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There are four major zone valve manufactures. Taco, Honeywell, White Rogers, And Erie. There are others out there, but I can't remember the last time I saw one that was not manufactured by these companies. This is what happens, you need heat so you turn on the thermostat this sends power to the zone valve, the zone valve opens, inside the zone valve is an end switch when the valve is opened this switch tells the aquastat relay  that you are calling for heat it send power to the pump and the pump sends the hot water through the pipes to your base board or radiators. and you have heat.    

Honeywell zone valve