How to choose

Some insurers offer a 5 percent discount if your home has smoke alarms. Install both types of smoke sensors and a CO alarm on every level and a smoke alarm in the attic and in all bedrooms and hallways. Our top picks focus on interconnecting models. Here's what else to consider:

Pick your power

Hardwired systems tie into a home's wiring and require professional installation (about $250 per unit). Battery-only alarms and plug-in CO alarms are easy do-it-yourself projects.

Look for key features

Smoke alarms should have a hush button, safer than disabling the alarm by removing the power during a false alarm. On CO alarms, look for a digital display showing CO levels in parts per million; levels as low as 30 ppm can harm heart patients, pregnant women, and children. Some show peak levels that occurred while you were out.

Balance convenience and cost

First Alert's SCO501CN and Kidde's KN-COSM-1B combine CO and smoke detection. But each covers only one type of fire and, like all CO alarms, should be replaced every five years; smoke alarms, every 10 years.