Many (most?) building codes now require the installation of
	smoke detectors in homes.  In fact, this has been made
	retroactive in many municipalities.

	There are many different types of smoke detectors.  Ionization,
	photo-cell, battery-powered, AC-powered etc.  The only thing
	we're concerned with here, is AC versus battery powered, other
	than to comment that most building codes are based around
	ionization detectors, photocell units being usually for
	somewhat more specialized purposes.  All things being equal, in
	a residential setting with the "ordinary fire", an ionization
	detector will detect smoke before a photo-cell will - indeed,
	in some fires, the smoke is almost invisible, and less likely
	to trip a photo-cell.

	There is another type of fire detectors - "heat detectors".
	These work usually by a small piece of special metal melting at
	110F or so.  These are much better at avoiding false trips.
	But they usually take much longer to trip than a smoke detector, and
	should usually only be considered for triggering sprinkler
	devices (where the consequences of a false trip are quite
	severe).  Heat detectors should not be used as primary fire
	detection.

	Most building codes that mandate detectors mandated AC-powered
	ones for new construction.  This is because the statistics show
	that, in houses equipped with smoke detectors, a lot more
	people were getting killed in houses with battery-only
	detectors that had dead batteries than were getting killed in
	houses where the breakers tripped and killed an AC-only
	detector.  It's also worth noting that some battery detectors
	are quite sensitive about battery condition.  Some even refuse
	to work if the battery is zinc-carbon (standard cheap battery)
	instead of alkaline (more expensive).

	Our building code discourages the installation of smoke
	detectors on circuits used for other purposes.  This means that
	only a main-panel breaker trip can kill the detectors.  A
	main-panel trip is unlikely even in a fire started by an
	electrical fault until well after the fire has really engulfed the
        home.

	These codes also usually require that the AC detectors be
	interconnected so that if one triggers, they all sound the
	alarm.  This is usually done by an additional wire between the
	units.

	The above suggests that the best way of doing things is to have
	one circuit dedicated for smoke detectors, and you run 14-3
	between each of the detectors - the red wire being the "gang
	trip" control.

	If you're still concerned about losing power and thereby losing
	your detectors, we suggest either the use of detectors that run
	off AC power with battery backup, OR, adding battery detectors
	into a system that's already adequately covered with AC detectors.

	Battery-only detectors should only be considered a stopgap
	measure in putting detectors into a house that doesn't have any
	detectors at all, or adding redundancy into a system that already
	has AC detectors.

	We also suggest that, if you have battery detectors, you make
	changing the battery a yearly (or semi-yearly) scheduled event.
	Some people change the batteries on their birthdays.  Others
	change the batteries during a "daylight/standard time change"
	maintenance pass.

	In Canada, the day before the standard/daylight time change
	(a Saturday) now seems to be officially called "smoke detector
	battery day" ;-)

	We don't recommend waiting for the detector to tell you that the
	battery is dead, unless you manually test the detector monthly.

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