Low coolant?
Bad thermostat?
Defective radiator cap?
System air bound?
Heater core plugged or restricted?
Temperature control cable not adjusted properly?
The water pump pushes the coolant through the engine block and head(s) then into the heater core.
Is coolant up to level? Are heater control cables functioning to their full travel Thermostat may be defective and opening too soon. Heater maybe either "airbound" and need to be bled or may be plugged
Most likely a replacement heater core
U JUST NEED A NEW WATERPUMP MINE WAS DOING THE SAME THING AND I PAID FIRESTONE $700 TO PUT MINE IN. YEAH WAY TO MUCH CAUSE THE WATERPUMP ITSELF IS ONLY BOUT $150 SO DONT GO THERE
The cooling fan at the radiator is not coming on. Running the heater take heat out of the coolant as it goes through the heater.
The heat valve is stuck open.
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It is a water heater, not a hot water heater. There would be little point in using a hot water heater. I assume that is your point.
Check: Heater controls, doors & door actuators, thermostat could be stuck open, heater core possibly plugged, might have a water shut off valve that's not working (depending on what year).