The 821 is not the heat cool relay. From what you are telling me, I think that you have the following: Power goes to the common of the heat cool relay, a wire from the NC terminal goes to the 821 relay power contact and from there to the medium high speed of the blower motor. A wire from the NO terminal of the heat cool relay goes directly to the high speed of the blower motor. When the furnace is running in heat mode, power passes through the NC of the heat cool relay to the 821 which will make after a delay when the gas valve is on and the blower runs on medium high. In A/C mode, 24 volts is applied to the coil of the heat cool relay which picks up and puts power from the NO terminal to high speed. If either relay sticks, the fan will continue to run. To find out which one, just pull off the wires, one at a time, to the motor.