Kitchen oven high temperature hack

[donstavely]

Hello all,
First time poster, so forgive me if this topic has been covered. 
There is a way to modify most any kitchen oven to go well above its normal maximum bake temperature.  Note that this is NOT a hack to disable the cleaning cycle interlock.  It fools the oven controller into thinking that the oven temperature is lower than it really is, so it will heat up, and then maintain, a higher temperature.  For example, my oven will stay at 650F when set to 450F.  (Confirmed with an IR gun on my baking steel.)  I can set it back to "normal" with the flick of a switch.   
I have done this on my original 25-yr old oven and my new Kitchenaid convection oven.  I can describe the technique in detail if there is interest in the forum.
Don

[Pizza_Not_War]

Go for it! I want to here it.

[02ebz06]

You might want to add that ANY hack will void a warranty, and do so at your own risk.

[donstavely]

Let me start with some caveats, along with caveats to the caveats:
 - First, yes absolutely, any hack to your oven voids the warranty.  This is why I had no hesitancy modding my 25-yr old oven, but my wife wouldn't let me touch our brand new one -- at first.  But after a few pizza nights with the new oven, even with a baking steel, the pies were not as good.  So unsolicited, she told me to do it.  Also, this mod is not very invasive and is totally reversible.  It can easily be undone without a trace for repair or resale.
 - Next, I am a retired electrical engineer, so this kind of thing is right in my wheelhouse.  Others may not be as comfortable, but it is certainly doable by anyone with some basic electrical skills. 
 - The circuits involved are very low voltage (like 5V or 3.3V) and low current, and thus totally safe.  On the other had, opening up your oven could easily expose you to dangerous line-level voltages nearby.  Always work with the breaker(s) off at the panel, and be careful.  Proceed at your own risk.
 - I personally think that this technique is way safer and less potentially hard on your oven that any kind of cleaning-cycle hack.  Of course a self-cleaning oven is capable of surviving any number of 900F+ cycles.  I certainly feel better about actually setting and regulating to a desired temperature, be it 600F or 900F.
 - Depending on your particular oven, the mod could be very easy or kind of a pain.  My old oven was easy, because I only had to remove a front cover panel.  My new oven was hard, because I had to pull in out from the cabinet to get at the right point, and it was a 300lb microwave/convection combo beast.
 - Lastly, I am confident that this works for most any electric oven.  It probably works on gas ovens -- I have never owned one, so I just don't know.  I think that the basic temperature control method is the same.

OK, here is the idea:  We fool the oven into thinking it is cooler than it really is, so that it will heat up to a higher temperature than it normally would.  We just need to understand how the oven senses temperature, and how to affect it.  On the back wall of the oven interior, there is a sensor containing a thermistor.  It is simply a resistor whose resistance value varies strongly with temperature.  A typical oven probe will measure about 1,080 ohms at 70F, but will measure say 1,700 ohms at 400F, and 2,050 ohms at 550F.  The oven controller measures this resistance, compares it to what it expects for the target temperature, and cycles the elements on or off depending if the measured temperature is too low or too high, respectively.  (This so-called "bang-bang" control is how lots of things work, from furnaces to curling irons.)
So now the foolery:  Let's say we take a plain ordinary resistor and put it in parallel with the probe's thermistor.  The combined resistance is lower.  The controller measures the lower resistance and thinks the oven is cooler, so it wants to heat it up.  The math for the combined resistance is simple:
1/R(combined) = 1/R(thermistor) + 1/R(added)
Let's say we add a 10K ohm resistor in parallel with the probe, and the oven temperature is currently 550F.  R(combined) = 1 / (1/2.05K + 1/10K) = 1.7K ohm.  This is the same resistance as the probe alone at 400F, without the parallel 10K resistor.  So with the resistor, the oven controller thinks that the oven is at 400F when it is really at 550F!  So when you set the oven to 400F, it will happily heat itself up to 550F and stay there.  And if you set it to 500F?  It goes to about 750F.  I should note that a lower value than 10K for the added resistor allows temperatures well beyond 750F, but this is plenty for my style of pizza, especially with a baking steel and the broiler element.                       
So the hack consists of some wire and exactly two components: Said resistor, and a switch to switch it out when we want the oven to behave normally.  We don't want our oven to always run 150F hotter than we set it, do we?  But ya gotta remember to switch back to normal after the last pizza!
This is probably sufficient information for the more intrepid hackers to proceed on there own.  I will try to post more specific descriptions of how I modded my two ovens in the next few days.
Don   
     

[barryvabeach]

Don,  interesting idea.  Years ago a few suggested wrapping the sensor in insulation so that it read cooler than the actual temp,  but there were some problems with that approach,  IIRC.  Your idea sounds much better. 

[donstavely]

As promised, here is a more detailed description of how I hacked my first oven to go to higher temperature.  This was a 25- or 30-year old Kitchenaid, standard electric wall oven - touch-screen and display, but otherwise pretty basic.  Please read my earlier post to understand how the hack works.
First, I shut off the breaker for the oven at the panel (attention!)  I located the temperature sensor in the back of the interior near the top.  I removed the two screws holding it, and could pull it out an inch or two, just to be able to see what its two wires look like.  They were unique-looking high-temperature wire, in a woven insulating sleeve. 
Next, I removed the screws holding the control panel, which were pretty obvious and accessible on this particular oven.  Pulling the panel out a little, I could see the controller printed circuit board behind it.  It had various wires attached via connectors coming to it.  Happily, one of these connections was clearly the other end of the wire from the temperature probe - all the other connectors had more than two wires, and/or the wires looked very different.  I unplugged the connector, and just to be certain I had the probe wire, measured the resistance at 1000-something.  (Advanced technique:  Reverse the meter lead polarity and read the resistance again.  If it is the same, you can rest assured that there is a passive component at the other end, not active circuitry.) 
Then I wrapped two lengths of telephone-type wire around the base of the male connector pins on the controller board.  The pins were long enough that I could then plug the probe wire back in securely.  I routed the two wires out from behind the control panel and into an adjacent cabinet.  I soldered my 10K resistor (1/4-watt, 10%, or whatever) and a miniature toggle switch in series to the wires.  I mounted them with a little bracket just inside the cabinet door, along with a cheat-sheet with the temperature conversions when the switch is in the high temp position:
Set   Act
325   475
350   510
375   545
400   580
425   615
450   650       
475   685
500   720 
That is it.  I turn on the switch, set the oven on "bake", enter the "set" temperature, and wait until it gets there.  Of course it will take a while longer for the steel to get to the oven temp, but I verified these actual values with an IR gun.  Again, don't forget to switch back to normal when finished baking pizzas!
So if you have an older oven, this procedure as described may very well work for you.  You can certainly do some non-destructive poking around to see if it can be this simple.  BUT, if your oven is more modern, like my now-two-year-old Kitchenaid convection oven is, it will likely be harder.  Not more complicated, just harder.  Please wait for my next installment, where I describe what I went through to hack it successfully.
Don

   
       

[donstavely]

Here is my experience hacking my new Kitchenaid wall oven to achieve higher temperatures.  Please read my previous two posts so this makes sense.
As with my old oven, I started by unscrewing and peeking behind the temperature probe in the upper back wall of the interior to see what the wires looked like.  Then I peeked behind the control panel, trying to find the other end of the probe cable where it should connect to the controller PC board.  Nada.  After checking a rechecking, i finally had to pull the whole unit out from the cabinet, in order to directly trace the wire from the probe forward.  (Remember, 300lb combo convection oven/microwave. Sad.)  Surprise, the wire lead to a daughter board on the left side of the oven.  So apparently some different cable takes the temperature signal from there forward to the main controller board.  No matter.  The good news is that I could, as before, tie in to the two probe pins on the connector, this time on that daughter board.  From here on, everything worked exactly as on my older, simpler, dumber oven. (10K resistor, switch, same cheat-sheet values even.)
My new oven does have some "smart" behaviors that affect my pizza-making.  One is that the broiler does not just turn on the upper element and leave it on, like the old oven.  A) it will not turn on if the temperature is at its 550F maximum temperature.  B) it will turn off if the door is open for more than a minute, and stay off for some time after the door is closed.  Another "smart oven" issue affects this hack directly:  If I flip the switch to the high-temp setting and then turn the oven on, the controller thinks that the probe is bad and displays an error code.  (It decides that a probe value corresponding to below 0F makes sense for a freezer, but not an oven, so it tilts.)  To work around this, I have to start preheating the oven up to some reasonable temperature first (anything above say 200F), turn it off, flip the switch, and then set the desired temperature according to the cheat-sheet.
One smart feature of my oven that actually works in the pizza-makers favor:  Buried in the setup menus is a calibration setting.  It allows you to increase the actual temperature up to 30F above the set temp.  So this oven can actually do 580F without modification.  This might be enough for many folks, especially with a baking steel.  The broiler issue still gets in the way though.
It probably goes without saying, but the push-ups I when through on my wall oven would be much easier on a free-standing range, since you don't have to tear your kitchen apart to get at the guts of the oven.  So I can recommend this hack on an old wall oven, and old or new range, but perhaps not a newer wall oven.
Don   
         

[dimarem]

See this post from 2009: https://www.pizzamaking.com/forum/index.php/topic,8730.0.html

[donstavely]

I bow to you, dimarem.  I don't know why I hadn't found your post.  I hacked my first oven back in that same time-frame, but only recently joined the forum. 
Interesting that I came to the same conclusion, right down to the 10K value for the resistor!  You did a better job documenting the mod, since I didn't take any pictures at the time. 

Thank you for pointing out your prior work.  I certainly don't want to take credit for other's contribution to the forum.  Let's consider my posts as an endorsement of your technique, and a bump for your thread.

[typicalsam]

See this post from 2009: https://www.pizzamaking.com/forum/index.php/topic,8730.0.html

13 years registered and 11 posts? you really make them count