Smoke detectors and carbon monoxide detectors
Let's begin with smoke detectors. I first started working in the electrical trade in 1996. The code that was in effect back then was that every house had to have one smoke detector installed per floor in the home. If you had a single-story house, you only needed one smoke detector total. If you had a two-story house, there would be a total of two—one on each floor.
Today, things have changed a lot. Current code requires one smoke detector on every floor, one in every bedroom, and one within 10 feet of each bedroom door (outside the bedroom). All smoke detectors will be interconnected as well. If one alarm goes off, it sends a signal to all the others.
Here are a few important things to consider about smoke detectors. Install all the same make and model because the detectors are all interconnected. They must recognize each other or you can end up with nuisance tripping of the alarms—they will start chirping. Normally chirping smoke detectors indicates a weak battery that needs to be replaced, but if you mix makes and models you can have a real hassle. Also, smoke detectors have a ten-year shelf life and need to be thrown away and replaced after ten years of the manufacture date—that date is found on the back of most models of smoke detectors. See the photo below as an example of where to find the manufacture date.
You can purchase a combination smoke/carbon monoxide detector that does both jobs all in one. However, there are things to consider if you decide to install those.
First, combination detectors cost twice as much as a smoke detector. If you are going to throw them away in 10 years, that's twice the money you'd spend compared to replacing a regular smoke detector.
Second, you can choose to plug carbon monoxide detectors into a receptacle, if there is one available outside the bedroom door within 10 feet. You don't need to wire anything this way and it might save your life better than a ceiling model. CO or carbon monoxide is heavier than air so it will accumulate at the floor level first. It would make more sense to thus put a carbon monoxide detector where the gas is more likely to be—near the floor. The instructions for the ceiling model explain that most people today have a furnace and that it needs to turn on and circulate several times a day. That will cause the air to be moved around enough to make the ceiling alarm go off if there is a high level of carbon monoxide. If it were my choice, I would install my carbon monoxide detector into a receptacle near the floor!