Heating loop waterflow without pump on

Flow Check Valves?

I will assume that the three circulators are soon after the expansion tank on the supply side of the system. Do you have a flow-check valve after each circulator? That is the first step. If the piping is large diameter (I am not a professional, but I would say 1/2" copper is not large), you might want one on the return side as well because you could get circulation in a single large diameter pipe.



It is possible that flow check valves are not required (though you might at least one ordinary check valves, such as what are in the Taco 00*-IFC circulators) if the piping is such that you do not get thermal circulation. I have radiant heat in a slab that is about 5 or 6 feet below the supply piping coming out of my boiler. The hot water is not all that hot (120F max), so it seems to have no reason to go down to the floor and push the cold water up. Hence, no thermal flow there. And upstairs there is no flow either, even with out a flow-check valve because over half the piping (about 75 feet of it) is 1/2 inch copper and is all horizontal. And here, too, the maximum temperature is 135F maximum.



Where I do have a flow check valve is to separate the heating circuit from the domestic hot water circuit. At that point, the piping is 1 1/4" and the flow check valve keeps the hot water heating the indirect heater from the rest of the house.