Is this a question of curiosity? Or is there a problem looking for a solution?

In the parts diagram shown below (1st view), a 1999 Camry 5S-FE vehicle (North American Produced, USA market) has a heater core part number of 87107-07010. The next view below is that from ToyotaPartsDeal.com, which tends to use photos of actual Toyota parts - so that heater core is what ToyotaPartsDeal shows for 87107-07010. The top tank appears to show a slight color difference in the tank between the two tube connections, which is typical when a separator is added within the tank to separate one part of the tank from the other. There is also nothing on the heater core that is present to hold two separate heat exchangers together along the sides, which should be present in a front-to-back setup. So putting a few pieces together, I would think the factory OEM heater core is a U-flow design. Using a ruler in each of the heater core inlet & outlet would provide a significant hint of what design it is, as you observed in your original post. Of course I've been wrong so many times before, and on this one I am totally not certain that the factory heater core is a U-flow design.

 

Is this a question of curiosity? Or is there a problem looking for a solution?

In the parts diagram shown below (1st view), a 1999 Camry 5S-FE vehicle (North American Produced, USA market) has a heater core part number of 87107-07010. The next view below is that from ToyotaPartsDeal.com, which tends to use photos of actual Toyota parts - so that heater core is what ToyotaPartsDeal shows for 87107-07010. The top tank appears to show a slight color difference in the tank between the two tube connections, which is typical when a separator is added within the tank to separate one part of the tank from the other. There is also nothing on the heater core that is present to hold two separate heat exchangers together along the sides, which should be present in a front-to-back setup.

I like these tips (look for seams indicating how the tanks would be divided), especially comparing your observations with what I see on the Audi B6 heater core.

A U. S. located 1997 Toyota Rav4 owner was having a "no heat" problem recently. He asked which coolant hose connected to which nipple on the Rav4 heater core. I told him what I had observed on my Rav4 (consistent with salvage yard Rav4s of the same generation). His Rav4 hoses were indeed reversed to what I had observed. He swapped the two hoses. He also (1) discovered a kink in one hose; and (2) suspects blockage-debris got kicked loose when he reversed the flow direction. The Rav4 heater now puts out full heat.

He thought the problem was not the swapped hoses but the kink and/or the blockage. I tend to agree, since going from no heat to proper heat seems more consistent with blockage.

If the hoses were connected to the wrong nipples on the heater core, then I think there would still be decent heat, even if the heater core were of the cross-counter-flow design. Edit: For the cross counter flow design, note that the air is at its coldest point, and the anti-freeze is at its hottest point, as air first passes over this heat core. Counterflow heat exchanges transfer more heat than non-counterflow ones. This is due to heat transfer being maximum when the temperature difference between the two mediums is as large as possible. If the supply and return hoses were swapped, would the cross counterflow heat exchanger still work? I think so, just not as well. Would the hoses being swapped cause a cold heater? I think not. Would the hoses being swapped be noticable, from a heat outupt standpoint? Maybe.

This was curiosity in the vein of "knowledge is power yada and helps me understand the world better."

I appreciate your looking into this and sharing your impressions.

 

And I appreciate your response. Most times this forum involves a problem looking for a solution. But it also serves well for those curious with the goal of expanding knowledge. Its great this forum provides a platform to do both.

 

Have access to a borescope? Know the manufacture of the Toyota originals?

 

I do not have access to a borescope. They have become inexpensive and it is something I consider adding to my tools.

I know Denso makes heater cores. One day maybe I will encounter a salvage yard late 1990s Toyota with a disassembled dash. I will try to check out the heater core, checking for seams, any offset on the connections and a manufacturer's name printed on it.

I edited my last post to add this: The wrong coolant flow direction in the cross counter flow (front-to-back) design should result in less heat in the cabin. Whether this reduction in heat is noticeable is the question I have. After all, adjustment of the air baffles (and not coolant flow) is how the cabin temperature is changed on the 3SFE Rav4 and 5SFE Camry.

 

Looking at the part closely it sure looks like a U type flow.
The above images, and some other images off the internet of Gen 4 Camry heater matrices (and the heater matrix from my Camry that I replaced a few years ago) you can see there is a separator in the top tank, dividing the "IN" from the "OUT" and at the other end of the matrix that tank is open, allowing the flow to make a U there.

Interestingly, from this research I found the heater matrix design was changed pretty dramatically for the '99MY, cutting its weight nearly in half while maintaining its original heat transfer rate.
Denso does this regularly: invents a way to make the matrix smaller and lighter while maintaining or increasing its ability to do its job, with many small improvements of the fin shapes, tube x fin ratio, tube pitch/spacing and so on.

Remarkable stuff, applied to something no one is ever expected to see or think about.

 

I searched youtube using the terms {heater core cut open}. A few videos do come up where folks took a hack saw or similar to an old heater core and cut it open. When the pipes/hoses are oriented/located like my first post shows, it seems the side-to-side U-flow design has been employed.

Sometimes the side-to-side U-flow design has two layers, like this one:

It seems like the flow direction would not matter in a side-to-side U-flow heater core.

Compare to this apparent cross counter flow heater core from a 1990 Renault:

Now I am thinking the cross counter flow, cylindrical-tube-and-shell heater core is used strictly in European designs such as VW, Renault and Audi.