (Clause 4.12.1), and a non-corrosive solution rather than household dish soap.
A leaking joint is never tightened under pressure: isolate and depressurize the section, repair it, then re-test.
After a system passes its pressure test, it is full of air, nitrogen or carbon dioxide. Gas cannot simply be turned on: where the incoming gas meets the air in the pipe, the mixture passes through the flammable range. CSA B149.1 Clause 6.23 (Purging of gas piping systems) controls how that air is displaced, and Clause 6.24 covers purging gas out of piping for repair, alteration or abandonment. The 2025 edition dropped "after leak testing" from the title of 6.23 because its rules apply to every purge, and it added combustible gas indicator monitoring.
1. Why Purging Is Controlled
- Natural gas burns between about 4% and 15% gas in air; propane between about 2.15% and 9.6%. As gas displaces air in a pipe, the interface zone inevitably sweeps through that range.
- Discharging that mixture into a building or near an ignition source can cause a fire or explosion. Propane's heavy vapour (specific gravity about 1.52) also pools in low spaces.
- Purging also carries moisture, scale and cutting chips out before they reach appliance controls.
2. When Must the Purge Go Outdoors? (Clause 6.23.1)
A gas piping system must be purged to the outdoors (using approved engineering practices or Clauses 6.23.2 to 6.23.4) when either:
- the gas pressure in the system is greater than 2 psig (14 kPa); or
- the system contains one or more sections of pipe, tubing or gas hose matching Table 6.4:
| Nominal size of pipe, tubing or gas hose | Length that requires an outdoor purge |
|---|---|
| 2-1/2 in. to less than 3 in. | More than 50 ft (15.2 m) |
| 3 in. to less than 4 in. | More than 30 ft (9 m) |
| 4 in. to less than 6 in. | More than 15 ft (5 m) |
| 6 in. to less than 8 in. | More than 10 ft (3 m) |
| 8 in. and larger | Any length |
The note to Table 6.4 sizes CSST of 2 in. or less the same way as piping. A typical house—1/2 to 1-1/4 in. piping at 7 in. w.c.—does not trigger an outdoor purge; a long 3 in. or 4 in. commercial header does.
3. Outdoor Purging (Clause 6.23.4)
The open end of the system being purged discharges directly outdoors, and the operation must meet all of these:
| Requirement | Rule |
|---|---|
| Attendance | The open point of discharge is continuously attended by a qualified person |
| Control | Controlled by a quarter-turn shut-off valve with an attached handle within 5 ft (1.5 m) of the open end; no other valve may control or end the purge, and the person purging controls the gas through that valve |
| Monitoring | The discharge point is monitored by a combustible gas indicator (new in 2025) |
| Purge line | Not smaller than NPS 1/2 |
| Ignition sources | Discharge at least 10 ft (3 m) from any source of ignition, after removing or shutting off ignition sources |
| Buildings | At least 10 ft (3 m) from a building or building opening |
| Air intakes | At least 25 ft (7.5 m) from a mechanical air intake |
| Bystanders | People not involved, and anyone smoking, are kept at least 10 ft (3 m) from the discharge |
Where the purge introduces fuel gas through a burner, a continuously burning flame is maintained at the burner ports until a stable gas flame is established, using a burner outside a combustion chamber with a continuous or intermittent pilot, two burners that reliably ignite each other, or an approved purge burner.
4. Indoor Purging (Clause 6.23.7)
Where Clause 6.23.6 allows the purge to stay indoors (in practice, systems that do not meet the outdoor triggers above), it may be done only in one of two ways:
- Burn it off at an appliance: ignite the gas at an appliance with an input of up to and including 400,000 Btu/h (120 kW) that has a readily accessible burner not located in a combustion chamber, with a continuous source of ignition, maintaining a continuously burning flame at the burner ports until a stable gas flame is established; or
- Use Annex H: for an appliance not equipped with a continuous pilot, follow the procedure in Annex H.
If there is an open point of discharge during an indoor purge, it must be continuously attended by a qualified person and monitored with a combustible gas indicator. Clause 4.10 still bans smoking and other ignition sources, and Clause 4.12 forbids using a match or flame to look for leaks.
Warning
Never vent purge gas into a basement, crawl space or combustion chamber. Propane vapour sinks and collects in low spots; natural gas collects under floors and in joist spaces above. Either can ignite when an appliance or switch operates.
5. Using a Combustible Gas Indicator
- Zero it in clean air. Switch on and warm up the instrument outdoors or in known-clean air; starting it in contaminated air hides the background level.
- Move slowly. Sweep the probe around each joint, close to the surface and slowly enough for the sensor to respond.
- Think about buoyancy. Natural gas (SG about 0.60) rises—probe the top of fittings and overhead spaces. Propane (SG about 1.52) sinks—probe the underside of fittings and low areas.
- Know its limits. Solvent vapours, pipe dope and primers can cause false readings; confirm with a leak-detection solution before repairing.
6. Leak Detection After Purging
Once the system holds gas at operating pressure, Clause 6.22.3 requires every appliance connection, valve, valve train and component to be checked with a liquid solution or a leak-detection device.
- Never use a flame. Clause 4.12.1 forbids using a match or other source of ignition to look for leaks.
- Use a non-corrosive solution. Purpose-made leak-detection fluids are formulated not to attack brass, copper or stainless steel, and winter grades work on outdoor meter sets in sub-zero weather. Household dish soaps can contain ammonia and chlorides that promote stress corrosion cracking in brass fittings and CSST; CSST manufacturers prohibit them.
- Give it time. A very small leak forms a slowly growing cluster of fine bubbles; watch each joint for a minute or more.
Never tighten under pressure
If a joint leaks: close the upstream isolating valve, depressurize the section completely, repair or remake the joint (re-applying certified sealant on threads), then pressurize and test again. Tightening a fitting under pressure can crack it or blow the joint apart.
7. Purging for Repair, Alteration or Abandonment (Clause 6.24)
Before piping that contains gas is opened for repair, altered, or abandoned, Clause 6.24 requires the gas to be purged from it. Vent the gas to a safe outdoor location away from ignition sources and openings, cap abandoned piping, and leave no open ends that could later be pressurized.
Under CSA B149.1-25 Clause 6.23.4, what is the minimum distance between an outdoor purge discharge point and a source of ignition or a building opening?
3 feet (1 metre)
6 feet (1.8 metres)
10 feet (3 metres)
25 feet (7.5 metres)
Each of these natural gas lines operates at 7 in. w.c. Which one must be purged to the outdoors under CSA B149.1 Clause 6.23.1 and Table 6.4?
A 3/4-inch residential branch 25 ft long
A 1-inch line 40 ft long
A 2-inch line 80 ft long
A 3-inch line 40 ft long
Why should a purpose-made, non-corrosive leak-detection solution be used on brass fittings and CSST instead of household dish soap?
Dish soap bubbles burst too quickly to show a leak
Dish soap neutralizes the mercaptan odourant in the gas
Dish soap is too thick to reach small thread gaps
Dish soaps can contain ammonia and chlorides that promote stress corrosion cracking in brass, copper and stainless steel
Sections you finish are checked off in the contents.