l have narrowed this problem down to being what l think is one of three things: The heater control valve in the engine compartment, which l have replaced but have no vacuum, odd to me.Possibly the temperature control valve in the dash, but l would need to know the voltage of the normal readings to know this.And the only other thing l can think of at this point would be something under the dash, but what that may be is beyond me. The manual l have isof little assistance, please offer any advice to what this could be.
It is definitely not the heater core due to no moisture upon startup, never once have l encountered any moisture whatsoever.
The control valve may not be getting any vacuum due to reasons beyond me, however, after using a vacuum pump to hold it open(and shut, l have tried both) still no hot air, very odd.The thermostat is fine, engine runs at optimal temperature, coolant level is fine. Have even tried driving it around with high RPM is as l thought this may produce more heat, still nothing.
Its a 1998 Mazda by the way, sorry for leaving that one out

It may be the heater core, which is typically under the dash.

when you replaced heater control valve are you sure the vacum is pulling the valve open to allow the coolant to enter the heater core ? is the engine getting up to opperating temputure l assume your in a cold climate its possible the thermostat is stuck open, i'm sure you have checked but is the coolant full, this will also cause the problem

the year of the car would be great, if ur car has electrical ac control the module that opens & close the core lid must be bad

or if was mechanical check the cable can be unpluged or the pin can be bad & dont hold the cable

another thing is.. new models has the lid made of plastic & with the heat the lid can be stick to the core & the electrical device dont have the streght to open the lid

you have take out the console & check the status of the heater core & the lid along with the device to open it

Definitely the heater core!