Even the plumber, his helper, & his boss didn't comprehend this until it was built & working. The yellow shows what happens if the gas heater isn't working, and the tankless electric is supplying just enough hot water for us to get by. When the gas IS working (normal), the top bypass valve is open, and the 2 shutoffs are closed.
http://www.amazon.com/dp/B003UHUSGQ/ 
