[bluGill]

The law requires you have both types of good reason. Either alone will detect less than half of all house fires.

Dual sensors are not and gated. While nobody will admit what algorithm they use, they detect most fires unlike the single sensor type.

[Syonyk]

Where does the law require both types? I'm not aware of any housing codes specifically requiring photoelectric types, and any house I've looked at, including mine, came with purely ionization types. Though it's been a few years, and it may have changed recently - this is less of a niche concern lately.

As for dual sensors and gating... do you actually trust your life to "nobody will admit what algorithm they use"?

My house has all the smoke detectors wired together (they're on an AC circuit, with battery backup, with a signal line running between them all), so I have some photoelectric and some ionization, depending on where in the house they are.

[riceart]

Lol “the law” .. what law? Maybe in some dumb ass jurisdiction - but you’re a bit full of yourself if you think where you happen to live is “the law”.

[bluGill]

Us fire code, though inspectors often don't check.

[riceart]

Oh that I happen to know something about - you’re making stuff up. I’ve got NFPA 72 right here if you want to point out where this alleged requirement for “multi-sensor” detector exists.

In fact there are specific requirements to use only single sensor type alarms - such as near cooking equipment.

You can go the the NFPA website though where the publicly facing website notes they are “recommended”. They are not and have never been required.

[bluGill]

I'll stand corrected.

But I'll still call you a fool if you ignore the recommendations even if you are allowed to.

[riceart]

Then call yourself a fool since you must ignore at least one recommendation as they are conflicting.

This is not even an official recommendation in the code, just some stupid public education NFPA website, which doesn’t carry the same weight and for good reason. If ionization alarms are dumb enough that the Europeans or anywhere else in the world including the IAFF don’t recommend them at all, I’m ok with just following that. Dual sensor alarms have been shown in real world testing to perform worse. I have seen no evidence they perform better, but I have seen the opposite.

Fire codes and electrical codes are as much driven by industry (both union labor and manufacturers) lobbying in the US as much as actual good evidence based practice. Someone has stuff to sell, that is all. About 20 years ago when the sensible big push was made to migrate to photoelectric alarms it wasn’t long after that a new money-making opportunity was seen by now selling these dual contraptions.

“ In June 2014, tests by the Northeastern Ohio Fire Prevention Association (NEOFPA) on residential smoke alarms were broadcast on Good Morning America program. The NEOFPA tests showed ionization smoke alarms were failing to activate in the early, smoldering stage of a fire. The combination ionization/photoelectric alarms failed to activate for an average of over 20 minutes after the stand-alone photoelectric smoke alarms. This vindicated the June 2006 official position of the Australasian Fire & Emergency Service Authorities Council (AFAC) and the October 2008 official position of the International Association of Fire Fighters (IAFF). Both the AFAC and the IAFF recommend photoelectric smoke alarms, but not combination ionization/photoelectric smoke alarms.”

From the IAFF:

Which one should you buy? The International Association of Firefighters (IAFF), the largest firefighter’s union in the US and Canada has adopted an official position recommending only Photoelectric Smoke Detectors and has stated that dual sensor alarms are no longer acceptable. The technology used in Ionization Smoke Detectors creates a delayed warning in smoldering fires which can lead to loss of life. Photoelectric Smoke Alarms are more effective at warning of smoke from smoldering fires and are less susceptible of nuisance alarms. The IAFF recommends replacing all ionization, dual sensor and unknown alarms with photoelectric smoke alarms.

Notably Iowa fire code had required dual sensor alarms and had to back pedal that last year. Apparently NFPA public outreach hasn’t gotten the memo.

> Either alone will detect less than half of all house fires.

This is nonsense. Ionization sensor may detect certain fires seconds earlier according to NIST testing and those are not even the deadliest types of house fires. No reputable body would recommend PE sensors only if that were true.

[Syonyk]

As far as I can tell, it's state by state, so... you want to cite some sources?