For a heater core, does it matter which of the two copper(?) tubes serves as the inlet or outlet? In other words, does the coolent flow direction matter?

Thanks.

 

Doesn't matter.

 

actually, it does matter. you want the coolant flow to enter the bottom of the core, and exit the top of the core. this will ensure the core is full, with no air pockets.
if the coolant enter through the top of the core and exits from the bottom, you can have air pockets in the core, and this can give you all kinds of noises. i have heard gurgling, banging, and a sound like running water with the flow entering through the top. as soon as i switch the lines, the noise goes away. i have never told the car owners what i did, because i like them to think i have magic powers.

 

OK. On my heater core both nipples come out the top. There is no clue (to me, anyway) which goes to the bottom or top.

 

what truck/ engine combo are you working on??
i have a few books, and may be able to check it for you.
when you change the core, you will see that one of the lines goes low, and one high.

 

Yes, on the firewall one nipple is higher than the other, but it's only because the heater core mounts at an angle. On the core itself both nipples come out of the same end at the same level and parrallel.