As discussed previously, the heater combines the pneumatic system (bleed air) with the electrical system (powered valves) and manual control valves. The function of the system depends on the speed of the airplane, the outside air temperature, the engine power, and even what the passengers choose to do with their gasper outlets. There's a lot that can go wrong.
If it's a hot day, everyone opens their gasper vents and then disembarks without closing them. Lets say the next trip takes us somewhere considerably colder, with a smaller passenger load, so there may be a number of unattended vents spewing cold air into the cabin. Remember, we have no control over that air supply. This problem can be controlled if I close all the gasper vents during the preflight inspection, during cold weather or before a high altitude flight.
If you get cold easily and want to annoy your fellow passengers with an excessively hot cabin, sit in row one on the right side, turn on your gasper vent and direct the flow of air towards the cabin air temperature sensor. It's a box on the forward bulkhead just behind the copilot (right seat pilot). It's sometimes just sticking off the wall and sometimes recessed behind a little grille. It will report a too cold cabin temperature and, assuming the heating system is on AUTO, the system will try to compensate by increasing the temperature. Similarly if cargo obstructs the temperature sensor, the cabin temperature may be reported as too warm, and you will be too cold.
The same air supply that provides cabin heat--nice to have, but not as essential to aircraft operation as, say, functional wings--provides air for pneumatic airframe deicing. A dual pressure switch ensures that sufficient air pressure is available for deicing. If the bleed air pressure from the engine drops below 25 psi, the cabin heat valve will not open. If the valve is already open, it stays open, but if the pressure drops below 20 psi, the valve will motor closed, and stay closed until the pressure is back above 25 psi.
On the ground, the deicing services are not needed, so in order to get heat in the cabin, the pilot needs to set enough torque to get that 25 psi of bleed air. Increase engine NG until the PNEUMATIC LOW PRESSURE light goes out (at 16 to 18 psi) then add another two pounds of torque. Make sure the vent fan is on.
On the ground, the ram air valve should be at least slightly open in order to move the heated air out of the silencer plenum into the cabin. Otherwise there isn't much point heating the air, unless the pilots just want a toasty warm cockpit floor. Once the air under the floor reaches 300 degrees fahrenheit a DUCT OVERHEAT light (5A CB L DC bus) illuminates to remind us to open the ram air valve.
In flight the ram air valve is usually kept slightly open, but if it's really cold, like latitudes greater than retirement age cold, the ram air vent can be closed, because the hot air entering the system acts like an ejector to drive air recirculation. The air recirculation intake is just above the floor by the base of the captain's seat. And all this time I thought that was just a convenient place to lose your pen. It also melts things you leave up against it, because the air doesn't always circulate exactly the right way.
Stale air normally vents out through ceiling grilles and leaves the aircraft throught he aft baggage compartment. If you have the Venezuelan burrito eating team on board, airflow can be increased by turning the heater to OFF, fully opening the ram air valve, fully closing the cabin air valve and increasing airspeed. In an emergency, the aircraft will be ventilated faster by opening the cockpit windows, but the fumes will come forward through the cockpit instead of out through the rear.