Heater issue! No hot air. Read many forums. Heater core hose not hot!

There isn't normally anything that fits that description in a coolant loop. The closest would be the rubber in the hoses. If one were failing badly enough to slough off that many pieces it should feel soft at that place (less rubber) and you might be able to see the damage inside from one of the ends with a flashlight. That said, I have never heard of one doing this. Blowing up or splitting, sure, but coming apart inside? Also, if chunks of rubber were coming off of the inside of a hose why would it develop voids which were not in the original rubber? Maybe if a section or all of it had overheated really badly? It's either that or somebody stuffed black foam rubber into your cooling system, which doesn't seem very likely either.

 

My brain still isn't able to go anywhere with this other than some odd undesirable chemical reaction, maybe between constituents of mixed coolants, or maybe between constituents of coolant and of the hose material.

I may have nightmares for a while about the dexterity of this foam.

 

If the crud came from a hose the first order of business would be to be sure said hose is no longer in the cooling loop. The link in #23 is interesting since it suggests some combinations of coolant and hose chemistries can produce problems like this. I don't know about you folks, but one black rubber hose looks very much like another to me, and surely many hoses have been replaced on Priuses with whatever the local auto shop had on hand without this sort of disintegration taking place. So at the very least give the current hoses a very thorough inspection, and if in doubt, replace them.

 

It was from the engine coolant circuit? I'd assume so, if it's affecting cabin heat.

If you've got any of that stuff, see if it'll dissolve in a 50/50 mix of white vinegar and distilled water. That formula is recommended for dissolving/flushing one stop-leak product, FWIW.

Maybe you're out of the woods now, but if you have issues down the road, something to try.

 

I recently had the same issue. I replaced the heater pump. I pray the inside of the thermos is not filled with that foam.
Did your pump clog again?

Gen 2 No heat. Flush heater core? | PriusChat