Dual sensors are not and gated. While nobody will admit what algorithm they use, they detect most fires unlike the single sensor type.
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.
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.
But I'll still call you a fool if you ignore the recommendations even if you are allowed to.
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.
So I guess you’ll have to call yourself a fool since you must ignore one of these.
Notably Iowa fire code had required dual sensor alarms and had to back pedal that last year. I guess the NFPA public outreach hasn’t gotten the memo.
There's been quite a bit of research done, generally easy to find if you look, that talks about the difference and tests them, but the short summary:
- Ionization type sensors detect the products of fast flaming combustion and "things cooking in the kitchen." Your oven, if a bit dirty, will reliably trip an ionization type. They are quick on the draw for this. The downside is that they're very, very poor at detecting the sort of slow, smoking, smoldering combustion that is associated with house fires that kill people in the middle of the night.
- The photoelectric type is very good at detecting smoke in the air - but it isn't nearly as prone to false triggers on ovens, a burner burning some spills off, etc.
They've been A/B tested in a wide variety of conditions, and in some cases, the ionization type is a bit quicker. In other cases, the ionization type is slower, by time ranges north of half an hour - I've seen some test reports where there was a 45 minute gap, while the photoelectric type was going off, before the ionization type fired!
In general, "rapid fires during the day" are somewhat destructive to property, but rarely kill people. If your kitchen catches on fire while you're cooking, it may burn the house down, but generally people are able to get out.
The fires that kill people are "slow starting fires during the night" - the sort that smolder for potentially hours, often slowly filling the house with toxic smoke, before actually bursting into open flames. On this sort of fire, the photoelectric type will fire long, long before the ionization type - in some cases, they get around to alarming quite literally "after the occupants are dead from the smoke."
Using smoke alarms as a way to talk about monitoring systems is nice, but in terms of actual smoke detectors, get at least a few photoelectric sorts in the main areas of your home.
Do not get the "combined sensor" sort, since these tend to be and-gated and the worst of both worlds.
Edited to add some resources:
A presentation on the matter from a while back by one of the experts in this field: https://wahigroup.com/Resources/Documents/Ion%20vs%20Photo%2...
Another paper: https://www.semanticscholar.org/paper/Detection-of-Smoke-%3A...
> Full-scale fire tests are carried out to study the effectiveness of the various types of smoke detectors to provide an early warning of a fire. Both optical smoke detectors and ionization smoke detectors have been used. Alarm times are related to human tenability limits for toxic effects, visibility loss and heat stress. During smouldering fires it is only the optical detectors that provide satisfactory safety. With flaming fires the ionization detectors react before the optical ones. If a fire were started by a glowing cigarette, optical detectors are generally recommended. If not, the response time with these two types of detectors are so close that it is only in extreme cases that this difference between optical and ionization detectors would be critical in saving lives.