my car will run for like 15 min. I turn the heater on, but it does not get warm.

If the engine temperature is warmed to operating temp, it is possible one of two things is occurring : 1. HVAC blend door actuator has failed and not switching vent from a/c to heat. 2. Heater core is plugged and needs 'backflushed' to clear blockage in the core......You will have to inspect to determine which. Why did you add antifreeze to it?? Do you feel any air blowing out of the vents?

The previous answer is correct but incomplete. Anything that prevents the flow of air across the heater core or hot antifreeze into the heater core can cause a no-heat condition. These situations are very challenging for customers to correctly diagnose. Our shop can diagnose this problem for you, the charge for that service is 45$. Let us know if we can help you- Prof. Auto diagnostics, 643-2559.

I totally agree with answer # 1!! .... IF you aren't savvy with that... See answer # 2 for hands-on testing, if desired .... seems to be in your zip code.

Does that vehicle come equipped with a temperature gauge , or just the 'idiot' light? --- WHY did you 'add anti-freeze'? was the system low? Did you simply notice the reservoir was low and 'topped it off'? IF it has a gauge , from a cold start in the morning , how long does it take for the gauge to move about 1/3 of it's distance to 'red'?? IF after 10 minutes , it's still barely moved , might I be so bold as to suggest a thermostat. ........**********DISCLAIMER********* THIS ANSWER IS INCOMPLETE AND IS SOLELY HERE FOR THE PURPOSE OF ENTERTAINMENT , IT IS NOT MEANT TO DIAGNOSE TREAT OR CURE ANY IMPENDING AUTOMOTIVE DISEASES!!!..........