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  Safety & Cooling Thermostats

Oven safety thermostats and fan initiation or control thermostats are normally small thermostats that are attached to the outer of the oven cavity to ensure safety, as in, your oven or cooker don’t get too hot so as to avert damage or, to control a cooling fan.

We will explain what they are and how they work.

  Oven Safety Thermostats

A typical oven safety thermostatThese small thermostats are almost exclusively very simple mechanical thermostats that have but two states, either on or off and therefore they are working or not and you get all or nothing.

This is great as it makes diagnosing a problem with one very easy and usually the symptoms are easy to spot as well.

What will usually happen in that the oven or, possibly ovens in double oven or multiple oven cookers, will simply go completely dead when these fail or are activated as they should be in the event of your oven overheating to a level where there is any potential for harm to the oven. Most often this will mean that all internal lighting in the oven, the elements and fan motor will cease to operate.

Many of these thermostats will kick in when you run your oven hot and the room temperature is high or if airflow is hampered in some way so please let your oven cool off and retry before buying a replacement. Then make sure that there is no external reason for the safety stat to trip again, before you buy a replacement thermostat.

Where these are used on a cooker it is very often the case that the oven clock or timer as well as the hob top, even when electric, will continue to operate as normal, just the oven or ovens will be without power.

Some of these are self-resetting meaning that, as soon as the oven cools down, the appliance can once more be used as normal. In very rare cases they are more akin to a thermal fuse and, if they fail then they must be replaced.

What they are doing in most cases is simple breaking the live feed to the oven in its entirety to prevent damage or danger. You only have a live connection to either side of the thermostat. When the preset temperature is reached that will vary depending on the design of the oven, the position of the thermostat and so on, it will simply open and kill the live side of the power feed to the oven.

If they fail they will normally fail open circuit so, the oven is dead.

That means you can check them for continuity with a multimeter very easily and, if you have a circuit then the chances are the thermostat is fine and, if not, then it’s almost certainly faulty.

Most modern built in ovens and cookers now have these safety thermostats fitted to protect you and your appliance.

  Ordering The Correct Safety Thermostat

Follow our golden rule for spare parts: If you’re not sure, ask!

The temperature used on your particular oven or cooker can be different from others, you need to get the correct safety thermostat for your model, it is essential to do so as these thermostats are not adjustable in any way at all.

The reason is that if it is to low a temperature used the oven will probably cut out too soon, which while not dangerous may render it next to useless in practice.

If the safety thermostat temperature is too high then the oven may not cut out as it should and this could potentially be dangerous.

Oven safety thermostats range from about 80˚C usually up to as high as 180˚C so there’s a lot of room to go wrong and, big penalties to pay if you fit the wrong one. And, as with most spare parts, they all look the same so don’t rely solely on images on any website other than for general guidance.

As you cannot adjust these thermostats you therefore need to fit the correct one or you face one of the two problems above so, don’t guess.

  Changing An Oven Safety Thermostat

Normally the biggest challenges faced in swapping out these small oven thermostats is in finding them in the first place along with getting to them.

Almost invariably they are fitted to the rear or top of the oven cavity. By oven cavity we mean the cavity where you put food into to cook, around that there is insulation and then the outer shell or panels. Very often they will be obscured by that insulation so not easy to spot on occasion but, most are relatively easy to find once you know to look.

Physically changing them is usually very simple and will normally all these types of thermostat will only have two wires connected to them so, they are not hard to swap out.

Let's show you a real world example.

Smeg range cooker with a fan initiation thermostat and a safety thermostat

The above is a parts blowout for a Smeg range cooker.

Item number 28636 is the fan initiation thermostat and item 39003, moounted on a brakcet to the rear of the cooker is the safety thermostat.

Sometimes though these will be placed directly onto the inner oven housing wall or, cavity or around that somewhere, it can vary from model to model as well as manufacturer and range.

If you are in a jam and absolutely need to you can normally bridge these out of circuit to allow the oven to be used however, in the sternest possible terms, it is a safety device and must be replaced as quickly as possible if it is faulty. Failure to do so can put you, your appliance or people at risk and this is to be done only if absolutely essential and only until the thermostat can be replaced.

If you do not understand the above, do not try to get around it!

There are a few that are slightly more complex of course and some safety thermostats that may require alterations to be made in order to fit them a they have been modified or changed. In most cases this is very minor but for some instruction is required on a specific type, which will usually be offered when ordered from our store if appropriate and/or required.

  Oven Cooling Fan Control Thermostats

These will, on older oven and cookers, sometimes have two thermostats but that’s not at all common. Most commonly there will be one thermostat to control both the on and off of the cooling fan, where one is fitted of course.

Basically these are almost invariably more or less a lower temperature setting variant of the oven safety thermostats described above, hence the including of them in this article.

Trouble is, they often look the same and it has been known for people (even the odd repair technician) to get the two confused. Especially on more complex ovens where there are multiple control stats fitted.

You can get a cooling fan initiation thermostat intended to start the cooling fan running when the temperature increases high enough to warrant the fan being operated and, one that is designed to operates until the oven cools down to another temperature and switches the cooling fan motor off.

The reason for this is that, on low temperatures of multiple cavity units, you may only be using the grill and unless it gets pretty hot, the cooling fan may not be required. But, in equal measure, it may be the case that in order to prevent any damage that you want the fan to run on, often for a considerable time, to protect the oven or cooker from any damage.

Okay, so lots of people can’t work out why this is so, we’ll explain.

In a number of ovens and cookers that have electronic control modules or other sensitive or delicate equipment in them they need to maintain a low temperature over those items and you have to account for any latent heat. If you think about it, most electronics will be at the top and, heat rises so, in order to prevent any harm coming to those items you will often find cooling fans running on for a good bit before the cooling fan will finally come to rest.

It is rather obvious once you understand why cooling fans will often run on after an oven or cooker is switched off, sometimes for quite a bit of time.

Much of the information and advice about oven safety thermostats applies to oven cooling fan motor control thermostats and, for the most part, diagnosis and changing them is exactly the same.

Also the same stands in respect to fitting the correct fan control thermostat largely only if you fit one that cuts out at too high a temperature you can fry the control board if one is fitted so, it is a very good idea to ensure that you get the correct one.

We do hope that this information helps you but if you have any problems please use our forums for technical and diagnostics help and our spares store for spare parts assistance and identification.

Richard Tallett
Beko cookmaster cooker ck90f232
The rhs oven cooling fan runs as soon as I turn the cooker on is the cooling fan thermostat faulty thank you.
Karl Foster
I have just received from you a ZN Limiter Cooling Fan Control No 3872079029 for my built AEG Oven PNC 944 185 047. The ZN Limiter has 4 connectors, 2 white and 2 blue but no other identification. How do I know how to correctly connect this device?Please respond soonest.

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