Originally Posted by
Tabasco
As roadster601 said "its all about the correct flow...."
Here is how I see it, let me know if I am wrong. If hooked up correctly, hot water flows from the engine directly to the heater control valve and if the valve is shut, then no hot water is in the heater core. If hooked up incorrectly, the hot water flows into the heater core then reaches the control valve so the core has hot water in it even when the heater is turned off.
and Rick replied:
You've nailed their logic. That means the heater core will have water with a temperature close to that of the cold side of the radiator. Maybe 30-50° cooler than the hot side of the radiator.