Buying Advice And Help
Principals Of An Oven
- Last Updated: Sunday, 22 May 2016 20:48
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One of the things we get a lot of as a repairer is complaints about poor cooking results from certain ovens, in particular gas ovens seem to be a problem for some customers especially if they have previously owned an electric oven. But in this article I will try to de-mystify some of the terms used and explain just how they actually work.
We all have an oven as a general rule, there's hardly a home in the land that doesn't have an oven in the kitchen in one form or another and, for such a common everyday device, there's very little information on how they actually work. This article aims to change that.
If we ignore microwave ovens, or cookers if you prefer, and the derivatives there are essentially only two different ways to cook in a normal oven with heat from an element. These are known static and forced air cooking, or fan cooking. I'll get to those later, but first let's explain an element to you.
In almost every instance in an oven an element is seen by you and I as a solid lump of thick metal wire formed to the shape required by the appliance, in fact it's a little more complex than that. What it actually is, is a thin gauge of wire often in a spiral shape or a single thick wire encased in a powder which is then encased again in a metal outer, as shown in the photograph to the right, with a terminal (for electrical connection) connected at each end of the encased wire. This is why you see the porcelain type bit just at the terminal as it insulates any electricity being passed onto the outer metal case of the element that you actually see and touch.
When you think about it this is blindingly obvious as many of the metal parts of the element touch the case, which is normally earthed, like the mounting brackets.
It is also why we hear that elements "explode"! Well, they don't really, the outer case splits (or similar) and the powder inside which is under pressure escapes and gives the impression of a puff of smoke before the element dies.
There are only so many types of elements in a cooker or oven being, lower base element, fan element, side elements and grill elements. But whilst that list is pretty short there are, quite literally, thousands of variations on the theme in shape, size and mounting which are all too often manufacturer, or even model, specific. Of course the lower the volume manufactured of such things then the more expensive they get, it's the economies of scale.
However elements are pretty dumb things in the general sense, they either work or they don't. You either have continuity through it or you don't, it's that simple.
On the odd occasion you'll get a hot spot on an element where the internal insulation is thin or the inner wire too close to the outer sleeve but these will quickly blow and go faulty. You can also, where the machine was stored in a damp or cold environment, get tiny droplets of water in the elements or terminals which will cause an RCD to trip, this is why that on occasion an installer will run an oven for a few minutes with the earth disconnected before finalising the installation as it dries this out and ensures proper operation.
But all in all elements are simple devices.
Bearing mind though that, like other spare parts oven elements also suffer from poor quality imitation items. Some elements, like the Ariston and Indesit one and some Hygena and Diplomat ones are extremely popular and we see many inferior copy parts. You can find out more about the differences by reading this article
Of course you need some way to control the heat, when to put it on and when to shut it off so we have a thermostat, or sometimes more than one.
In a normal domestic oven the thermostats are pretty rudimentary affairs that use a capillary in the oven to read the air temperature. A gas expanding or contracting in that capillary operates a switch which will switch on or off the element/s as required.
These thermostats, in a domestic oven, are generally pretty inaccurate the truth be told and as the prices drops, so does the accuracy of the thermostat. It is generally accepted that +/- 10-15% is perfectly acceptable in a domestic oven. But they can be fooled by improper placement and fixing, especially if they are not sited correctly after repair or replacement.
One thing that you never, ever do (I've seen it done) if you do have call to replace one, is to cut or bend over the capillary as, if the gas doesn't move in there, the thermostat won't work!
There are also now a good proportion of ovens fitted with a safety stat, often mounted on the top of the oven cavity below the top cover. This is there to prevent the oven just heating and heating if the main thermostat goes faulty. They do however fail and can cause issues like, no heat or cutting the oven out early, this can also happen if the oven is not correctly installed or ventilated correctly.
You can of course buy a new thermostat or oven switch from our online store, we have listed the most common cooker and oven thermostats which you can find from this link and of course, for anything other thermostat (or any spare part really) all you have to do is ask and we will source it for you.
The Oven Box
This is quite important and very often overlooked when choosing a new oven. People tend to look at an oven or cooker and think, "well they're all the same really, they all heat up and cook food, I'll pick the cheapest one I like" well, yes and no.
Apart from the build quality and the quality of the components used you also have to think of how good the insulation is. By that I mean how well, how good and how much insulation is around the inner cavity that you put food in, it's like youÃ're home when you're trying to save energy, the better the insulation the more efficient your home is. Same principal here, poor insulation and door sealing means you loose heat that should be retained in the oven cavity and it's likely going somewhere you don't want it to go as well.
As touched on in the previous paragraph this applies to the door area. Remember you have a lot of hot air in there wafting around and it need to remain pretty stable temperature wise, so the less you have to open the door and the less heat escapes through it then the better the performance. Obviously a double glazed door is required and triple, even quadruple glazed these days insulate against heat loss even better.
The nice thing about better insulated doors is that it also reduces the external temperature of the door, so if you have kids the more insulation the better from a safety point of view.
You can find out much more about oven surface temperatures from this article including the current legal requirements
Different Cooking Methods
On though whole cookers, or ovens in particular are really pretty simple devices, the concept is extremely simple, it's the execution of the concept that can often become rather complex. The basic idea is to provide as even a heat as possible in a formed cavity, the heat (both direct and reflected) then evenly cooks the food placed in the cavity. Simple isn't it?
Well it is simple enough as a concept as I stated, but there's a few ways of doing the same basic thing and I did say I would get back to this earlier.
Static Ovens Or Conventional Cooking
The first, most straightforward method we'll look at is known as "static" or "conventional" cooking. As shown in the diagram below there is an upper element, which will most likely also serve as a grill element and a lower element below the base of the inner lining of the oven. What happens is that the two elements are brought on at full blast until the thermostat detects that the oven has reached the set operating temperature, when it does the power to the elements is removed and they cool off, as does the cavity temperature over a period of time. After the temperature falls below the preset temperature the elements are switched back on again and the cycle starts over.
This is the basic principle on which almost every oven out there in the field operates on irrespective of the methods employed to achieve the goal.
Static cooking is also the oldest method of cooking out there, fan driven heating came along a lot later and many people that cook swear by this method for results, especially for baking.
This method of cooking is ideal for slow roasts and the likes and delicate baking as it is quite a relaxed and gentle method of cooking although not the most efficient it has to be said. You will also normally find this method available as an option on a multi-function oven.
Also remember that, because heat rises, the top of the cavity in a conventional oven is usually warmer that the bottom. This is why many recipes will state something along the lines of "place in the central baking tray of a pre-heated oven". The reason they want it in the centre is so as it gets about the right temperature that you have set and also as that is the most stable area for heat giving a reasonably even heat. The reason for pre-heating is that generally, especially with pre-packed meals and the likes, that the cooking time is measured at that temperature for the time given however there can be huge variances in the performance of different ovens so for best results we always avise customers to experiment with the particular machine to get the best from it.
Fan Ovens Or Forced Air Cooking
A fan oven currently seems to be the single most common type of built-in oven that we see as well as being standard on many range or free standing cookers. But not all fan ovens are equal and not all perform the same.
The idea is that there is a fan in the rear of the cavity that is encircled by an element, which can glow bright red in some models. The fan inside the element runs and cools the element whilst driving the hot air around the inside of the oven cavity, hence it is sometimes referred to as a "forced air fan oven".
If the appliance is good execution of the principle then the temperature around the inside of the oven cavity will remain pretty stable, it is important to note however that thermostats in domestic appliances are not that accurate and setting the stat to say, 180?C, will cycle up to 15% either side of the set temperature giving an average of 180?C. It is also worth noting that there are differences, sometimes quite pronounced, between different appliances due to the airflow pattern and the rating of the element/s involved in the cooking process. This can adversely affect the time that the oven takes to heat up as well as produce "hot spots" in a fan oven. Where this can be seen quite well is on some less well engineered machines the top of a pizza will be burnt whilst the base remains soggy.
Of course not all cooking problems are down to the appliances, much of it comes down to the uses to which functions are put and the lack of explanation on how to use and, indeed, when to use the various options on many modern ovens. Using the above example of a pizza, especially a frozen one, they are far better cooked with a lower heat only or on a pre-heated pizza stone, so that the base gets crisp and bakes (it is bread after all) whilst the topping cooks more slowly and remains moist. As I'm sure you've realised by now, especially if you have a basic fan oven, the option to do that simply isn't there!
You can get around the problem to a degree by following the manufacturers recommendation as most will state that for fan cooking you do not pre-heat the oven before use (personally I allow mine to warm a bit first) and then reduce the time and temperature by 10% from the instructions or recipe. Bear in mind that pre-packed food with instructions given are usually for a static oven and not a fan oven although the likes of Marks & Spencer do generally give guidance for both conventional and fan cooking.
But for fan cooking it is very much a case of trial and error until you find the settings for the oven that suit you.
As a little tip, try defrosting food with the heat off and the fan on in the oven, many have this option usually denoted by a snowflake type symbol on the function dial, as it defrosts pretty quickly and you do not get any "burns" as you often get when using the microwave.
In a true fan cooker you also do not get the transfer of flavours from one dish to another, so you can be cooking garlic mushrooms whilst roasting a salmon and the flavours will not migrate whereas in a conventional oven they would do.
Multi-Function Cooking Explained
Customers get confused by this term when it is in fact very simple and allows you a lot more flexibility in the method that you choose to cook with.
It simply means that the oven has many different ways to cook, usually with at least six or seven different options including both static and fanned cooking.
The other options that it may allow could include;
- Lower heat only
- Upper heat only (grill)
- Upper heat with fan
- Fan only
- Fan with upper and lower hear
- Fan with all elements
You may ask why anyone would wish to have all these different options, but it's really quite obvious once it is explained.
Lower heat only can be used, as described earlier in the pizza example, for any sort of pastry based pies, such a quiche or pizza or just about anything that you only wish to cook the base whilst leaving the top moist.
Upper heat only is generally the grill function which is, in continental appliances not always as powerful as we like in the UK. The reason is that whilst we cook with the grill (broiler I think in the US) most Europeans only heat with the grill, so on lower cost appliances in particular this can be pretty poor. It is also worth noting that not all grills are infra-red, i.e. they will not glow bright red on many EU appliances and with some only the centre section will glow whilst the outer is warm and is cooking it is not infra-red. If you use your grill to cook a lot, I know that some people require this due to dietary concerns, then it is worth asking before purchase if the appliance is suitable for your needs.
Upper heat with fan is basically fanned grilling, much like fan cooking this is faster and takes a bit of getting used to. I still burn the sausages every time I use this function after three years! Often this function is referred to as Turbo Grilling.
Fan with upper and lower heat is generally more gentle than fan cooking with the single element at the rear and allows most of the benefits of conventional baking but also the advantages of fanned cooking. Again it is best to reduce time and temperature until you are familiar with the function's use.
Fan with all elements is really only an option I've seen so far on Smeg cookers, it brings on the rear, upper and lower elements and it really is very fierce indeed. It is however extremely useful for heating the oven very quickly when you're in a hurry!
For more on the various functions that are available in a multi-function oven please see this article