Heater Core Flow Direction

[JZLAJeep]

Been driving cold all winter because the heater core apparently hasn't been getting much flow. Concluded to delete the Heater Control Valve, but came across the question of heater core flow direction. Stock Renix closed system flowed into the bottom and out the top. By 97 on XJs with the then stock open system, the flow was into the top and out the bottom. Were these different heater cores or the same? Does the direction matter?

 

I found the following old threads very useful:

HCV deletion discussion

 

Discussion at the end of page 1 touches on flow direction, but doesn't address directly

 

The '99 4.0L picture shows that the coolant hoses into and out from the heater core were reversed from the earlier Renix closed systems

 

[JZLAJeep]

No takers on this query? Maybe shoveling snow from this weekend's mega-storm is keeping many of you busy...

 

Unless one of you responds with a good reason not to do so, we've decided to go ahead and reverse the direction on the heater core when we put the cooling system back together. Jeep appears to have done just that in 97 on Cherokees and it does keep the 3/4" diameter ends and the 5/8" diameter ends together. I've flushed the heater core and water runs through seemingly equally well in either direction, so it seems like an OK thing to do.

 

In other news, got the cooling system torn down and flushed (not a warm proposition in Idaho in January) and the parts are ready to pick up from the parts store. Decided to go ahead and replace the radiator and make the open system swap. The existing system was full of rust and the heater core was clogged, which shouldn't have been the case after a flush this summer. So out with the original '89 stock radiator and in with new radiator, thermostat, hoses, and HCV delete. Just a little bit of scope creep. 

[Eagle_SX4]

I have always thought the flow should go in the bottom and out the top to prevent air getting trapped in the heater core. It is easier for the air to get out of the heater core when pushed out the top vs the bottom.

[JZLAJeep]

22 hours ago, Eagle_SX4 said:

I have always thought the flow should go in the bottom and out the top to prevent air getting trapped in the heater core. It is easier for the air to get out of the heater core when pushed out the top vs the bottom.

That all makes sense. Planning to flip this (like Jeep in '97) anyway. Wish us luck!

[boxyjeep]

I don’t think it matters so much.  I haven’t tested it personally, but likely both routing options are fine.  HOWEVER, if you have an old heater core and switch it without replacement, it may loosen up some residual gunk that may end up in your cooling system…

[JZLAJeep]

Made the open system swap this weekend and I did reverse the flow on the heater core. So far so good. Heats the cab up quickly and nicely now. Too hot now!

 

Prior to all of this I had flushed the heater core very well in both directions until it ran clear, so pretty confident that it's as clean as it's going to be without being new. 

[ghetdjc320]

I’d assume that the flow restriction on the return line would help slow down the coolant to heat soak the core better . It will probably work fine either way as we have an XJ heater core in the tiny cab of our MJs. The other thought would be trapped air but you can easily eliminate that by vacuum filling the coolant. It’s by far the easiest and best method I’ve ever come across to make sure all the air is gone in pretty much any cooling system. I would delete the bypass valve though in the heater core. 

[JZLAJeep]

2 hours ago, ghetdjc320 said:

I’d assume that the flow restriction on the return line would help slow down the coolant to heat soak the core better . It will probably work fine either way as we have an XJ heater core in the tiny cab of our MJs. The other thought would be trapped air but you can easily eliminate that by vacuum filling the coolant. It’s by far the easiest and best method I’ve ever come across to make sure all the air is gone in pretty much any cooling system. I would delete the bypass valve though in the heater core. 

Bypass valve deleted. When flushing, water seemed to go through either direction equally well making me think that direction didn't matter? The 97+ XJs reversed the direction through the heater core which goes back to my original question of whether the heater core was the same or different on 96 and earlier and 97 and later?

 

I can report that the cab warms MUCH better now, to the point of being hot, with the HCV deleted, the system flushed, new radiator and hoses. This is with the heater core direction reversed, which is further evidence to me that the direction through the heater core doesn't matter.