Which contacts on a normal fan center could be jumped to activate the furnace blower?

Jumping the R and G contacts on a normal fan center is the method used to activate the furnace blower. In a typical HVAC control circuit, 'R' corresponds to the power supply from the thermostat, and 'G' corresponds to the fan relay, which controls the operation of the blower motor. By connecting these two terminals, you supply power to the fan relay, thereby energizing the blower and enabling it to run. This action can be particularly useful for testing the blower operation independently of the heating cycle or during system troubleshooting.

Other combinations such as R and Y involve the cooling circuit (where Y controls the compressor), while Y and G or G and W involve different functions that do not directly activate the furnace blower in the same manner. Understanding the specific functions of these terminals is crucial for effectively diagnosing and servicing HVAC systems.