On Thu, 17 Jul 2008 23:30:47 -0700 (PDT), Aaron Fude
wrote:

I would guess they need three: one for cooling part, one for the fan,
and one neutral.


It's hot not neutral. The common wire to a group of switches (which
the thermostat is) is normally hot. In the case of normal thermostats,
that's 24VAC 1A max.

So why 5?


How many wires are actually connected to the thermostat? Maybe the
installer had some 5-conductor wire, and used that to save money. The
wire coming out of my furnace is 8-conductor. The other 4 aren't
connected to anything.

Thanks!

I usually see 4: 24VAC supply, heat, cool, fan.

An additional wire might be for a multi-speed compressor, or heat-pump
changeover valve.
--
Mark Lloyd
http://notstupid.laughingsquid.com

"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy