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memarcus

Truth about 'True Convection'

memarcus
15 years ago

Hello all first time posting. My wife raves about these forums and we're currently deciding on appliances for a new home build and had a discussion on the issue of "true convection."

Can anyone explain the differences between normal convection and true convection? I am a mechanical engineer by profession and fairly knowledgeable of basic heat transfer mechanisms and don't understand the differences. I am not a chef but is there any advantages of one over the other such as browning faster due to the higher power?

Comments (41)

  • weissman
    15 years ago

    The big difference is the source of the heat. With regular convection, the heat comes from the bottom of the oven and is circulated by a fan on the back wall. With true convection the heating element is around the fan on the back wall so all the heat comes from the side. The supposed advantage of true convection is that you can bake things like cookies on multiple racks of the oven at the same time and they will all get done evenly since the ones on the bottom don't get any direct bottom heat as they would otherwise.

  • memarcus
    Original Author
    15 years ago

    Weissman, I understand that part but can't understand how the heat source makes much difference. Convective heat transfer is a function of area, fluid/film temp, and heat transfer coefficient which is largely related to Reynolds number and specifically fluid velocity. No where in the heat transfer physics does the heat energy source location come into play. Assuming the fan moves a reasonable amount of air for circulation, the cooked item should not "see" any difference because the food doesn't know where the hot air came from, just that's it's hot.

    The only possibilities for advantage I can understand are 1) the fan draws in fresh unheated air 2) the additional element allows higher power output thus allowing food to brown more quickly or 3) the additional element allows heating above the oven temperature. With situation 1 I would think this would be a horrible idea design efficiency-wise and secondly it would make a cold spot (relatively speaking) in front of the fan. With situation 3 I would think this would cause a hot spot in front of the fan.

    Has anyone out there used both? Does anyone know of any manufacturers that list the fan flow rate?

  • edlakin
    15 years ago

    i've used both and don't see any difference. i'm a chef and use 'true convections' at work and have used multiple home convections where the heat comes from the bottom and is circulated by a fan in the back.

    neither is superior or inferior in and of itself. there are too many other variables--quality of oven, calibration, ability to hold temp, quality of cookware used, what's being cooked, temperature setting, etc--to claim that 'true convection' makes any difference all by itself.

    i've had professional 'true convection' ovens that heated unevenly. i've used residential ovens with back-mounted fans that heated very evenly and didn't require rotation of whatever was being cooked.

  • maxdel
    15 years ago

    It has been a while since I took physics and similar classes but in the end it is just hot air that is being moved around. As the air impacts surfaces (food, containers, etc...) it cools down by releasing its heat into the surfaces.

    Perhaps "true convection" heats "better" because it does not pass the bottom shelves on the way to the rest of the oven. The physics you stated would not take this into account as it would treat the cavity as a whole. I suspect the differences are minor but are there and blown out of proportion by marketing.

  • memarcus
    Original Author
    15 years ago

    This is what I suspected. In terms of heat transfer the importance of the velocity of the fluid is second only to the thermal gradient driving the transfer so I'm surprised the manufacturers don't brag about fan size. Assuming they are appropriately sized to turn over the oven cavity several times per minute this should create enough turbulence to overcome free convection currents and standing hot/cold spots. Of course I guess you wouldn't want the fan too big because then you'd get flat souffles and cookie sheets would blow around.....

    Edlakin-Does the extra air current tend to dry foods more or does the speedier cooking rate allow juices to be locked in by a browner outside?

  • edlakin
    15 years ago

    well....yes, to both, memarcus. it really depends on the food you're cooking.

    the air blowing can dry food out, especially when you use it at lower temps. we've used ours as makeshift food dehydrators for that reason.

    it can also aid in excellent hi-temp roasting, especially with meats, as you've alluded to, yielding well-browned exteriors and very juicy interiors.

    it all depends on how you use it and on what kind of food you're using it on.

  • rococogurl
    15 years ago

    This explains it quite well IMO

    Here is a link that might be useful: All About Convection

  • writersblock (9b/10a)
    15 years ago

    Thanks for this thread. I've been looking at several ovens that offered both convection or true convection and have been puzzled by the difference.

    roccocogurl, one thing in that article puzzled me: it said conventional oven for a roasting pan, yet I always hear about how great convection is for roasting meats?

  • capecodcook
    15 years ago

    Wouldn't the radient heat of the heating element have to be taken into account also? Seems like that might tend to brown the bottom tray of cookies a little quicker than the trays being baked by convection alone. For other than an oven full of baking sheets this is probably not too much of a factor.

  • weissman
    15 years ago

    writersblock - the article talked about not using a DEEP or COVERED roasting pan with convection. The roast needs to be exposed to air for the convection to be effective. I have a roasting pan with a V-shaped rack in it. The rack raises the meat or poultry above the bottom of the pan so that the juices drop into the pan and the roast is more exposed to the convected air. It works great for turkeys, ducks, etc. - crispy brown on the outside, moist on the inside.

  • writersblock (9b/10a)
    15 years ago

    Oh yeah, I wasn't thinking about the rack inside. Thanks. Duh.

  • chefkev
    15 years ago

    For roasting medium to larger sized cuts of meats/poultry, I'm a proponent of a combination method starting out with moderately high heat convection until desired browning is reached and then switching to low heat conventional mode to allow the meat to cook through and reach desired doneness. This gives the best balance of flavor from browning and maintaining juiciness/reducing shrinkage. Although this takes a little longer than straight convection roasting, a little time is gained back in reduced resting time after the meat is removed from the oven. It's basically applying the same concept I learned in restaurants about cooking a thick steak where it is "marked" on the grill for browning and finished in the oven for even cooking.

  • memarcus
    Original Author
    15 years ago

    Rococogurl- Thanks for the link it does explain it in the basic form but it confirms my suspicions and the text about mass transfer is self contradictory. In one area it says convection makes things juicy but in another it says it helps evapoation as it should (higher Sherwood #).

    This is from the page.
    "Total heat transfer = (Food surface area) x (temp. difference) x (transfer coefficient)"

    Food surface area is the same regardless of oven design.

    Temp difference is the same regardless of oven design (the controls will energize the element until the probe reaches the appropriate resistance, this is the little metal tube with pinched end poking into the cavity from the back wall)

    That leaves the transfer coefficient, which is dependant on fluid velocity. So where does the location of the third element come in? I believe the third element does nothing but generate heat in a different location (which doesn't matter) and is hyped for marketing purposes. This really irritates me when manufacturers do this.

  • rococogurl
    15 years ago

    writers -- way back in the dark ages I had one of the first small Farberware convection ovens -- about the size of a microwave. I used to make the best chicken and leg of lamb in there just by putting the meat directly on the rack and letting the drippings fall into the pan below.

    My current oven has a pan that slides in right under the rack. I cook on that the same way. No pan at all unless something will give off a huge amount of liquid (I do use a shallow pan for turkey if over 12 lbs).

    With or without convection, I start most roasts (even turkey) around 400-425. My oven has an autoroast feature that's perfect for me -- it sears and then lowers the temp slightly. What chefkev was explaning except I don't need to make the adjustment myself.

    The advantage of convection roasting over radiant (non-convection) in my mind is crispness -- the way circulating air dries things.

    When baking, I'm careful not to mix the dark and light cookie sheets since the dark ones will brown things more and it's recommended to reduce the temperature by 25 degrees when using those.

    What I like about my oven is the heat combinations. F.ex. my cobbler has a thick biscuit dough that needs to cook through, plus fruit mixed with cornstarch and sugar that needs to simmer to thicken and dissolve.

    In the old oven, the top cooked ok but sometimes the bottom would not heat up enough and I'd get pockets where the cornstarch didn't cook with the fruit. Or, I would not get the top cooked through as well as it should be.

    With the current oven, can use a surround setting, which heats from the top and bottom - not only the top -- so the bottom bubbles plus the heat penetrates the top crust much better. Alternately, I could use bottom heat plus the fan.
    There are more settings, which annoy some people, and there is a learning curve (at least with my oven - Miele).

    Sometimes I have to look at the instruction book to figure out which setting to use but I like it because I ultimately get much more control.

    Depending on where the heat is coming from and whether or not the fan is going I can get a hot/crisp effect or more gentle heat, or more even, or searing + automatic lowering.

  • capecodcook
    15 years ago

    FYI, I was referring to "regular convection" in my post above. It would seem to me that the radient heat from the heating element in the bottom of an oven using "regular" convection as opposed to "true" convection would tend to brown the bottom of the tray or pan above it as well as heating the air which is circulated. This would provide a different cooking experience than just circulated hot air.

  • rococogurl
    15 years ago

    memarcus - I'd discount some for marketing. Explanations of convection ovens often differ slightly with the source, manufacturer.

    I've had 2, the little Farberware one and now the wall oven. My observations are drier and crisper with the air. Searing does seal in juices if there are any.

    Also found there is a "regular" bake without air that's like any other oven -- ok for things like baked potatoes or casseroles. Then there's the top/bottom setting without air, or the bottom heat setting with the air. The effect of using them is trial and error, I'm finding.

    Bottom line: this is my best oven so far. Most nuanced in terms of what I can set and how it cooks and with a very accurate temp and digital readout (a big advance for me over temp averaging and imprecise knobs). It's a step beyond one type of heat with one heat source -- to me.

    Turbochef is the only really new technology with dramatic results I've seen in the lat 15 years.

    capecod-- if using the heat from the bottom element with the fan, I put things in the center of the oven to equalize the browning. Have had good luck and even browning with multiple racks, too (special setting).

    But can only speak for my oven in re results. Not sure I understand the difference between "regular" and "true."

  • oruboris
    15 years ago

    I made the case here a year or so ago that 'true convection' is calculated by the marketing dept. to imply that anything else is 'fake convection'. It would be more accurate to call it 'third element convection' if we want to discuss it without parroting the marketing mantra.

    Betcha there's a certain class of buyer who go with 'true' because they wouldn't want the appliance salesman to think they are the sort of people who'd accept anything 'fake'.

    My suggestion that the source of the heat was a pretty small issue compared to the circulation itself was met with a level of self righteous indignation on the part of 'true' partisans rarely encountered outside the political sphere.

    But seriously, I think its a bit of a 'more gourmet than thou' thing: some people insist that the difference in outcome is very noticable to their educated pallets, but I'm rather skeptical that they could actually tell the difference in a blind test.

    Worth noting that any difference between the two is only in play when the elements are actually firing. Once the oven is at temp, the fan is just blowing hot air. When the elements are firing, the placement of the elements would matter only if the air stream of the fan is significantly hotter than the oven chamber itself. No doubt it is a bit warmer in a third element situation, but considering how fast the air is moving and how small the chamber is, I have a hard time beleiving the difference is significant.

    I do like having convection, and if _everything_ else was equal, might use third element convection as the tie breaker. But I wouldn't pay more than a few bucks for the 'upgrade'.

  • heartsurgeon
    15 years ago

    if you start with an incomplete understanding of the facts..you end up with an inaccurate "analysis"

    "tru-convection" uses a heating element shielded behind the fan shroud, to heat the oven. Hence, as CapeCodCook pointed out above, NO RADIANT HEATING of the food/pan/pot takes place.

    "regular" convection cooking uses the heating element below (and sometime above) the food/pan/pot. Radiant heating therefore will play a roll in the cooking process.

    using your engineer logic, broil and bake should be the same...not. The source of the heat makes all the difference in the world, because of the radiant effects.

  • rococogurl
    15 years ago

    oroboris -- The thought did occur to me that if there's "true" convection there must be some "false" around too.

    It would, I think, be very tough to discern a convection-roasted something vs a non-convection roasted something on a plate. Maybe chicken skin (preparation being the same)? We should do a throw down for Top Chef!

    heartsurgeon -- my oven uses the fan heat plus radiant on several settings.

    Here's an explanation of the variations from the Britannia range folks in England -- helped me understand it to the degree that I do. My oven settings are similar.

    Here is a link that might be useful: Convection But Verify

  • ya_think
    15 years ago

    My gut reaction was to agree with capecodcook and heartsurgeon, but I just did a crude experiment that to me actually proves they are right:

    I heated a cast iron skillet on the range to really hot. Then I held it perpendicular to a running window air conditioner with my left hand. I moved my right hand closer to the skillet, and despite the cold air blowing between the pan and the skillet I clearly felt the "radiant" heat on palm at about 8-10 inches away. Meanwhile my face felt no difference in the temperature of the cold air blowing into it, regardless of whether the skillet was there or not.

    Now while I have no basis to say what practical effect the various modes have (though I think common sense would say it matters) I have no problem accepting that different heat sources result in different heat patterns, even with a fan blowing air around.

  • ya_think
    15 years ago

    typo - I meant to say "despite the cold air blowing between the pan and my hand..."

  • heartsurgeon
    15 years ago

    radiant heat is not the only factor in your oven.even with convection cooking, you can end up with air circulation patterns that create hot spots or cool spots. hence the debates about single fan convection versus "European" convection with multiple fans. It gets kinda crazy, and meaningless after a while...

    however, i will say that every oven i've owned, has ended up browning the skin of a turkey, slightly differently, inspite of identical cooking technique..

    i had a GE convection oven, that would leave some spots on the turkey pale, while other areas of the turkey were burnt..
    i had a thermador oven that produced the most uniform, beautifully browned turkey i'd ever seen..
    the viking i have now, browns one side of the turkey, slightly faster that the other side...

    they were all convection ovens, they all cook slightly differently...

    i use to just monitor the meat temp with a thermometer, but know i'm monitoring the oven internal temp as well...it's not nearly as accurate, or as stable as you might believe!!

    try it yourself and see...

  • oruboris
    15 years ago

    I'm not saying-- haven't ever said-- that the heat source doesn't matter _at all_. I'm just saying the importance of having a third element is overhyped, and the phrase 'true convection' is a bit of a cannard, more about marketing than cooking.

    When I've done a roast [in a roasting pan with rack] on convection, the bottom is only a little more cooked than the top and sides. Not a great deal more, as I'd expect if a lot of the heat were being transmitted radiantly. To me, this prooves that the location of the element in convection cooking does matter, but not to a great degree.

    Perhaps with third element, the meat would be perfectly even, I don't know. I just don't think its worth much more money to have the underside of my roast a bit less brown.

    The difference between regular and third element convection isn't remotely equivalent to bake v. broil, which are about technique as much as the location of the heat source. Baking elements on the bottom create passive convection currents that heat the oven more evenly than a top element would. But broiling is all about radiant heat, minimizing the hot air as much as possible. The location of the heat source doesn't matter, as we are all happily aware during grilling season...

  • cpovey
    15 years ago

    RE: ""tru-convection" uses a heating element shielded behind the fan shroud, to heat the oven. Hence, as CapeCodCook pointed out above, NO RADIANT HEATING of the food/pan/pot takes place."

    No correct. Once the walls of the oven are hot, they create radiant heat.

    Only three ways to move heat: Radiant, convection, and conduction. ALL at work in ALL ovens.

    Conduction-baking pan gets hot, bottom of cookies brown faster than tops. Conversely, if you used a cast iron baking pan, cookie bottoms would not brown as fast, because cast iron is a poor conductor and heats slowly because of it's large mass.

    Radiant-once oven walls get hot, they radiate heat (in the form of infra red waves) to the food.

    Convection-heat transfered by the movement of molecules in another media, in the case of ovens, hot air.

    In reality, all ovens use a combination of all three methods of moving heat. Since ALL ovens use convection, calling a particular oven a convection oven is really another mistaken marketing term. Using a fan in an oven to aid convection should properly be called 'forced convection'.

  • jpreisman
    15 years ago

    I've been following this thread with interest. While I can't contribute to the physics and mechanical engineering aspects of "true convection", I think, as a user, I can contribute to the practical aspects.

    Having owned a convection, regular radiant, and a "true or european" convection range within the last 6 years, I can tell you that I have seen a significant difference between the first two and the last. When we moved to our current home, I replaced the existing range. Having used convection for the previous 5 years without seeing any real advantages, I had a radiant range installed....less money and I had decided that "convection" was indeed a marketing ploy. I had seen no real differences between the two. The convection range that I had had just the fan, no third heating element.

    A year ago, we changed all appliances in the kitchen. This time, we chose a Bosch range with European convection....mainly because I liked the looks of the all stainless model and had always been pleased with their dishwashers. This range has a separate heating element around the fan in the back wall of the oven. My initial reaction was.......WOW, what a difference.

    I think the easiest way to explain the results is this: use the convection settings (bake, roast, broil) whenever the food item needs to be "sealed" to keep moisture in...like cookies that are crisp outside & moist inside, roast chicken with a crackling brown skin and very juicy interior, broiled fish with a lovely delicate crust and incredibly moist interior. Do NOT use convection for anything that has to rise in the oven......cakes, bread, brownies, etc. The fan causes that exterior "skin" to occur much too quickly.

    All that being said, the results I'm getting with this range bear no resemblance to those with the other two. Since "I" haven't changed, my methods haven't changed and I don't think that chickens and various other foods have changed, the only conclusion is that having that third element around the fan is making all of the difference. If there is any "marketing ploy", it's those companies who market their product as "convection" and just throw a fan in the oven.

  • ya_think
    15 years ago

    cpovey - I have never studied thermodynamics, so these comments are based solely on my limited understanding and what would at least seem to be common sense. Please correct me if I am going wrong somewhere:

    "In reality, all ovens use a combination of all three methods of moving heat."

    Agreed, but different modes and heat sources would have an impact on the predominance of each type.

    "Once the walls of the oven are hot, they create radiant heat."

    In "true convection" the air is heating the walls, not the other way around. Heat moves from hot to cold. So it would seem to me that when in a true convection mode radiant heat is a minimal factor at best, possibly most noticable only when the heat source is off during the cycle. Quite a different scenario from when oven walls are heated to purposely act as the heat source, as would most notably be the case in an oven such as a traditional Aga.

    My understanding therefore is that true convection attempts to provide the most even heat distribution possible: The primary heat source is convection - yes, forced convection. This would be desirable in situations like heating multiple racks of cookies, hors d'oeuvers, etc., so that your top and bottom racks are minimally impacted by radiant heat and will bake at the same rate as the middle rack which is by comparison fairly shielded from radiant heat.

    Am I making any sense at all, or am I being brainwashed by marketing?

  • guadalupe
    15 years ago

    True convection (European convection) is over rated nonsense. The burner being in the back wall around the fan gives you a hot spot in the rear of the oven, the claim that you can do fish and merangue at the same time is also bogus. Convection broil is a real dog.

  • ya_think
    15 years ago

    "The burner being in the back wall around the fan gives you a hot spot in the rear of the oven."

    Do you speak from experience? In theory that sounds entirely logical. But I have a difficult time believing that companies would/could get away with marketing and more importantly charging more all these years for a feature that is actually a detriment. Negligible benefit I would accept, detriment I have a hard time with.

    "the claim that you can do fish and merangue at the same time is also bogus" - Now THAT sounds like hype to me. If air is being moved so thoroughly in one cavity than how can the merange not taste like fish?

    "Convection broil is a real dog." I never really understood the logic behind that, either. Are you trying to roast it or broil it? Make up your mind!

    Again I have to qualify that these are just my gut instincts, so I would be interested in all opposing thoughts.

  • guadalupe
    15 years ago

    European's did not produce self cleaning ovens, they produced continuous clean ovens with coated liners. Convection heps to quickly seal meats and poultry to reduce spatter and lessen the need for self clean, it was an energy decision not a cooking decision.

  • memarcus
    Original Author
    15 years ago

    Glad to see these reponses. Unfortunately I cannot be swayed from my initial reaction when I first encountered this. The third element makes no difference maybe except for the first 10 minutes or so of cooking.

    heartsurgeon - using your engineer logic, broil and bake should be the same...not. The source of the heat makes all the difference in the world, because of the radiant effects.

    The broil and bake function do work the same the only difference is that there is a shield between the element and heat source. This is why things are so easy to burn with the broil. If you could suspend your food over the bottom element without using a cookie sheet you'd get the same effect. Net radiant transfer is calculated as follows:

    Q=A*sigma(Tsurface^4-Tsurroundings^4) where A is the radiating area, sigma is the Boltzmann constant, and the temp are in thermo scale like Kelvin or Rankine.

    This means that radiation is only a factor when there is a temperature difference between objects causing a net heat transfer. This means that once the oven is up to temp mostly its conduction going on since the surface of the oven interior is likely about the same temp as the air and food. There will be a net radiative transfer from the oven walls until the surface temp of the food reaches the same temp. In reality there is no real pure radiation transfer from the bake element due to the viewing angle. I suppose you could get some reflection from the walls but the enamel usually used is likely has a low emissivity so I can't think this would be significant.

    However add a fan and now you have a motive fluid and get forced convection. I am firm believer that the largest factor in how well this works is dependant on the flow performance of the fan, more turbulence means more uniform thermal distribution. I was out looking at some Wolf ranges and they actually have a pair of fans in the cavity. Not only would this increase air flow but they also went one up and added a control algorithm that changes their direction to really stir things up. I'm sure hot spots aren't a problem with this oven although it prolly looks like the Wizard of Oz inside when everythings working.

    My conclusions are the same from the beginning:
    Convection=good
    Third element=doesn't matter as long as fan moves sufficent air to create turbulence.

    Of course the more important thing to consider is whether you like the third element or not you're pretty well stuck with it because it seems to be fairly ubiquitous in the convection ovens. The thing I don't understand is how this could get past the designers at the appliance manufacturers. Adding a third element would add all kinds of cost. A cal rod usually cost a few bucks, extra insulation, controls for the fan, fail safe device, etc. Seems this would add up for no real benefit other than marketing hype. Guess I'll have to ask my buddies at GE consumer to see if this was a fight between them and marketing.

  • ya_think
    15 years ago

    "This means that radiation is only a factor when there is a temperature difference between objects causing a net heat transfer."

    "Only" being 100% of the time, right? I certainly hope your chicken never reaches equilibrium temperature with the oven! Also, your statement holds true of all forms of heat transfer, not just radiation.

    You sounded impressed with Wolf's dual bi-directional fans - did you read the owner's manual?

    "How Convection mode works: Dual convection fans, each with a heating element, operate from the back of the oven to move the heated air throughout the entire oven cavity. Uniform air movement makes it possible to multi-level rack cook in this mode. The heat is cycled on and off to maintain the temperature, resulting in evenly browned food."

    "How Convection Bake mode works: This mode combines heat from two convection elements with some heat from the bake element that is located beneath the oven floor. Two convection fans circulate this heat within the oven cavity. The added heat source from the bottom of the oven makes this mode ideal for pie baking."

    "How Convection Roast mode works: Heat from both convection fans, with additional heat from the broil element, intensifies the convective and radiant heating in this mode. This combination gently browns the exterior and seals in juices, making it perfect for roasting tender cuts of beef, lamb, pork and poultry."

    So even though they have got "Wizard of Oz" convection going on, they are telling us that the additional and varying heat sources impact. Marketing hype? If so, they have got me hook, line and sinker.

  • cpovey
    15 years ago

    "tru-convection" uses a heating element shielded behind the fan shroud, to heat the oven. Hence, as CapeCodCook pointed out above, NO RADIANT HEATING of the food/pan/pot takes place.

    Well, if that were the case, you should be able to put your hands on the inside walls of the oven when it is operating in true convection mode. Don't do it, as you will get a nasty burn, because if the oven is set to 350 degrees, then when the oven is warmed up, the walls, door, racks, everything inside the oven is at 350 F, and is radiating heat (via invisible infra red radiation) at 350 F.

  • memarcus
    Original Author
    15 years ago

    Ya think - You are right that all heat transfer is driven by thermal difference and to a certain degree there will always be some radiative transfer to the food as a 350F turkey would be very dry.....and black prolly too

    If you figure a turkey has a surface area of 2.5 square feet (pretty generous given that it'll be in a pan) and the surface temp gets up to 250F then the net radiation transfer is about 267 watts, about 1/5 the power of a typical hair dryer. If you assume a 180 F its like 330 watts.

    Beings that the typical bake element is >2000 watts that would mean there is quite a bit of conduction going on, into the turkey and out the oven. Although this is a bit exaggerated because the element doesn't burn continuously.

    I didn't read the Wolf manual just looked inside and the salesman mentioned that they change direction. As far as forced convection goes it looks like Wolf is the only one serious about it.

    Has anyone ever heard of a manufacturer divulging the fan flow rate? I've looked on line and not found much. It may be listed in an owner's manual.....

  • mindstorm
    15 years ago

    memarcus, At the risk of restating what may have been written, I just wanted to say that you are correct in saying that the convective transfer mode is much the same between both oven types - with fan-element and without. As will be let's call it the "passively radiant" (oxymoronic, I know) that will be similar between both oven types because the walls will equilibriate to the hot air temperatures. The key difference between the two is the dominant radiant and hence directional heat source between the two oven types - that with the bottom (or top broil!) element activated. Heartsurgeon and capecodcook are right about this.

    Convective transfer being virtually identical in both cases as a function of aerodynamic conditions (and let us incorrectly and inaccurately assume that they are identical in different ovens) and heating element power (which we CAN assume to be the same in any two ovens since they are usually rated quite similarly). We can safely say that the two thermal sources perfectly superpose and hence are perfectly additive, yes? (since radiant heating is independent of any transfer medium while convective relies exclusively on it). Radiant is also directly proportional (inversely) to the distance from the source; with convective - as you rightly pointed out - there is no such dependence. Well therein lies the rub - you do need to consider the radiant part.

    In my oven, I do have a true-convection mode as well as a bottom-heat + convection fan mode. I can assure you that there is an observable difference between the two. The difference is exactly what you'd expect if you understand that the directional radiant superposes the convective transfer - namely, the sensitivity of the food at the bottom of the pan and hence its tendency to cook from the bottom up as well as the sensitivity of EVEN THE BOTTOM of the pan to its distance from the heat source (cooking rack). This is consistent with radiant heat xfer. I did this with multi-rack cooking placing two racks of the same things just where I'd place 'em for pure-convection cooking viz. on my first and third racks.

    Not to say that you should buy one oven over the other - but since you wanted to analyze in scientific terms, I'm expounding on the difference. Well, am trying to expound on the difference. I make no claims of the lucidity of my writeup - I returned yesterday from a transcontinental flight and am definitely feeling the effects of longitudinal transfer.

  • ya_think
    15 years ago

    mindstorm - Thanks for putting some scientific jargon around what my common sense was telling me. But my favorite part was:

    "I can assure you that there is an observable difference between the two."

  • Timer69
    11 years ago

    Obviously the heat will be most intense around the source as the air moves away and cools, a fan helps even this out quite a lot, a second heat source would help a little more, so on and so on.
    I would speculate that a turkey roasting in the centre of the oven would not see the same effects that a oven full of cookie racks would. Eg: a std convection oven would heat the bottom pan first as the air is pulled past and into the back fan then circulated around the oven cooled and returned to be heated again. Adding a heat source to the fan would help even out the heat but concentrated it a little more at the back too. An extra heat source at the top would also help even it out more, so would one in the front. The question is how much will it effect the cooking you do, and is it worth the extra costs and possible repair bills. K.I.S. (keep it simple) invest in quality not quantity.

  • Timer69
    11 years ago

    Maybe once the oven is at temperature all that is needed is the fan heater to maintain the temperature, this would concentrate the heat at the back centre for all those cookies. My preference would be a convection oven over the old conventional, still on the fence whether a fan heater is worth it, for me. (not too many cookies or cakes made here).

  • dodge59
    11 years ago

    I would start a new thread Timer69, note the date of the last post on this thread, until you posted, Aug 4, 08

    There are newer threads
    on convection and "Real or Euro Convection too.

    Gary

  • wekick
    11 years ago

    This is a great thread and raises some excellent questions. This thread really made me start thinking back when it was written.

    Ultimately I think that the third element convection has the potential to increase the evenness in heating in an oven that is full. It probably makes less difference when baking an isolated cake or one tray of cookies. There is also the consideration that additional variations in baking vs roasting modes are usually available along with the third element convection. The elements that are used during different modes and how the fans work and the air moved and baffles all are proprietary and it is very difficult to compare ovens unless you have some way of comparing results. In my experience I had one that did make a hot spot in front of the back element, but the two I have now(Wolf and Electrolux), bake very evenly. I used one last summer that was kind of spotty and the fan seemed to come on and off. If I were buying something like that now I would definitely buy something with a long positive track record. Designing an oven is very much an art and there are many variables to consider. The bulk of the oven also is very important in determining the radiant heat, which could be as important as the FA convection. Again more variables. This is why some ovens take so long to preheat and equilibrate. It might not matter for cooking something low and slow but may be important for baking cakes and pastries. Each oven is unique in how these variables work together and is a learning experience. Some people will appreciate the variations and it won't matter to others. Then you have the differences between gas and electric!

    All the fuss about burners-ovens are much more complex.

    One misconception in these posts- the dry air does not seal in juices.

  • Daryl Cornelius
    7 years ago

    I love to cook and have for a time cooked professionally. I have used both professional ovens as well as plain ovens with convection. I love convection oven cooking it is faster and cooks more even for the most part however, I do disagree with some of the comments on here as to where the heat comes from. In an oven with the heating element on the bottom, (even if it does have the fan in the rear), you can create hot spots depending on how close the vessel is to the bottom element. If you have a large pan which may require you to lower the shelf to accommodate it, you can very well burn your food due to the pan being so close to the burner. As far as convection vs true convection, I've used both and I would not pay extra for true convection. As long as the air is moving, your temperature is right and your pan located properly, you'll have no problems. Just keep in mind, your food does cook faster with convection. I'm no expert, just giving you my opinion.