heater core

[Chamberpains]

Most truck of yester year and my 2006 had hand valves of some sort on both sides, supply and return. This kept hot coolant from circulating in from either direction. The heating system wasn't factored in to how the engine is cooled. Its an auxiliary add on and should be able to be completely cut out on demand if one wants it.
Unless the truck manufacturer designed it as a part of cooling the engine because they stuffed it under a very tight, well insulated engine compartment. Check with your truck maker before installing any shut off valves.