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Backup boiler piping for solar drainback system
Posting here because it's more of a boiler question than a solar question. Existing drainback solar system with 800 gallon nonpressurized tank. Tank has two copper HX coils, one for DHW and one for space heat. Space heat is currently direct piped out of HX to floors with no ODR. Barrier tubing embedded in concrete on ground level, suspended below (not touching subfloor) of 2nd level. Backup heat comes from a W-M CGi on LP using manual changeover valves to redirect the boiler output from the floors to the tank. Needless to say, it's been giving the owner fits for the entire 13 years since it was built.
We are tearing off the ground floor ceilings and installing Watts FlexPlates on the existing PEX (8" on centers.) I plan to install separate motorized ODR-controlled mixing valves for each floor. Still looking at control options but the question at hand is the best method of piping in the boiler. We want long boiler run times, storing whatever is not needed for the floors in the tank.
Since the tank is an open system we have to work with the existing HX coils (no way to add a third since the tank is located on the other side of the house from the boiler room.) I'm thinking about injecting the boiler water between the HX output and the hot side of the mixing valves, then allowing any excess to go back into the HX and heat up the tank, but not quite clear on the best way to do this. I could add motorized three-way valves on either side of the HX coil but I suspect there's probably a simpler way.