I have a quite similar one coming up here. My plan is to use a 50 gallon electric water heater as storage, but disable the elements so that when the temp sensor in the tank calls, it kicks on a pump drawing from the bottom of the tank and runs it across the plate HX and then through the electric boiler before depositing back to the tank. In both our cases, I think an aquastat in/on the line between HX and boiler/heater and wired in to override the flow switch unless the water is under 120 degrees or whatever, would work. Flow switch will make right away when the pump kicks in but if the aquastat is in series before getting back to the tankless, it should override the flow switch and keep it from firing. I think. Right? One could also put a 3 way right underneath the tankless in a P/S fashion operated from an aquastat that would divert flow through the tankless when temp gets low but otherwise would flow straight through the valve and tee. Most pumps have integral checks so that shouldn't be a huge dilemma. My application will leave the tank hot, with the mixer on the output of the tank to elongate the storage capability, and in a non-storage application I'd still put the mixer after the tankless regardless