We have no heat in our car.We flushed it.Still no heat.

What is your question?

After we had the car flushed,we still didn't have heat,until the next day, we had heat, just until the weather got cold again. The heater works fine above 20 degrees, anything below 20 degrees, it will not heat up.If the car heats up in warm weather, doesn't it mean the heater core is okay? What else could we try?