Gas Leak Investigation in a Residential Basement

The CGD reading of 10% LEL near the floor joists indicates a gas concentration of about 0.5% by volume (since LEL for natural gas is 5%, 10% LEL = 0.5% gas). This is a significant leak and must be addressed immediately. The bubble test on visible piping showed no leaks, but that does not rule out hidden leaks. Possible sources: 1) The flexible connector for the furnace is not listed for indoor use and may be leaking. Unlisted connectors may not be gas‑tight or may be damaged. 2) The furnace connector lacks a thermal shutoff, which is required in some jurisdictions, but that is a code issue, not a leak source. However, the connector itself could be leaking at the fittings. 3) The condensate drain line near the gas piping could be causing corrosion of the piping, leading to a pinhole leak. 4) Hidden piping in the ceiling or walls could be leaking. 5) The loose bonding clamp is not a gas leak source, but it is an electrical hazard. 6) The water heater connector could be leaking. 7) The gas valve or manifold on the furnace could be leaking. 8) The gas dryer connector could be leaking.

To locate the leak, the gas fitter should use the CGD to trace the concentration to its highest point. Start at the area of highest reading and move the detector slowly along the piping and connectors. Use a soap bubble solution on all fittings, including those hidden behind panels if accessible. If the leak is not found, the gas fitter may need to isolate sections of the piping using the shutoff valves. The gas fitter should also check the furnace and water heater by turning them off and observing if the CGD reading changes. A manometer can be used to perform a pressure test on the piping system. The gas fitter should also check for leaks at the meter and the main shutoff. The bubble test is limited because it only detects leaks on surfaces where bubbles can form; it may miss leaks in hidden areas or very small leaks. Also, the bubble test may not detect leaks if the gas is migrating through the soil or behind walls. The CGD is more sensitive but can give false positives from other sources (e.g., solvents, exhaust). Therefore, the gas fitter should use a combination of methods. The gas fitter should also ensure that the area is ventilated and that no ignition sources are present. If the leak cannot be found, the gas fitter should shut off the gas and advise the homeowner to evacuate until repairs are made.

The flexible connector for a gas furnace must be listed for the application and must be installed per the manufacturer's instructions and the National Fuel Gas Code (NFPA 54) or local code. In many jurisdictions, flexible connectors must be listed to ANSI Z21.24 (for indoor use) and may require a thermal shutoff device (also known as a excess flow valve or thermal shutoff) if the connector is not rated for the temperature. The connector in this case is not listed for indoor use, which is a violation. It must be replaced with a listed connector. The correct repair procedure: 1) Shut off the gas at the meter or the appliance shutoff valve. 2) Disconnect the old connector. 3) Install a new listed flexible connector of the correct size (1/2 inch) and length, with a thermal shutoff if required by local code. 4) Use new flare fittings or approved fittings. 5) Leak test all connections with soap bubble solution or a CGD. 6) Ensure the connector is not installed in a location where it could be damaged. 7) Check that the connector is not kinked or twisted. 8) Verify that the connector is not used as a structural support. 9) Ensure that the connector is not concealed. 10) After repair, perform a pressure test if required. Also, the connector must be installed with a drip leg and a shutoff valve upstream. The gas fitter should also check the water heater connector and the dryer connector for similar issues.

Bonding issue: The gas piping is bonded with a 12 AWG copper wire to the cold water pipe, and the clamp is loose. This is a violation because the bonding conductor for gas piping must be sized per the Canadian Electrical Code (CEC) or NEC. For a typical residential service, the bonding conductor must be at least 6 AWG copper. A 12 AWG wire is too small. Also, the clamp must be listed for the pipe material and must be tight. The bonding connection must be made to a clean, bare metal surface. The gas fitter should inform the homeowner that the bonding must be corrected by a qualified electrician. The gas fitter should not attempt to repair the electrical bonding unless qualified. The loose clamp could cause arcing and is a fire hazard. The correct bonding would be a 6 AWG copper conductor connected to the gas piping with a listed clamp and run to the grounding electrode system. The gas fitter should document the deficiency and advise the homeowner to have an electrician correct it before the gas is turned back on. In the meantime, the gas fitter should ensure that the gas leak is repaired and that the appliance is safe to operate.