i'm trying to put a aprilaire 500a humidifier on a thermal zone furnace.i've tried attaching the call for heat wires on the common terminal and the hum spade connector on the board.the solenoid stays open all the time? i checked the terminals with my volt/ohm meter and found i have a 24volt signal all the time even when the furnace is'nt running? the same is true for the w and c terminals? last i tried the current sensing relay from aprilaire and put it around the common wire from the fan motor and this does'nt work either? i don't understand why nothing works.the furnace works fine otherwise.