In article , Jerry Stuckle writes:
This is one reason why ionization detectors are not recommended any more
- too many false alarms. That, plus photoelectric detectors are much
faster at detecting real fires.

And to avoid everyone just repeating whomever their favorite
pontificator is, let's inject something a bit more authoritative.

Courtesy Wikipedia:

According to the National Fire Protection Association (NFPA),
"photoelectric smoke detection is generally more responsive to fires
that begin with a long period of smoldering (called smoldering
fires)." Also, studies by Texas A&M and the NFPA cited by the City of
Palo Alto California state, "Photoelectric alarms react slower to
rapidly growing fires than ionization alarms, but laboratory and field
tests have shown that photoelectric smoke alarms provide adequate
warning for all types of fires and have been shown to be far less
likely to be deactivated by occupants."

Although photoelectric alarms are highly effective at detecting
smoldering fires and do provide adequate protection from flaming
fires, fire safety experts and the National Fire Protection Agency
recommend installing what are called combination alarms, which are
alarms that either detect both heat and smoke, or use both the
ionization and photoelectric processes. Also some combination alarms
may include a carbon monoxide detection capability.

[...]

A 2004 NIST report concluded that, "Smoke alarms of either the
ionization type or the photoelectric type consistently provided time
for occupants to escape from most residential fires," and, "Consistent
with prior findings, ionization type alarms provided somewhat better
response to flaming fires than photoelectric alarms (57 to 62 seconds
faster response), and photoelectric alarms provided (often)
considerably faster response to smoldering fires than ionization type
alarms (47 to 53 minutes faster response)."[13]

Regular cleaning can prevent false alarms caused by the buildup of
dust or other objects such as flies, particularly on optical type
alarms as they are more susceptible to these factors. A vacuum cleaner
can be used to clean ionization and optical detectors externally and
internally. However, on commercial ionization detectors it is not
recommended for a lay person to clean internally. To reduce false
alarms caused by cooking fumes, use an optical or 'toast proof' alarm
near the kitchen.[42]


Ionization has its uses, and photoelectric is better in at least one very
important way, but the US NFPA recommends a combination of both types.

And, after all, the best reason to avoid old detectors is to get updated
to the newest technologies.

Oh, and if you look at the more garish anecdotal reports you will find
the cases where ionization detectors outright missed smouldering fires
until it was too late. But the jury cases that followed appear to be
overreactions (somebody died because some technology was not perfect),
and when the Firefighter's Association begins pushing for tighter
standards than fire protection professionals (NFPA), it might be
understandable - one first hand anecdote can in a first responder's
mind trump a thousand studies.

Find out what is recommended, folks, and update to that.

[I should talk - that combined detector I bought years ago, one that is
also dual powered, is still sitting on the workbench].

George