I need the electrical diagram for a central heating system?

Answer 1

A specific brand or just general?
One circuit is thermostat as switch, power supply transformer for low voltage, and relay at furnace.
Another is the motor blower circuit, which is normally set up to be turned on by a heat sensor - when the flame heats the furnace, the blower comes on.
Another is the gas ignition and safety circuit. This is the most complicated. It has to have timing aspects so the gas is turned on, the ignition fired, the flame sensor checked to see if the flame has caught.

Answer 2

TX INSTRUMENTS 10859828 FAN CONTROL BOARD