mr. spam wrote:

i have an old furnasman gas furnace.

It has AC

there is a honeywell transformer screwed on to the electrical connection
enclosure i suspect it was installed when the ac was installed.

I want to replace the fan motor with a 2 speed one

i want to add a switch to the furnace so i can have the fan on all the
time in slow speed. then when the burner or the ac comes on it will
change to high speed.

is there a relay i can install to do this or is there something else i
need to do.


Hi,
You need extra single pole double throw relay with proper coil rating.
This relay has to be controlled by original fan control signal/power.
When idle relay will supply power to lo speed lead on the motor, when
fan is called for by heat, the relay will energize to supplu power to hi
speed lead.