This is the Taco zone valve. The head on yours may be blue green instead of gold or brown. The top (#1) terminal is the power to the zone valve The middle (#2) terminal is the common terminal. The button (#3) terminal is the end switch. 24 volts to 1 and 2 opens the zone valve. If the thermostat is turned all the way up and you do not have heat in that zone. test to see if you have 24 volts AC at terminals 1 and 2.