Water pump maybe?
air block in heater hose most likely bleed it
the gas stays on but the water still doesn't get hot
you might want to check and see if the heater core is bypassed
You have to install the thermostat with the jiggle valve in the 12 o'clock position.
im having the same problem with my 1994 I've replaced the heater core,thermostat,radiator,i checked the coolant, and it's not over heating or nothing and still no heat lol
it is a hyndai that's why it is a hyndai that's why
Sounds like the heater core may be plugged or the heat control valve may not be fully opening.
Broken blend air door/actuator....replaced many of them
If everything on your car ( thermostat, heater core, heat controls, water pump) has been changed and you are sure that it all works, check the water flow. A clog or an air bubble in the heat lines will cause coolant to not flow and have no heat output.
Check the thermostat to see if it was installed correctly. They can get put in backwards.
Are both lines hot that go into and out of heater core? Plugged heater core will result in no heat, or other restriction.