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Flow/Check Valves on an Indirect
I am installing an indirect water heater as the only zone off of a steam boiler. I've looked at a bunch of installation diagrams from various manufacturers, and most show one or more flow check valves to prevent gravity circulation. I am a little confused about the necessity, and since they add significant head loss, I don't want to add them for no reason. On the boiler supply leg, hot water exits the boiler below the water line, drops close to the floor to the circulator, and then gets pumped up to the boiler supply input on the indirect. Since both the indirect and the boiler have an immediate downward leg, it would seem that both have effective heat traps, eliminating the need for a flow check (or even an internal flow check on the pump itself). On the return side, the indirect exit and the boiler return are essentially at the same level. Since the indirect is going to have some stratification, the water at the bottom of the tank near this exit is likely to be much cooler most of the time, so I wouldn't expect to see significant losses to a cooled off boiler. If the boiler is hot, it's actually not a bad thing to get some flow in the return leg to the indirect since I'd rather have the heat in the indirect as opposed to sitting in the idle boiler. So I am not sure why I need the flow check on the return leg either. I must be missing something here, but I can't figure out what. Any help would be much appreciated.