heater

Two possibilities. The blend door behind the dash, which moves one way or the other depending on the hot/cold setting is not working or moving completely for the hot setting. You're therefore getting a mixture of air out the dash. Check the blend door actuator and blend door for proper movement. Second, on the inlet side of the heater core hose, there should be a control valve. They normally default to the open position allowing hot water to flow through the core at all times. Normally vacuum operated. Sometimes they can get stuck in the closed position. Check for proper operation when set for heat. Both hoses, inlet and return hose should get hot when in heat mode.