Low coolant, bad coolant, blockage in the coolant line, bad heater core or element, bad thermostat
Possible causes are, low on coolant, thermostat stuck open, clogged heater core or vacuum scorce not getting to the heater control.
Air in the water system.
Low coolant. If the coolant is low then the heater core is not able to transfer heat in to the car.
Your coolant level is low.
Your heater will blow cold air only.
It doesn't really blow it out it just warms the cold air.
The temperature blend door may be at fault.
If the car is setting in the sun.Heat builds up in the ducts. With my car, it was Heater matrix clogged with radiator sealant. The workshop cleaned it and it's start working fine!
Could be low coolant, stuck thermostat, bad heater control head (if climate control)
you need to replace your heater core
it wont blow heat itll blow cold air