HEATER IS NOT GETTING HOT, IT GETS WARM BUT NOT HOT

I HAVE FLUSHED THE COOLING SYSTEM OUT AND PUT ON NEW HOSES. I HAVE CHECKED AND THERE ARE NO LEAKS IN THE HEATER . IT GETS WARM BUT NO REAL HEAT IN WINTER TIME. PEOPLE ARE TELLING ME IT IS A HEATER CORE. BUT I NEED TO KNOW BEFORE I P[UT ANYMORE MONIES INTO

THANKS,
FLOYD L. LINDSEY