Ordinary smoke alarms will only do this one thing
Protecting your home from fire is a chief concern for homeowners, and smoke detectors play a critical role in shielding your family. However, smoke detectors have their limitations. To illustrate, they can only detect smoke, not heat. When there is an emergency in your residence, you may not be cautioned before the smoke reaches the smoke detector. Although there are some other telltale signs of fire -- such as a sharp increase in warmth -- if there’s a lack of smoke, you won’t have a triggered smoke detector.
Additionally, smoke detectors only sound the alarm if they find enough smoke. When a fire creeps along slowly at first, you might not be warned until it's too late. Various smoke detectors use more than one sensor, meaning they will be able to perceive smoke from both a raging fire and a more gradual burn. Then when they start beeping, it falls to the resident to contact the fire department after they safely leave the residence.