A detector that is mounted in the wrong place or never tested is a decoration. Installing smoke and carbon monoxide units is a short job if you handle it in the right order: plan the locations, confirm the type, mount to the manual, test, and then write down who owns the routine.
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Map the rooms before you drill
Walk the house with a pencil and a sheet of paper. Mark each sleeping area, the hallway that serves it, each level of the house, and any space where a fuel-burning appliance sits. The manual that comes with each unit sets the mounting distances from walls, corners, vents, and kitchens, so follow those numbers rather than any rule of thumb. For questions about what local code requires, a St. Louis area fire district or the local building office can answer, and Local Help in St. Louis lists where to start.
Confirm battery or hardwired before you buy the bracket
A battery unit mounts with a bracket and screws. A hardwired unit connects to household wiring and often to other units in the house so that when one sounds, all of them do. Hardwired work is electrical work. If the unit needs wires, hire a licensed electrician or have one do the connection under the local rules. Do not improvise the wiring to save a visit.
Mount to the manual
Use the bracket supplied with the unit, set the screws into solid framing or the anchor the manual calls for, and keep the unit away from vents and the spot where the kitchen steam reaches. Check that the unit is level and that nothing blocks the sensor opening. Write the mounting location on the sheet as you go so the next person knows what was done.
Test each unit the way the manual says
Press the test button on every smoke unit and listen for the full alarm. Use the test method the manual gives for each carbon monoxide unit, which is often a button and a waiting period. Confirm that interconnected units all sound together. Replace any unit that fails the test and note its serial or model on the sheet.
Name the owner of the routine
Testing, cleaning, and replacement are not one-time jobs. Write the testing routine on the same sheet, with the name of the person responsible for it. Keep the install paper, the model numbers, and the location map in one folder that you can find without a search. See Sources for how we check manufacturer instructions before we rely on them.
Next move: Walk the house tonight with the manual in hand, mark each unit location in pencil, and confirm that every sleeping area has a unit outside it. Then order only the brackets and units the map calls for.