On Tue, 30 Nov 2010 12:12:50 -0600, The Daring Dufas
wrote:

On 11/30/2010 11:35 AM, Mark wrote:
On Nov 30, 12:24 pm, wrote:
On 11/29/2010 2:37 PM, Sam Takoy wrote:

Hi,

Looking for a cheap but reliable solution.

I want a fan to turn on when my thermostat is on. Is there a gizmo
that would do that for me. My fan is line voltage.

Many thanks in advance,

Sam

I put a 120V duct fan in one heating duct in my old house. I hooked it
up to the furnace fan controller in parallel with furnace fan . When
the furnace fan came on the duct fan came on.

LdB

there is at least one gottcha with this plan if you have a multi-speed
furnace blower.

The extra outlet will have 120V when the speed tap that you connect to
is energized. But if your furnace at some time sets a different
blower speed the exra outlet will have more or less then 120V.

If you wired the outlet to the low speed winding for example, you'll
get 120V when the blower is runing low speed. But when the blower is
running high speed you'll get more then 120 V which could be dangerous
to your fan.

I think if you wire to the high speed winding, then the voltage will
always be 120V or less which is probably safe. Moral of the story,
check the voltage at your extra outlet at ALL the speeds that the
blower might operate.

Mark

The speed of a furnace blower with a conventional multi-winding AC motor
is not controlled by the voltage. The speed is determined by
which winding is energized. There are usually three speeds available
via different taps. Low, medium and high with only two connected to
the control board at one time. The speed for AC is usually higher
than the speed for heating.

TDD
That was my first thought too - but that darn stator acts as a
transformer, perhaps? Never tried it, and I don't have a 2 speeder
around to check with any more.

On my old furnace, though, the low speed was ALWAYS connected and
powered - and the high speed just overtook it when powered up. Never
had a problem in over 12 years running it that way (low speed never
"open circuit"