Theo,
You say,
<<If you block either the inlet or the outlet, there is no flow, and no hot
air from the heater core. So it makes no difference which side of the core you
put the control valve on, from a heating perspective>>
It does make a difference which side the heater valve is on for two reasons.
First, when the coolant flows through the valve in one direction it tries to
open the valve which makes it leak hot coolant and if it flows in the other
direction it tries to close the valve which helps keep it sealed. You can
figure
this out by trying to blow through each end.
Second, has to do with core durability which is too long a topic and probably
boring to discuss here.
John Logan
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