Dear Telluride Friends, I have amazing news. I had this exact issue of the heat blowing warmish but NOT HOT air at idle on my Telluride 2022 S AWD. Remote start was useless as it could not defrost my windshield in frigid Michigan, even after multiple remote start sessions! It also had extreme amounts of exhaust steam coming out of the tailpipe which I now understand was caused the same problem as the heating issue. After a long ordeal, I'm thrilled to report and share with you that this is now fixed!
The first dealer service center I brought the car to found that the coolant had an air pocket. They bled the coolant, gave it back to me and still had the same problem. They took it back and found the thermostat was stuck halfway open. They replaced the thermostat along with the gasket-coolant inlet, then admitted that this didn't solve the problem. They then told me that the manufacturer's official techline engineers told them that this was "normal operation for this model and year", and that they would not authorize any more work on it under warranty.
I then opened a Lemon Law case with Lemon Law Group Partners at lemonlawgrouppartners.com. These guys were awesome, and completely free to open a case. They only collect if you win a cash settlement. Keep in mind that even if for any reason you may be exempt from Lemon Law rights (e.g. not the first owner, etc.) there are other laws at play here. These guys told me that manufacturers claiming a clear defect is now considered "normal operation" is something manufacturers do all the time, and that it's nonsense. They went after Kia America for breach of their manufacturer's warranty and sent a harsh letter demanding a fix for the car or a replacement vehicle.
In the meantime while this was pending, I then brought the car to a different Kia dealer/service center nearby. They additionally found that my radiator cap was only holding 12psi instead of of the 16psi that it should. This could have been one cause of air bubbles in the coolant that the other place found. They replaced the cap and told me it was fixed. It was still not fixed!! They then took it back in... and finally they replaced the heater core which they found to be clogged. This is apparently a big (~$2K) job involving removing the entire dashboard, but it was covered under the manufacturer's warranty. They had it for over 1 week to get this done, but I just got the car back today and the issue is completely resolved! I have good strong heat when idle again!
So just to sum this up, if this is a perpetual problem make sure to have them validate the proper functioning of the following:
1. No air pockets in coolant
2. Thermostat door is not stuck open
3. Radiator cap is holding the proper pressure (16psi)
4. Heater core is not clogged
5. Also note: IF none of those above pan out, a mechanic friend of mine told me to make sure they validate that the water pump (that moves the coolant) is operating properly.
Finally, I'm not sure if opening a legal case against Kia America helped, but it didn't cost anything and I knew that if they couldn't fix the issue I had some other potential recourse.
Best of luck to you all. Please reply if you need any info from my service shop to make sure that your vehicle gets the proper fix as well!