[djimellis]

Jim, thanks for this very clear reply. Is the one lower right connected to a boost pump inlet or do you not have a boost pump?

I can feel the boost pump working and there is hot water flowing through it. I'm trying to figure out if the flow changes with engine speed.

My air intake ports make little to no difference in the temperature of the hot air output whether they are open or closed. What seems to matter the most is the inverse relationship of engine RPMs and heat out the vents.

Yes, the lower right shut off is connected to a boost pump. The boost pump does most of the water flow and if you have hot water coming out of the pump I believe your core would be your next concern, unless for some reason one of your 3 blower motors is failing. Have you tried reversing the electrical leads to the pump to see if that makes any difference? You won't hurt anything by trying that.