I don't have a check valve on mine. Maybe I should. The way I look at it, there is no flow through the sidearm on the boiler side when the wood boiler water is below 160 degrees. And since there's a flow control on that line and the water is entering the sidearm from the top and flowing down, there's no place for the boiler water to thermosiphon to. That's with a direct feed from the EKO. I haven't really thought through the implications of using it with hot water storage, but I don't think it would be much different. As a practical matter, the boiler can shut down for hours and we don't run out of hot water.