heater hose

[Tabasco]

I doubt the heated air coming thru the heater cares which way the hot water is flowing thru the heater core..

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.