A variable-speed blower in a heating, ventilation and air-conditioning installation is controlled by signals from the thermostat via a dedicated blower board. Because the thermostat uses comparatively low voltage to control the blower, a transformer is also part of the system.
A correctly wired thermostat is vital for the proper operation of the blower. The thermostat communicates to the furnace’s control panel, which in turn issues instructions to the blower. Thermostats typically have nine dedicated terminals at the control board, only six of which are used in almost all applications. The terminals are labeled Hum, R, C1, EM, W1, O, Y2, Y1 and G.
The connections to the control board from the thermostat -- all at 18-gauge -- are as follows: red to R, blue to C1, black to EM, white to W1, orange to O, yellow to Y1 and green to G; Hum and Y2 remain empty. The connections from the control board to the components controlled by the thermostat -- again, all at 18-gauge -- are as follows: red from R to the transformer, blue from C1 to the transformer, black and brown from EM to blower terminals 8 and 7 respectively, white from W1 to blower terminal 6 and green from G to blower terminal 5; Hum, O, Y2 and Y1 remain empty.
Blower Board Connections
Blower connection boards typically have 12 terminals, only eight of which are used in almost all applications. In addition to the connections for terminals 5 through 8, detailed in Step 1, the other four used are as follows: a 14-gauge black wire from the transformer to terminal 1; a 14-gauge yellow wire from the transformer to terminal 3; an 18-gauge blue wire from the transformer to terminal 4 and an 18-gauge red wire from the transformer to terminal 2.
Blower Motor Connections
Blower motors typically have five terminals, only three of which are used in almost all applications: an 18-gauge green wire to ground goes to terminal 3, an 18-gauge yellow wire from the transformer goes to terminal 4, and an 18-gauge black wire from the transformer goes to terminal 5.
Many furnaces feature integral blowers, and such units typically come with the various connections to the variable-speed blower already made. In such cases, connecting the thermostat and the main supply -- its gauge as mandated by the furnace manufacturer -- is all that's required. The black wire in the supply connects to the black wire in the furnace’s connector panel to create the hot line in, white connects to white to create the neutral or return line out, and green or bare connects to its mate to complete the ground circuit.
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