Choosing between photoelectric and ionization smoke detectors

If you’re serious about residential fire safety, early detection and escape are obviously high on the list in your fire protection strategy.  The smoke and flame detection problem is more complex than just verifying that a smoke detector is on the ceiling.  One has to consider the types of fires to detect, features, and frequency of nuisance alarms.

Innovation in the smoke detector market has been minimal with Nest Protect (2nd gen) being the notable exception.  If you’re like me and avoid cloud-enabled security devices your choice is limited to First Alert and Kidde.



There are no residential detectors on the market that fulfill all the desired requirements of:

  • Photoelectric smoke (smoldering fires)
  • Ionization chamber (flaming fires)
  • CO detection of 50ppm for 60 second period
  • Interconnected (when one alarm triggers, all detectors sound)
  • Low/no false alarms
  • Voice announce (e.g., WARNING: FIRE!)
  • Location announce (e.g., WARNING:  FIRE IN BASEMENT!)
  • Escape light (bright light when alarm goes off)

So making the least amount of compromises is desired.


What Kills You

  • Most likely you’ll die from smoke inhalation long before the heat from flames reach you.  House fires create smoke particulates, heat, and gas effluents (water vapor, CO2, CO).
  • Synthetic materials release smoke faster than natural materials.
  • While furniture treated with fire retardants resist going up in flames, it creates more smoke as it smolders longer.
  • In the last 20 years home contents smolder more and flame less


Photoelectric vs. Ionization

In a perfect world there would be a UL-listed triple-detector unit combining a photoelectric chamber, high-sensitivity ionization chamber, and CO sensor.  Unfortunately this doesn’t exist on the market yet.  In case you are unaware or forgot the differences between detectors, here’s a snapshot:

Photoelectric Ionization
smoke-photoelectric smoke-ionization
Faster Detection For… Non-flaming fires (smoldering) Fast-flaming fires
Technology LED in smoke chamber Americium-241 isotope in chamber
Method Detect light scattering caused by smoke particulates Detect changes in ionization field from smoke
Strength Triggers much (5-30 minutes) faster for smoldering fires Triggers a few seconds to 2 minutes faster for some flaming materials
Weakness Will trigger for flaming fires, but slower than ionization Won’t trigger for many smoldering materials
Material Examples Pine, mattress foam, polystyrene Newspaper, carpet, douglas fir

Look at the photoelectric performance for a smoldering fire — often 30+ minutes earlier detection!  What a difference that makes between life or death and massive or minor property damage.

Most homes have the cheapest featureless ionization detectors general contractors can find.  If you don’t know what detector type is installed, look for a “P” (photoelectric) or “I” (ionization).  Worst case, do a DuckDuckGo search for the model number and find the specs.

If you have to pick one type, choose photoelectric.


DOs and DON’Ts

Prefer photoelectric detectors over ionization Only have ionization detectors
Use both detector types, if able Buy combo photoelectric/ionization detectors
Use wired or wireless interconnected system Disable your detector. Move its location to avoid false alarms or replace it.
Conduct periodic tests
If source of smoke is unknown (e.g., not burnt toast) and detector triggers, GET OUT


Determine Your Requirements

Ideally your home has AC power to each smoke detector location and the wire contains a hot (black), neutral (white), and interconnect (red or orange).  In order of preference:

  1. Hardwire AC with battery backup and wired interconnect (using the existing red or orange wire)
  2. Hardwire AC with battery backup and wireless interconnect (e.g., First Alert “OneLink”)
  3. Battery-only with wireless interconnect
    • Some models include a 10-year non-servicable battery, but this limits your options to fully power down the unit when troubleshooting nuisance alarms


Make a Plan and Purchase

Here’s an example plan.

Floor Location Mfg Model Type Action
1st Kitchen Kidde KN-COSM-IBA CO, Ionization Smoke (existing) Keep
1st Media Room First Alert SA521CN Photoelectric Smoke, Hardwire + OneLink gateway.  This interconnects to the CO511B. New
1st Garage First Alert CO511B CO, Photoelectric Smoke, Battery only, OneLink New
2nd Hallway First Alert SC7010BV CO, Photoelectric Smoke, Voice, Hardwire New
2nd Bedroom First Alert SC7010BV CO, Photoelectric Smoke, Voice, Hardwire New
2nd Bedroom First Alert SC7010BV CO, Photoelectric Smoke, Voice, Hardwire New
2nd Bedroom First Alert SC7010BV CO, Photoelectric Smoke, Voice, Hardwire New

Note:  As of 1/2016, First Alert has NO highly rated simple hardwired photoelectric detector.

If you have no issues with cloud-enabled devices in your home, the 2nd Gen Nest Protect has compelling features and reportedly great performance.


Final Thoughts

I really like the location announcement feature.  I immediately know which detector triggered the alarm which is great information to have when planning an escape — especially if woken up in the middle of the night.  Is the fire upstairs?  Downstairs?  Which exit should I take?  All questions answered easier if you know location.

I’ll be adding a 2nd unit upstairs that is a battery-only ionization detector so that I’ll have at least one on each floor.

Selecting new smoke detectors for your home takes a bit of time, especially with poor reviews on the models you need.  However, the additional protection is worth it.


Further Reading

FPRF / UL Smoke Characterization Project (2007)

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