I had an old chevy when I was young that we all thought was just cold blooded and had a poor heater. Tried the 1970's fix of cardboard in front of radiator and it still sucked.

Finally one day I decided to flush the core out with a garden hose. Took both hoses of engine an just hit it with moderate pressure every so often till it flowed freely. And what do you know I ended up with a heater that would melt snow of the car.