I am installing a Lux TX9000TS digital thermostat. The wires I have are Red, White, Blue & Green. On the Thermostat I have W, Y, G, B, O, RH RC (Jumper) & C. My question is where does the Blue wire go? My father in law put it on the B, but when the furnace kicks on, the AC fan also kicks on. I have a Bryant Plus 90 Gas Furnace and an old Central AC unit (don't know the name). From what I've read, the Blue is either going to go to C or Y, but which one is it? Thanks!

Red to R
White to W
Green to G
Blue to Y

wmproop nailed it. Long time no see wmproop. How you been?

Old wiring is blue,red,green and white,rh and rc wire is black.What matches what

It really depends on how things are wired at the furnace. But the standard is red to rh with a jumper between rh and rc, white to w, green to g, blue to y. However, if you have a black to rh-rc, you need to look in the furnace and let us know were wires are hooked.

Blue could also connect to C. Blue is 24 VAC Common wire.