Gas leak detection, but leave the building first
If you can smell gas, nothing on this page matters until you are outside and have called the utility. Read the rest afterwards, when the property is safe.
What to do right now
If you can smell gas, do these things in this order and read nothing else until they are done.
- Get everyone out of the building. Including pets. Do not stop to gather things.
- Do not touch anything electrical. No light switches, no thermostats, no garage door openers, no appliances. Do not use a phone inside the building.
- Do not light anything. No matches, no lighters, no candles.
- Leave doors open as you go if it is easy to do so.
- Do not try to find the leak or turn appliances off.
- From outside, well away from the building, call your gas utility's emergency line and 911.
- Do not go back in until the utility or the fire department says the property is safe.
The utility responds to suspected gas leaks as an emergency at no charge, twenty-four hours a day. They will make the property safe, which may mean shutting off the supply at the meter, and they will identify whether the leak is on their side or yours.
Once the property is safe and the fault is on your side of the meter, that is when a licensed gas fitter repairs it. That is what we dispatch.
Outside the building too. If you smell gas outdoors, see bubbling in standing water, notice dead or dying vegetation in a line across a lawn, or hear a hissing or roaring sound near a gas main, move away and call the utility and 911. Underground leaks migrate along pipe trenches and can enter buildings some distance from the source.
How gas leaks present
Smell. Natural gas is odourless in its natural state, so utilities add mercaptan, the distinctive rotten-egg or sulphurous smell. It is deliberately unpleasant and easy to recognise. Propane is odourised similarly but is heavier than air, so it pools at low points rather than rising.
Sound. A hiss, a whistle or a roaring near a pipe, meter or appliance.
Visual signs. Bubbles in standing water or wet ground; dust or dirt blowing from a hole in the ground; dead or discoloured vegetation in a strip over a buried line; a white cloud or mist near a pipeline.
Appliance behaviour. Pilot lights that repeatedly go out; a yellow or orange lazy flame where it should be crisp and blue; soot or scorching around an appliance.
Physical symptoms in people or pets that improve on leaving the building.
An unexplained increase in gas consumption, which can indicate a slow leak.
One important caveat: not everyone can smell mercaptan reliably. Age, medication, congestion, smoking and simple olfactory fatigue all reduce sensitivity, and prolonged exposure dulls it further. A gas detector is therefore worth having even in a household with good noses.
Property already made safe by the utility? We repair the fault.
Get a gas fitter dispatchedCarbon monoxide is a different danger
These get conflated and they are not the same thing. A gas leak is unburned fuel escaping, it smells, and the primary risks are fire and explosion. Carbon monoxide is a product of incomplete combustion, it is odourless, colourless, and it poisons people.
CO is produced when a fuel-burning appliance does not burn cleanly or cannot vent properly: a blocked or damaged flue, a cracked heat exchanger, inadequate combustion air, a badly adjusted burner, or a chimney that no longer drafts correctly. It can come from a gas appliance, an oil appliance, a wood stove, or a car left running in an attached garage.
Warning signs at the appliance: yellow or orange flames instead of blue, soot or staining around the casing or flue, excessive condensation on windows in the boiler room, and a pilot that keeps going out. Warning signs in people: headaches, nausea, dizziness, confusion or unusual drowsiness that improves when you leave the building, and several occupants or pets unwell at the same time.
If a CO alarm sounds, treat it as real. Get everyone out, leave the door open, call 911 from outside, and do not re-enter to investigate. Once the property is cleared, the appliance must be inspected and repaired before it is used again.
Massachusetts law requires CO alarms on every level of a residence with fuel-burning appliances or an attached garage. They have a finite service life, typically five to ten years depending on the model, and an alarm past its date is not protecting anyone. Check the manufacture date on yours.
Where leaks happen
Interior leaks concentrate at connections rather than in the middle of sound pipe. The usual locations: threaded joints in black iron that have loosened or corroded; flexible appliance connectors that have been kinked, stressed or reused from a previous appliance; shut-off valves with worn packing; unions and transitions between materials; and the appliance connection itself.
Older systems accumulate risk. Pipework that has been extended repeatedly over decades has more joints, and some of those were made by people who should not have been making them. Capped-off branches left behind after an appliance was removed are a particularly common find, and a capped tee with a loose plug is a leak waiting to be noticed.
CSST that was never properly bonded, or that has been pierced by a nail or screw with no striker plate, is another category worth knowing about: see gas line installation and repair.
Exterior and buried leaks come from corrosion on older metallic services, damage from excavation, and ground movement stressing connections. Those are usually the utility's responsibility up to the meter, which is why their free response matters.
How leaks are located
Once the property is safe, locating the source precisely is straightforward for someone with the right equipment.
Electronic gas detectors, combustible gas indicators, sense methane and propane at very low concentrations and are used to sweep along pipe runs, joints and appliance connections. They find leaks far too small to smell.
Leak detection solution applied to joints under working pressure bubbles at the leak point. Simple, reliable and still the standard confirmation method.
Pressure testing isolates a section, pressurises it and watches a gauge. A pressure drop proves a leak exists in that section, and progressive isolation narrows down which part.
Never a flame. The old trick of testing joints with a lit match belongs to a category of advice that has killed people.
For buried exterior lines, tracing and surface sampling equipment is used along the route, and that work usually sits with the utility.
Alarms, detectors and prevention
Three pieces of hardware are worth having, and they do different jobs.
on every level, as Massachusetts requires, with the manufacture date checked and replaced at end of life. These protect against combustion problems, which is the more common domestic killer.
A combustible gas detector near gas appliances detects unburned gas, the thing your nose is supposed to catch but may not. Inexpensive, and genuinely worthwhile in a household with older occupants, anyone with reduced sense of smell, or a basement mechanical room nobody visits.
Smoke alarms, obviously, and interconnected where possible.
Beyond hardware, the preventive measures are unglamorous and effective. Annual service on every fuel-burning appliance, which catches combustion and flue problems before they become CO events. Keeping flue terminations clear of snow, ice and vegetation, which in Massachusetts winters is a real and recurring issue. Not storing anything against or around appliances that need combustion air. Having any gas work done by a licensed gas fitter with a pressure test and a permit. And knowing where your gas shut-off is, though remembering that in an actual gas smell, leaving the building takes priority over operating it.
See boiler repair for appliance faults and annual service, and water heater repair for flue and combustion issues on water heating.
CO alarm sounding? Everyone out, 911 from outside, do not go back in.
Get the appliance inspectedGas safety questions
Get everyone out of the building without touching any switch, phone or appliance, and without lighting anything. Leave doors open as you go.
From well outside, call your gas utility’s emergency line and 911. Do not go back in until you are told it is safe.
No. Utilities respond to suspected gas leaks as a free emergency service, twenty-four hours a day.
They make the property safe and establish whether the fault is on their side of the meter or yours. Repairs on your side are then a licensed gas fitter’s work.
A gas leak is unburned fuel escaping, it smells, and the risk is fire and explosion. Carbon monoxide is a product of incomplete combustion, odourless, colourless and poisonous.
They need different detectors, and a CO alarm will not detect a gas leak.
Typically five to ten years depending on the model, and the sensor degrades whether or not it has ever alarmed.
Check the manufacture date printed on the unit. An expired alarm is not protecting anyone.
It is worth investigating rather than repeatedly relighting. It can indicate a failing thermocouple, but it can also indicate a draft or venting problem.
Venting problems are how carbon monoxide events start, so it should be diagnosed properly.
Incomplete combustion. A gas flame should be crisp and blue; yellow or orange and lazy means the appliance is not burning cleanly and may be producing carbon monoxide.
Have it inspected before continuing to use it.
A plumber will use an approved leak detection solution on joints under pressure, and soapy water works on the same principle for a visible bubble test.
Never use a flame. And if you can already smell gas, do not test anything, leave the building.
Move away and call the utility and 911. Underground leaks travel along pipe trenches and can enter buildings some distance from the source.
Other outdoor signs are bubbling in wet ground, blowing dust from a hole, and a strip of dead vegetation over a buried line.
Located, repaired, retested and signed off
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