Knowing what’s going on inside an oven when it’s working properly is a definite asset the day everything coming out of it is either burnt, not cooked, or unevenly cooked. A basic understanding of how the appliance is supposed to operate is a step in the right direction in deciding what to look for while troubleshooting to find the defective component. The following explains how different types of ovens work and what components are used to make it work.
There are basically four types of ovens—conventional, fan-assisted, fan with electric thermostat, and digital electronic with PCB and NTC sensors.
The Conventional Oven
The most basic oven on the market is the conventional oven and it has a very simplified circuitry with a selector switch, a thermostat, a cooling fan, a bake element at the bottom of the box, and a broil element at the top of the oven cavity for broiling.
With the selector switch set on bake, the bottom element controlled by the thermostat will come on to warm up the oven. The top element might also come on in conjunction with the bottom element, but in short bursts only, just to even up the heat throughout the oven cavity and cook more evenly (Figure 1).
In some oven models, a bake selection which turns on the bottom element only, also controlled with the thermostat, this selection can be used for specific cooking where that is more useful such as cooking a casserole.
The broil selection on the switch will activate the top broil element only, also controlled by the thermostat. Some ovens might have a dual top element and possibly two settings for the broil selection, one setting for the outer element only, the other for both of the dual top elements (Figure 2).
There is also a cooling fan on the outside of the oven at the top with a thermostat of its own keeping the electronics from getting too hot by blowing the heat out from inside.
Conventional Oven Particularities
The bake (or bottom) element is not always obvious just sitting on the floor of the oven cavity. Some brands and models of ovens have their bottom element concealed underneath the floor of the oven. That makes it a lot easier to clean the spills and particles of food that would otherwise have lodged themselves around the element or its supporting legs.
Another detail to take into account concerns the top or broil element that often is a dual element assembly, as previously mentioned, that on certain models has the outer section of the dual element activated while baking, to even out the heat distribution in the oven cavity. Two models are shown in Figures 3 and 4.
With a dual broil element, both sections are working when it is switched on broil.
A thermostat that is failing could either cause the food to take longer to cook or not cooked inside, but also to burn the food. The food burning, however, could also be caused by a faulty bake element, leaving the top element only working.
A failed selector switch is usually caused by electric sparks. A common cause is the selector switch being turned while the thermostat is on and creating sparks that gradually deteriorate and burn the contacts inside the switch, greatly shortening its lifespan.
In design, the fan assisted oven still has a bake element (bottom), a broil element (top), a thermostat to control both and a cooling fan keeping the top cooler, but in addition, a fan with another element is added to the back wall to circulate the heat from all three elements making it much more even throughout the oven (Figure 5).
It still has a selector switch, a thermostat, a timer with a relay to allow turning on and turning off at certain times for auto cooking, a bake element, a broiling element at the top of the oven cavity, a fan and element in the back wall to generate and circulate the heat in the oven and a cooling fan on top of the oven box to protect the electronics. Although the thermostat still controls all the heater elements, it does not affect the operation of the oven fan.
Here are some specifics about the cooking selection:
On the convect bake selection, all three elements are activated but at specific preset intervals, each in turn, as the oven fan never stops going.
On the bake setting, all three elements are activated but at specific preset intervals and each in turn but without the oven fan ever running.
On the convect roast setting, all three elements are activated but at preset intervals and each in turn as the oven fan never stops going.
On the broil setting, only the broil element is ever activated and without the fan.
It should be mentioned that if the convect baking and convect roast setting seem to work the same way, the convect roast setting lets the meat probe control the operation of the oven instead of the oven’s thermostat.
If the oven fan isn’t working properly, the food might be burning at the back of the oven and not cook at the front.
Fan-Assisted Oven with Electronic Thermostat
This oven basically operates like the previous fan-assisted oven, but this is a fan-assisted oven without a bottom element, and it has a selector switch with the thermostat built-in.
It works the same as the previous ovens, except when it's set to broil, the thermostat on the selector switch is bypassed to a separate electronic thermostat which takes over by reading the resistance of the element that changes as it gets hot, and turns the element off, and then back on as it cools down.
With a fan oven without bake element, if the oven cooks and works properly but the broil setting doesn’t work as it should, the electronic thermostat is the likely problem.
The Electronic Oven with PCB
The electronic oven with PC board is basically an oven with a PC board/timer assembly with relays onto the board each of which specifically controls one of the heater element, fans, and other components based on the signal received from the NTC sensor which replaces the temperature-sensitive capillary tube in the other oven types. The electronic timer circuit determines what is turned on and off depending on which cycle it’s on.
As far as cooking or baking goes, the oven is designed with the same layout and operates the same as any other fan assisted oven.