I have a coal fired boiler, 4 zone , baseboard hot water system. Last year I changed it from a single circulator with 4 zone valves to 4 separate taco 007 pumps. Everything works fine except two of the zones appear to be circulating hot water without the pump running over a period. Ex: When the thermostat is set at 70 the circulator will pump to 70 but over the course of the day the temp will gradually rise to 75 or 78. When I look at a Taco 007 pump I realize that water can flow through the pump without the impeller circulating . Could the heat of the boiler pushing the hot water be causing this? Do I need to install some type of valve to control this? Thank you.

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You need to install a check valve or a anti Thermosiphon device on any zone experiencing this problem.