One of the zones in my house is always hot. Even when the circulator pumps are off (The pumps will turn on fine if you crank the thermostat up.) I have a hot water boiler system with baseboard radiators. There are two taco circulator pumps(007-5F, No IFC) on the return side, and two taco flow check valves(221-1) on the supply. The only thing I can think of that would cause this is that the flow check valve is leaking or stuck open. Is there anything else that would cause this? If it is the flow check can one replace just the stem or does the whole assembly have to be replaced?