Carbon monoxide alarm, digital display

Carbon monoxide alarms are required to stay silent below thirty parts per million, which means a house can sit at a level that causes headaches and nausea for months with every alarm in it behaving exactly as designed. A digital display shows the current reading, and peak-level memory shows the highest concentration recorded since it was last cleared — which is the number to give a technician.

Electrochemical sensor, seven-year service life with an end-of-life warning, and a battery backup that keeps it alive through an outage.

Fitting and care. Carbon monoxide mixes with air rather than rising, so height is less critical than distance: mount it at breathing height in a hallway outside the bedrooms, at least fifteen feet from a fuel-burning appliance to avoid nuisance readings on start-up. Clear the peak memory after any reading so the next number means something. Test monthly, vacuum the vents twice a year. A reading of 30 ppm or above that persists is a call to your gas supplier, not a wait-and-see; the display exists so you can give them a number.