Should I replace smoke detectors in an older home with hardwired alarms?

Short answer

Why people ask this

Because the home is older, owners worry about how hardwired alarms fit with legacy wiring, plaster-and-lath ceilings, and code requirements during a retrofit. They also wonder if they must open walls or if exceptions apply when upgrading detectors without a full remodel. Hardwired, interconnected alarms can alert the whole house faster, but older homes may have knob-and-tube or brittle cloth wiring that complicates the project. People want to know if permits, existing box depth, and locations (hallways, bedrooms, and basements) will pass inspection under NFPA 72 and local code.

When it might be safe

• Use hardwired alarms with battery backup specifically rated for retrofit and interconnect on a dedicated or lighting circuit per local code
• Run new cable using surface raceway or attic/basement access to avoid excessive demolition of plaster-and-lath
• Use listed wireless interconnect bridge units to tie hardwired zones together when one area can’t be hardwired

When it is not safe

• Extending or tapping into knob-and-tube or deteriorated cloth-insulated wiring to power new alarms
• Installing hardwired alarms without the required battery backup or interconnect where code requires it
• Burying splices behind plaster without junction boxes and covers
• Mounting alarms in dead-air spaces (within ceiling peaks or too close to bathrooms/kitchens) contrary to NFPA 72 placement rules

Possible risks

• Disturbing plaster-and-lath, lead paint, or possible asbestos-containing materials when fishing cable or cutting openings
• Power outages or a tripped breaker disabling non–battery-backup units, reducing protection until restored
• Code compliance failures if locations, interconnect, or power source don’t meet local amendments to NFPA 72/IRC
• False or nuisance alarms if placed near older poorly vented kitchens or damp bathrooms in vintage layouts

Safer alternatives

• Wireless interconnected smoke/CO alarms (10-year sealed battery) where running new cable is impractical
• Smart alarms that provide app alerts and interconnect wirelessly while meeting UL/NFPA listings
• Partial hardwire: hardwire where accessible (basement, hall ceilings via attic) and use wireless interconnect to reach closed-off rooms
• Professional alarm system with monitored smoke detectors using listed wireless sensors tied to a control panel

Bottom line

Yes—hardwired, interconnected alarms with battery backup are a strong safety upgrade for older homes, but plan for retrofit realities: avoid legacy wiring, follow NFPA 72/local code for placement and interconnect, and use raceways or wireless interconnect where opening plaster isn’t feasible. Pull permits and have a qualified electrician assess wiring conditions before starting.

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