I have a 2004 Lexus ES330 with 114k on the. I still have yet to do the 100k maintenance (because the car was barely driven and all the belts look good)

I notice that when I turn the heat on and adjusted the temp to 75 it didn't feel like 75, so i turned it up to 80 and that didn't feel like 80 but it did get lil warmer. I turned it up to "HOT" and it got very hot.

I decided to go ahead and do a coolant flush to see if maybe that would fix the problem. So that wasn't the solution. So what else could it be? Thermostat?