It's important not to ignore the chirping, that's why it's so annoying. If you ignore it long enough, it will stop -- because the battery is dead and you're no longer protected!
So when the chirping starts, pick up a battery at the store and swap it out. If you're not sure how to do it, contact the manufacturer, ask the guy at the hardware store or call the nonemergency line at the fire department for advice.
Do not call 911 because your battery is low.
Carbon monoxide alarms are a little different than smoke alarms. When a smoke alarm starts beeping, you can usually see the smoke that's causing it. When a carbon monoxide alarm starts beeping, there's nothing to see. So it's important to call 911 for a beeping carbon monoxide alarm -- just not a chirping one. If either kind of alarm starts beeping continuously, it's time to get out of the house.
So, to sum up:
- Get out of the house and call 911 if either the smoke alarm or the carbon monoxide alarm starts beeping loudly.
- Change the battery if either alarm chirps one quick little beep every few minutes.
- If you need more advice, call the nonemergency phone number for your fire department.
To avoid this problem entirely, change your alarm batteries twice a year. The fire service recommends you change batteries when you change your clocks. Keeping fresh batteries in your smoke alarms and carbon monoxide alarms promotes peace of mind, not to mention avoiding that annoying chirping sound.