There's an article on line at www.izook.com on cleaning out the fan compartment to get better heat. Try there.
from below the heater
An air sensing or air flow switch will do the job to prevent the heater from operating is there is no air flow from the fan.
To visually determine if there is enough air flow across a duct mounted electric heater, you should tape a piece of cardboard to it. A small piece that can flutter in the flow of the air. If it moves wildly, there is enough air.
No air flow from the heater duct of an 88 Chevrolet Corvette is often caused by a defective blower motor. A blocked air inlet or defective switch can also cause this issue.
The heater does not blow air because it emits infrared rays that have heat molecules within them, thus releasing heat, not blowing it. A heater does not blow air becasue there is no moving air source within the heater. The heater emits infrered rays that cause heat to slowly flow out of the heater.
If there is no fan to pull the air in, fresh air must come into a convector heater from below. It is drawn in because the air heated by the heating element is warmer and therefore lighter than the colder air below it. So the warmer air rises to leave the convector heater from the top.
As the water begins to flow within the system, the components inside the heater compartment ( the pressure switch)senses that flow and if your heater is calling for heat it will fire. When that water ceases to flow those same sensors tell the heater to stop firing. Built in safety devices. Ken
on top of heater housing
from the air in the vent to the air in the room
The best bet is that the valve that controls the flow of water to the heater core is not functioning, but on some cars it's the flaps that control the air flow through the core.
If there is a restricted air flow through a heater, yes a heater can get hot enough to start a fire.
there is a little electric motor located under the dash on the heater box it opens a little door in there that directs the air to the defroster, vents, under the dash etc. those are bad about going out. If air flow is reduced when a/c is on- could be an incorrect refrigerant charge - causing the evaporator to "Freeze" When you shut of system evaporator has a chance to defrost restoring air flow. Try turn vent only on when air flow is reduced to see if evaporator defrosts and air flow restored