On Monday, December 11, 2017 at 10:17:13 AM UTC-5, wrote:

Or like Gfre said,
very old furnaces just used thermostatic switches in the furnace plenum
to turn the blower on and off.

Not in the plenum, in the thermostat.

no in the furnace .

the air handler blower in my furnace can be turned on 2 different ways.
1) by the wires in the thermostat which you already discussed
2) by a bi metallic switch in the furnace

the cycle goes like this
1) thermostat calls for heat
this turns on the burner AND the blower
2) furnace heats up the house
3) thermostat stops calling for heat
this switches off the burner and the blower BUT.....
4) the blower keeps running because the bi metallic switch in the furnace is still warm

this keeps the blower running until the heat in the furnace has been removed.

the bi metallic switch in my case also is the hi limit and will turn off the burner if the temp in the furnace gets too high.

there are two tabs that can be adjusted

If the blower tab is adjusted too low, the blower will never turn off.


it is something like this

http://www.supplyhouse.com/Honeywell...C0qsQ AvD_BwE



mark

and this
https://www.youtube.com/watch?v=jg0nK4QLpZo

mark