Replies

1. 
   DTHA | Oct 23, 2008 03:56am | #1

   I think we need to know the wiring scematic of the "old" and "new" blowers as well as the voltage(s) etc...

2. 
   BoJangles | Oct 23, 2008 04:21am | #2

   If you have the new furnace blowing in to the plenum of the old furnace, the temperature sensor in the old plenum will usually turn on the blower in the old furnace when the air is hot enough to trigger that. 

   That's why the blower on your old furnace will run for awhile after the flame shuts down after satisfying the room thermostat.

3. 
   DanH | Oct 23, 2008 04:22am | #3

   There are several ways to do this. If the old furnace has a socket for an AC fan relay, the simplest approach is to use that. It can be powered from the new furnace several different ways, depending:

   -- Just wire it to come on with the thermostat.

   -- Use a current transformer in the new furnace to detect when the fan comes on. (Won't work if the new unit has a DC motor.)

   -- Use the "humidifier" output of the new furnace, if there is one.

   Corporation: n. An ingenious device for obtaining individual profit without individual responsibility. --Ambrose Bierce