My name is XXXXX XXXXX I am a professional here at Just Answer. I noticed that your question was not getting a response and thought I would see if you still need help with this.
In some cases this can be caused by a restricted heater core,or possibly an actuator on the heater box not switching from cold to hot.Locate your heater hoses going into the firewall for the heater core and warm up the vehicle completely,then turn on the heater on full hot and feel both of these hoses.If one is hot or very warm to the touch and the other is cold or allot cooler then the heater core is restricted needing either flushed out or replaced.
If both hoses are hot or very warm to the touch and you only have cold air coming out of the vents this may be a fault with an actuator on the heater box or the control to switch the door from cold to hot in the heater box.This would need to be checked into/tested further to locate the fault.
I hope this has helped and if you need anything else let me know.
both pipes are hot going into the bulkhead , where is the actuator located , ? can i check this ?