Cooker And Oven Thermostats
A lot of people do not understand oven thermostats which are, in essence, very basic electro-mechanical switches and how they work as well as how to test that an oven thermostat works and how to replace them. Often people think that these simple thermostats are much smarter than they actually are.
At its core an oven thermostat is a simple switch with two positions, on and off. That's it, it will do one of two things, close a simple switch or open it internally no more and no less.
Here we explain what they do, how they do it and how to replace and oven thermostat.
How An Oven Thermostat Operates
Understanding the general principles of how an oven thermostat works will allow you to determine whether or not the thermostat is actually faulty or not.
This is why that there is often only two electrical connections on the thermostat as shown in the image as all this one does is make or break the live supply to the fan oven element. If the temperature drops below the set level, it closes and switches the live supply to the element on and, if it rises above that level it opens the switch, breaking the connection and the live supply to the oven element is shut off. This cycle is repeated many times during the course of cooking once the predetermined oven temperature is reached cycling the element's power repeatedly.
Due to this cycling and, that is exactly what it is known as in the industry, there is a degree of error in a normal thermostat and this can be up to plus or minus 10-15% depending on the quality of thermostat used. Obviously the more expensive a thermostat is the more accurate that it will generally be so do not expect cheap thermostats used in cheap ovens and cookers to be fantastically accurate as, usually, they are not.
The way that the temperature is sensed is by using what we call a phial that is filled with gas, more on that below.
The gas in the phial expands and contracts with the heat causing a bellows inside the thermostat to move which opens or closes the internal contacts allowing current to flow or not as the case may be.
Oven Thermostat Phial And Capilliary
The phial is the "wire" that you see in the image which is actually known correctly as a capillary will normally have a long bulbous end that is inserted into the oven cavity itself to detect the temperature.
That capillary and the phial are vital to the oven getting to and maintaining the correct temperature that you set the oven to. So, when you replace the thermostat, you have to ensure that the phial goes back near enough exactly where it was sited in the original or you may get inaccurate (more!) temperatures in the oven.
The capillary can be strewn pretty much anywhere, under the oven insulation, over it, it really isn't of any great concern. Great if you can get it back exactly as the original was but in a great many cases most engineers would just say, "good luck with that" and add a knowing wink. Basically, we don't bother, so long as the phial is in the correct position and the front end of the thermostat is correct the capillary isn't of any real concern so far as position goes.
You must not bend the capillary over on itself leading to sharp turns as, if kinked, you've just ruined the thermostat and it wil never, ever work again. You cannot cut the capillary as the gas inside will escape and, again, it will never work ever again.
You need to gently unravel it on the new thermostat without bending too severely, kinking or cutting it. If you have leftover capillary (you often will when replacing an oven thermostat) just roll it up or don't unravel too much and it will be absolutely fine.
This is all because if you stop that gas held in the phial, capillary or the bellows in the thermostat from moving and, bear in mind these are minute, fraction of a millimetre movements, then the thermostat will no longer function and no supplier will cover damage caused by incorrect fitting under warranty. If you do not know how to fit an oven thermostat, call in a professional engineer.
Universal Oven Thermostats
You will see in our store universal oven thermostats. for an engineer it is a case of, "don't leave home without one" (or several in winter) as they fit a vast range of ovens and cookers and you can kill 90% plus of all thermostat jobs with one of these thermostats. Just search the store for "universal oven thermostat" to see the selection available.
However, like a lot of original thermostats when manufacturer's change them (they do that a lot, really annoying) you sometimes have to use a bit of common sense, skill and sheer determination to get them to fit. What is often the problem will be either the mounting points for the thermostat (where it screws to the front panel of the oven or cooker) or, the shaft offset.
The first problem is easyily sorted usually with a drill and a bit of skill or, failing that, a couple of trusty self tapping screws as most cookers (especially the cheap ones) have metal panels that are about as solid as a tin can. Can be more of a challenge on a decent cooker or oven though so beware of that.
The second problem, where the shaft offset is different can be a pain although it's not often a problem these days is where the "flat" on the shaft for the control knob is set at a different angle to the original. This can be a pain and usually involves the use of a drill, patience and bad language but it will always be doable, if not exactly the easiest job in the world.
But if you are stuck and cannot get the original thermostat or, the original one is a ludicrous price, these are often absolutely ideal.
More Help With Oven Thermostats
For more help with oven thermostats and diagnosing oven or cooker problems please see the rest of this section and also the cooker and oven section of self repair.
If you need technical support on a particular thermostat, cooker or oven then please search the forums for more information or, if you cannot find what you are looking for, simply register and post your questions there where someone will help if anyone can.