Who needs to extract heat from the exhaust in order to heat up the coolant?
Exactly!!
The water shutoff valve that you don't have
And, exactly where does one place that? You've got super heated water each end of that manifold...shut the flow off from one way, what happens to the coolant that still residing in the tubes in the manifold? gonna turn into steam pretty damn quickly! Then what?
https://www.ebay.com/itm/225619817059
Oh No! Another one! Good grief!