I had a heater core silt up in an old Focus I once had. Driving home 15h in blizzards with the kids in the back finally got me off my backside to fix it. Took a lot of forward and back flushing to wash it all out. If your coolant looked bad after only a year, might be worth back flushing the radiator and the heater core separately next time. Flushing in the normal coolant direction only gets the easy stuff out, as that's the way coolant is normall flowing anyway. Flushing it in the opposite direction knocks all that deposited silt out. I was amazed - forward flushing ran clear with the garden hose after a few minutes, back flushing after that took 20min or so before the water ran clear. Best heat in any car I've ever had after that!