I have a old White Rodgers thermostat model # 1f56w444. At the connections the wires are. W, Y, G, RH, jumper to RC, and B
All the new ones that I have seen have.
W, Y, G, RH, jumper to RC, but no B
My question is what is the be wire for and what kind of new digital thermostat can I use.
Does not have to be the most expensive one I would not like to spend more the $40.00
And still get a good one.
Thanks for your time.

If this uses batteries do not need the B. The B is for common wire and not needed on battery thermostats