Carbon monoxide: the alarm every home needs, and where it goes

Carbon monoxide is what you get when a fuel burns without enough air. A furnace with a cracked heat exchanger, a water heater whose flue has come apart, a fireplace with a closed damper, a generator run in the garage, a car left idling with the door down. Every one of those puts an invisible, odorless gas into the house, and the symptoms it causes, headache, nausea, tiredness, confusion, are easy to mistake for the flu. People go to bed to sleep it off.

The fix is a ten-dollar alarm, and most states now require them. Here is what you need to know.

Where to put them

• One on every level of the house, including the basement.
• One outside every sleeping area, close enough that it wakes you.
• Within ten feet of the door to an attached garage.
• Not in the garage itself, not within fifteen feet of a fuel-burning appliance (it will nuisance-alarm on startup), and not in a dead corner or behind a curtain.

Carbon monoxide mixes evenly with air, so height does not matter the way it does with smoke. Follow the mounting height on the package; knee-high plug-in units work fine.

Which kind

A plug-in unit with a battery backup is the simplest. Combination smoke and carbon monoxide alarms cut the number of devices, and hardwired interconnected units that all sound together are the standard in new construction. A unit with a digital readout tells you the level, which helps the responders.

Every alarm has an end-of-life date, typically seven or ten years from manufacture, printed on the back. The sensor wears out whether the alarm has ever sounded or not. Write the date on the front when you install it.

When it sounds

Get everyone out, including pets. Call 911 or the fire department from outside. Do not go back in to open windows or hunt for the source. Anyone with a headache or nausea should be seen; the treatment is oxygen and the earlier the better. The fire department has meters and will find the cause; do not run the appliance again until a Pro has looked at it.

Where it comes from, and the Pro who checks

• The furnace. A heat exchanger cracks with age and every annual service should include a combustion test. If your Pro does not carry a combustion analyzer, find one who does.
• The water heater. Its flue can disconnect above the draft hood where nobody looks.
• The fireplace. A blocked chimney or a damper closed too early.
• Anything with an engine. Generators go outside, twenty feet from the house, exhaust pointed away. Every storm season brings deaths from a generator run in a garage with the door open. The open door is not enough.

The habit

Test every alarm on the first of the month, replace batteries when the clocks change, and replace the alarm on its date. The whole thing costs less than one service call and it is the cheapest insurance in the house.