Dave, before you flush it do the following to confirm the heater core is the issue.
First, confirm the coolant is full in the radiator (engine cold). If it is and in the reservoir, start the engine and turn the heat on high. Allow the engine to reach operating temp. Once it does, turn the engine off. Feel both heater core hoses to see if both are hot. The return will be a little cooler but should still be hot.
If that is the case, we need to focus on the blend air door. If one is hot and the other is cool, then I would recommend the flush. Make sure to backflush it as well.
Here is a link you may find of interest and helpful:
https://www.2carpros.com/articles/car-heater-not-working
Let me know what you find or if you have other questions.
Take care,
Joe
Tuesday, October 25th, 2022 AT 7:18 PM