304V - Hydronic Heating Zone Valve Control - Four Zones with PriorityTekmar 304V Hydronic Heating Zone Valve Control Four Zones with Priority The Zone Valve Control 304V connects up to four thermostats and operates zone valves to provide heating to a zoned hydronic heating system. The 304V operates a system pump whenever a zone valve opens. Zone 1 can operate either a zone valve or a circulator to provide heat to an indirect domestic hot water tank with optional priority over zones 2 through 4. The 304V includes