In an interconnected system, when one alarm is triggered, every alarm in the house sounds. That extra warning can make a critical difference. A single battery-powered alarm sounding in a distant hallway while people are sleeping may go unnoticed until the fire gets closer. But when every alarm simultaneously activates throughout the home, occupants are alerted immediately. Which is essential when every second counts.
When a Massachusetts homeowner wanted to upgrade her home’s outdated smoke detection system to meet current safety standards, master electrician Heath Eastman stepped in to help pick out and install an updated system.
Because fire alarm code compliance and requirements vary by location, homeowners should always verify specific requirements with their local fire department.
Types of Smoke Detectors
Residential fire detectors are classified by their sensing technology (how they detect fire) and their power source.
Smoke Detection Technologies
Photoelectric Detectors
Detect smoke particles in the air using a light sensor. They are especially effective at detecting slow-burning, smoldering fires that produce thick smoke before the flames begin. For example, those involving overheated appliances, electrical wiring, and mattresses.
Photoelectric alarms tend to produce fewer nuisance alarms from things like cooking smoke or shower steam.
Ionization Detectors
Use a small amount of radioactive material to detect changes in air particles caused by smoke. They respond fastest to rapid, flammable fires, such as those involving wood, paper, and other combustibles.
However, ionization detectors are more prone to false alarms, especially near kitchens.
Because of advances in photoelectric detector technology, some jurisdictions are phasing out ionization detectors and even dual-sensor combo units that combine photoelectric and ionization.
Heat Detectors
Heat detectors do not sense smoke. Instead, they activate when temperatures rise quickly or exceed a set threshold.
They are commonly installed in areas where smoke detectors might trigger false alarms, like in garages, workshops, and attics.
Because they respond to heat rather than smoke, heat detectors are typically used in addition to smoke alarms, not as replacements for them.
Combination Smoke and Carbon Monoxide Detectors
Carbon monoxide (CO) is a colorless, odorless gas produced by fuel-burning appliances like gas stoves, furnaces, and boilers.
A combo smoke and carbon monoxide detector protects against both hazards while reducing the number of devices installed.
Power Sources: Battery vs. Hardwired Smoke Detectors
Battery-Operated Smoke Detectors
Battery-only smoke alarms are common in older homes; however, they have a few drawbacks:
- Batteries must be replaced regularly
- A dead battery can set off the alarm. Leading people to just remove the battery instead of replacing it.
- Individual alarms do not alert the entire home.
Hardwired Smoke Detectors
Hardwired smoke alarms connect directly to a home’s electrical system and include a battery backup in case of power outages. Most modern code-compliant systems are also interconnected, meaning if one alarm detects smoke, all alarms in the house sound. This provides an earlier warning throughout the house. Hardwired interconnected alarms are required in most new construction and major renovations across the United States.
Where Should Smoke Detectors Be Installed?
Exact smoke alarm placement depends on local building codes, so homeowners should always confirm requirements with their local fire department.
However, most U.S. building codes follow similar guidelines.
Protect the Escape Route
Smoke alarms should protect the path people will use to exit the home during a fire. The goal is to ensure smoke will trigger an alarm before it blocks the escape route from bedrooms to exits.
Inside Bedrooms
Most codes require a smoke alarm inside every bedroom.
Outside Sleeping Areas
Smoke alarms should also be placed right outside bedrooms. In the segment, the local fire chief recommended 3-5 feet away from bedroom doors.
Every Level of the Home
Install at least one smoke alarm on every level of the home, including basements and finished attics.
Look up local requirements on how many smoke detectors need to be installed per square feet of living space.
Stairways
Smoke detectors should be installed at the base of every stairway.
Attached Garages
Smoke detectors are generally not recommended for garages due to dust, humidity, and vehicle exhaust, which can trigger false alarms.
Instead, interconnected heat detectors are typically recommended.
In-Law Apartments
If an in-law apartment is part of the same structure as the home, its alarms should be interconnected with the rest of the house.
Keep Detectors Away from Vents
Avoid placing smoke detectors near supply vents or areas of strong airflow.
Air movement can:
- Push the smoke away from the detector
- Cause dust buildup inside the sensor
How to Replace Battery Smoke Detectors with Hardwired Alarms
Upgrading battery-operated smoke alarms to a hardwired, interconnected system typically requires running new electrical wiring through walls or ceilings. Because this involves electrical work and must meet local building codes, it’s recommended that a licensed electrician handle the installation.
Below is an overview of how the installation process typically works:
- Plan Detector Locations. Determine where the new detectors will be installed. Placement should follow the local fire code requirements.
- Remove Old Detectors. If existing detectors are battery-operated, remove them by:
Twisting the detector off its mounting base
Remove the mounting bracket from the ceiling or wall - Choose a Power Source. Hardwired smoke detectors need a constant power supply. While the system doesn’t need its own dedicated circuit, it is best practice.
If you are wiring them into an existing circuit, choose a reliable, minimal-load 120-volt circuit to wire all the detectors into. Lighting circuits are typically good options. The goal is to connect the alarms to a circuit that will not be accidentally switched off.
The wiring from this power source will serve as the starting point for the smoke alarm circuit. - Plan the Wiring Route. Determine the easiest path for running cable from the power source and between detectors.
In many homes, like the one in the segment, the attic provides the most accessible route for running cable above the ceilings. Heath used a fiberglass fish stick to find a clear route from the basement to the attic.
Planning the route in advance helps minimize drywall cutting. Turn off the power at the electrical breaker before beginning any work that involves cutting through walls and ceilings. - Check for Obstructions for Each Electrical Box. Each new detector needs an electrical box installed. Before drilling a hole, check for obstructions such as joists, pipes, or wiring above the drywall.
Drill a small pilot hole in the center of the proposed detector location. Heath used a piece of insulation support tie wire, bent at a 90-degree angle, as a feeler bit. Once inserted into the hole, he rotated it to check for obstructions in the space above the drywall he was about to cut out.
If the area is clear, the hole can be cut safely. - Install Electrical Boxes. To cut for the box, Heath used a drill, a 3-¾” hole saw, and a dust collection bowl attached to the drill to catch drywall debris.
Heath installed 1-gang 3-¾” round old work ceiling boxes for this installation. Old work electrical boxes are designed for retrofit installations and include flip-out mounting tabs or clamps. When the screws are tightened, the tabs swing outward and clamp against the back of the drywall without needing to nail into a stud.
Once the electrical boxes are installed in each location, run and pull cable through each. - Run Wiring Between Detector Locations. Hardwired smoke alarms must be connected to a 120-volt power source and wired together so they can communicate with each other
Heath used a 14/2 NM-B cable from the power source to the first smoke detector. 14/2 has two conductors, one black (hot) and one white (neutral). Plus, a copper (ground).
He used 14/3 NM-B to connect the remaining alarms. 14/3 has three conductors, one black (hot), one red (second hot/traveler), and one white (neutral). Plus, a copper. The red wire serves as an interconnect, allowing all the detectors to communicate with each other.
From the first smoke detector, the 14/3 cable will be run to each detector to daisy-chain the alarms. - Make Wiring Connections. Once the cable has been run to each electrical box, remove the outer jacket and strip the insulation from the individual wires using a wire-stripping tool.
The wires from incoming and outgoing cables are connected together using wire nuts/connectors. Attach the hot, neutral, and interconnect wires. - Install Detector Mounting Bracket. Each smoke alarm comes with a mounting bracket and wiring harness.
Attach the mounting bracket to the electrical box using the provided screws. Then plug the smoke alarm harness into the back of the detector.
Once connected, the detector is twisted onto the mounting bracket until it locks into place. - Restore Power and Test the System. After all the alarms are installed, restore power at the electrical breaker.
Each detector should then be tested using the unit’s test button. In an interconnected system, pressing the test button on one alarm should trigger all alarms throughout the home.
This confirms that both the power and interconnect wiring are working properly.
Resources
Codes vary by location. Check with your local fire department to learn the local requirements, including questions about which detectors to purchase and where to install them.
Smoke Detectors Installed
- Combo Smoke & Carbon Monoxide Alarms = BRK SMC0110V-AC – Heath installed at least one on every floor, and one installed within 10 feet of each bedroom door.
- Smoke Detectors = BRK SM110V-AC – Heath installed smoke detectors inside the bedrooms, at the base of each stairway, and made sure one was installed within 1,200 square feet of living space for each floor level.
Cut Holes for Detectors
- Tape Measure
- Nonconductive Fiberglass 4 ft. Ladder
- Safety Glasses
- Hearing Protection
- Insulation Supports – Heath used as a feeler bit to make sure the area above the ceiling was free of obstructions before drilling.
- Drill
- 3-¾” Hole Saw
- Arbor Bit with Pilot Drill Bit
- Dust Bowl for Hole Saw
- Plastic 1-Gang 3-¾” Round Old Work Bracket Box
- Screwdrivers
- Wire Detectors
- Fiberglass Fish Stick
- Electrical Tape
- 14/2 NM-B Wire – to run from power source to the first detector
- 14/3 NM-B Copper Building Wire — to run wire between all detectors
- Universal Wire Stripping Tool
- Linesman Pliers
- Diagonal Cutting Pliers
- Tan/Red Wire Connectors
