Guide · Home safety

Combustible gas detectors for homes near wells

Methane, propane and hydrogen sulfide behave differently, and a detector in the wrong place is close to no detector at all. Here is what to buy, where to mount it, and what it will not do for you.

Educational guidance · Follow your manufacturer's instructions and local code

If you smell gas right now

Leave the building first. Do not troubleshoot.

Get everyone out immediately, including pets.
Touch nothing electrical on the way. No light switches, no appliances, no garage door opener, no phone inside the building. A spark is all it takes.
Leave doors open as you go if you can do it without delay.
From outside and well away, call 911 or your gas utility's emergency line.
Do not go back in until a professional tells you it is safe.

SoCalGas emergency 1-800-427-2200
PG&E emergency 1-800-743-5000
Verify the number for your own utility — these can change.

If you live near an oilfield, an abandoned well, or a pipeline, there is a second source of flammable gas beyond your appliances. Gas can migrate through soil and enter a building through foundation cracks, floor drains and utility penetrations, and it collects in the parts of a house people spend the least time in. A detector is cheap, and it catches what a nose cannot.

The gases, and why placement follows density

The single most consequential decision is mounting height, and it is dictated by whether the gas is lighter or heavier than air.

Gas	Common sources	Vs. air	Main risk	Mount
Methane CH₄	Natural gas lines and appliances, oil and gas wells, landfills, sewer gas	↑ Lighter	Explosion. Displaces oxygen in confined spaces.	HIGH 4–12 in below ceiling
Propane / LPG C₃H₈	Propane tanks, rural heating and cooking systems, barbecues	↓ Heavier	Explosion. Pools in low spots and stays there.	LOW 6–18 in above floor
Hydrogen sulfide H₂S	Sewer and septic gas, some oil and gas operations, decaying organic matter	↓ Heavier	Acutely toxic well below the flammable range. Poisoning, not explosion, is the danger.	LOW 6–18 in above floor
Other combustibles	Hydrogen, gasoline and solvent vapours, refrigerants, ethane	Varies	Fire and explosion	By density

Mount for the gas you are actually worried about. A methane detector at floor level and a propane detector at ceiling level are both close to useless.

Hydrogen sulfide deserves its own warning.

H₂S smells of rotten eggs at low concentrations, but it deadens your sense of smell as the concentration climbs. An H₂S odour that fades on its own is a reason to leave faster, not to relax. It is also dangerous at concentrations far below the point where a combustible gas sensor would alarm, which means a standard explosive gas detector can miss a genuinely harmful level entirely. If H₂S is a real concern where you live, buy a dedicated sensor or a multi-gas monitor rather than relying on a combustible unit.

Choosing a unit

Look for a device explicitly labelled for combustible gas, explosive gas, natural gas and propane, or LEL — lower explosive limit. A carbon monoxide alarm is not a substitute and will not alarm on any of the gases above.

Covers methane and propaneMost decent home units cover both, since the sensor responds across a range of hydrocarbons.

Plug-in with battery backup, or a long-life sealed batteryContinuous monitoring matters more here than with a smoke alarm, because migration is often slow and steady.

Certified to a recognised standardUL 1484 covers residential gas detectors. Check the housing rather than the marketing copy.

Loud audible alarm plus a visual indicatorYou need it to wake you.

Consider a combination CO unitConvenient, though verify it genuinely detects combustible gas and is not a CO-only alarm with vague packaging.

A portable detector as a supplementFixed units protect continuously. A portable one with a probe is what you use to sweep a crawl space or check around a foundation penetration.

Where to put them

Basements, crawl spaces and utility rooms first. These are the highest priority near wells and the places migrating gas collects.
Near gas appliances — furnace, water heater, stove, dryer — but three to ten feet away, and never directly above a burner.
At least one audible from where you sleep.
Attached garages containing gas meters, lines or appliances.
Near entry points: foundation cracks, floor drains, sumps and utility penetrations.

Avoid drafty spots near windows, doors, vents and HVAC registers, which dilute gas and delay an alarm. Avoid dusty, greasy or extreme-temperature locations, and anywhere directly above a cooking surface, which causes nuisance alarms and eventually gets the unit unplugged.

Maintenance

Test with the button on a set schedule, monthly at minimum.
Keep the sensor clear of dust, grease and paint. Never paint over one.
Replace the whole unit at end of service life, typically five to ten years. Sensors degrade whether or not they have ever alarmed.
Note the manufacture or replace-by date on the housing when you install it.

What a detector will not do

No consumer detector catches everything. Slow seepage, unusual airflow, or concentrations below the alarm threshold can go unnoticed, and a unit placed for the wrong gas may never alarm at all. More importantly for anyone reading this because of a nearby well:

A detector tells you gas has already arrived. It tells you nothing about what is underneath the property.

It cannot say whether a well exists beneath the lot, when it was drilled, how it was sealed, or whether that seal would satisfy current standards. Those are records questions, and they are answered from the state well file rather than from a sensor on the wall.

Pair a detector with regular professional inspection of gas appliances, a search of the state well records for the property, sealing and ventilation work where migration is a concern, and your utility's leak survey if one is offered.

Frequently asked questions

Is a carbon monoxide detector enough?

No. A CO alarm detects carbon monoxide only. It will not alarm on methane, propane, or hydrogen sulfide. If you want coverage for flammable gases you need a combustible gas detector, either as a separate unit or as a combination CO and explosive gas alarm.

Should I buy a methane-only detector or a combustible gas detector?

A combustible gas detector covering methane and propane suits most homes. These sensors respond across a range of hydrocarbons, so one unit covers natural gas, propane and many solvent vapours.

Where should the detector go in a basement?

High on the wall, roughly four to twelve inches below the ceiling, because methane rises. Add a second unit low to the floor if you also have propane, or if sewer gas is a concern, since those are heavier than air and pool at floor level.

Do combustible gas detectors detect hydrogen sulfide?

Sometimes, but not reliably and not at the concentrations that matter. Hydrogen sulfide is dangerously toxic far below the level at which it becomes flammable, so a combustible gas sensor may never alarm before the gas has become harmful. If H2S is a real concern, use a dedicated sensor or a multi-gas monitor.

Does a gas detector mean I do not need to check for wells?

No. A detector warns you about gas that has already reached your living space. It tells you nothing about what is beneath the property or how it was sealed. The two are complementary.

How long do these detectors last?

Sensors degrade. Most units carry a service life of roughly five to ten years and should be replaced entirely at the end of it, not just re-batteried. Check the date stamp on the housing.

What if the rotten egg smell goes away on its own?

Treat that as more serious, not less. Hydrogen sulfide deadens the sense of smell at higher concentrations, so an odour that fades can mean the concentration has risen past the point where you can detect it. Leave and call for help.

Find out what is recorded under the property

Search any California address against the state oil and gas well database. Free, instant, no account required.

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This page is educational and reflects general industry and manufacturer guidance. It is not a substitute for the instructions supplied with your detector, your local building and fire code, or advice from a qualified professional or your gas utility. Detectors are one layer of protection and do not replace appliance maintenance, well research, or an emergency plan. In an emergency, leave the building and call 911 or your utility from outside.