We’re thinking of buying/installing a carbon monoxide detector.
It’s a two-story house (no basement). We have pets (three cats). Where is the best place to install a detector? First floor? Second floor? Near the ceiling or near the floor?
Where is the furnace, and how is it fired (oil, gas, electric)? Is the hot water heater electric or gas, and where is it located? Are there any other appliances which use oil or gas, and where are they located?
The answers to these questions will help determine the best location for your CO detector.
Gas furnace and gas water heater located in the garage (attached to house). Oh, yeah. A gas fireplace on the first floor as well.
Honestly, you probably don’t need one at all. The water heater and furnace seem to be well isolated, unless there are large-ish openings for CO emissions to escape the garage into the house. Gas fireplaces are very safe as well, provided your chimney is properly maintained, since the natural draft action of the rising hot gases suck all the emissions right out of the house. However, if you want an extra level of safety, I’d install one on the ceiling at the top of the stairs on the second floor, assuming that’s where everyone sleeps. If there are bedrooms on the first floor as well, put another one on the ceiling near them.
Here is good article on proper placement of CO detectors which will guide you further.
Super.
Thanks!
Super.
Thanks!
You only had to thank me once. 
Give your local fire department a call. They can tell you what is required by the current code.