Natural gas and propane detector

A carbon monoxide alarm detects a product of incomplete combustion. It will not detect a gas leak, because unburnt methane or propane produces no CO at all — and a great many houses are covered for smoke and CO and completely uncovered for the leak that causes an explosion.

Semiconductor sensor calibrated for methane and propane, with a plug-in body and a battery backup. Mount high for natural gas, low for propane; the instructions say which and why, because getting it upside down makes the unit useless.

Fitting and care. Height depends on the gas and getting it wrong makes the unit useless: natural gas is lighter than air, so mount high, within a foot of the ceiling; propane is heavier, so mount low, within a foot of the floor. If you have both, you need two. Keep it at least ten feet from the appliance to avoid nuisance alarms on ignition. Do not fit it in a cupboard with the boiler. The sensor has a service life of around five years with the date on the body.