Hot Pot: Experiments in Stove Burners and Empty Pots

Blog Comments
1

Have you ever wondered about how hot electric stove burners run, or how hot you could get an empty pot on an electric burner if you insulated it? I’ve been trying to find answers for this for a while, I haven’t had any luck, so I’ve Done a Science with a thermal imager to answer my questions!

In short: An electric stove burner in open air will run around 550C (about 1000F), will rise in temperature when covered by a pot (to around 750C / 1400F), and I was able to get a stainless steel pot to around 450C (850F) after wrapping it in insulation!

Why on Earth… ???

It’s not the historical norm for this blog, but these questions relate to some of my recent work with charcoal production (with a specific focus on biochar). I’ve decided to sell biochar locally, which requires me to be able to produce high quality biochar locally. I’m starting out with small batches until I sort out the demand (which may require far larger retorts - some of us joke about finding my place by “Oh, yes, drive south until you see the flare stacks…”), and I know I can produce good charcoal with our firepit - but I’m also set up with basically “free electricity” out by my office. Between the array on my office and the solar trailer, I’ve got access to large amounts of zero-marginal-carbon electricity, and using that to either produce or refine a charcoal down is of great interest to me (keeping production costs low, among other reasons).

But these days it’s impossible to find data on even basic things like “How hot an electric stove burner runs” - the Internet, ever useful, gave ranges from “195F to 1500F.” Fine. I’ll do my own work and document it. Hopefully this is helpful to people who might want to do similar things.

Stove Burner Behaviors

You’ve probably, at some point, seen “coil burners” for electric stoves. They’re not great to cook on, they tend to be a pain to clean, and they’re cheap. So they’re everywhere, and once you get used to them, it’s not awful to deal with… mostly. They’re a big coil of resistance wire - usually something like 80/20 Nichrome, which melts around 1200C. On most stoves, the small burners are around 1500W (5000 BTU/hr), and the big burners are around 2500W (8500 BTU/hr). I’ve had to replace one of the controls on my stove, and taking apart the existing one demonstrated that there’s nothing fancy to them - no thermostatic control or anything else. Just apply voltage, pulsing it if needed for less heat - and “high” is a little detent that forces them to run at a 100% duty cycle.

Easy enough… I ordered a few (they’re stupidly cheap, and easy to find) and set out probing and prodding their behavior.

I started out by building yet more SketchyAdapters(TM). If you own an EV, you’ve probably got one of these already. This translates a 20A 240V outlet (L1/L2/G) to the normal 120V plug wiring (L/N/G). Doing this allows me to use a regular 120V extension cord for carrying 240V around. It’s fine - they’re insulated to 600V, and almost all my extension cords are 12AWG - so no problems carrying the expected 10A for extended periods of time. I hope the “double red wire” would help clue people in, at least if they know wiring conventions, as to what exactly this thing does. But I’ve also warned my kids, repeatedly, to not use random wiring adapters they may find around the hill without asking me first. This isn’t SCSI!

I built another chunk of adapter to go from a 120V plug to the burner - which, I’ll note, allows me to run the burner at far lower power on an actual 120V circuit, should I care to. My small burner pulls a relatively constant 5.2A at 240V - so right around 1250W. I’d wondered, before I started this project, if burners were self-regulating in terms of temperature by some behavior like “resistance rises as temperature rises.” Incandescent light bulbs regulate this way, but I’ve not seen any evidence of this in stove burners. They pull a nearly constant power, and their temperature just relates to how well insulated they are.

Sitting outside at about 40F, mostly out of the wind but with some breezes in the carport, it eventually stabilized at around 560C. Neat! My data here is coming from the P2 Thermal Imager I reviewed some while ago.

The large burner pulls a steady 9.8A - so about 2350W. I might see if I can find some higher power ones, but these are reasonable enough values as a “general burner ballpark” for now.

This burner ends up right about the same place - a peak coil temperature of around 550C. It’s got more coils, and pulls more power, but the actual “open air temperature” seems to be right about the same. However, outside some basic academic curiosity, the actual coil temperature isn’t useful to me. I want to know what I can get a pot to!

Hot Pot Experiments

I’ve been using this pot for a while as a charcoal burning retort - though I recently upgraded it with a BBQ grill thermometer that claims to be able to report up to 1000F. Well, that’s a tall claim, and as you’ll see later, cheap BBQ grill thermometers may not quite handle their ratings.

I don’t have anything in the pot - this is just “open air” heating to find out what the best case limits are for something like this. I’m sure I could sit down with a thermodynamics textbook and see what it maths out to, but that’s way more work than just trying to melt down a pot and see what happens. From the firepit, I already know this pot handles a dull orange glow just fine.

What happened is that the pot’s temperature rose up to about 280C (550F), and then sat there. This is, apparently, the point at which the pot is radiating out heat at the same rate it’s flowing in!

Normally, shiny pots don’t make good targets for thermal imaging - they don’t radiate well, and reflect their environment. Fortunately, my pot is far from shiny. It’s got a layer of rust, ash, and carbon on it from quite a few evenings in the firepit, so I have some confidence in the sanity of my imager here. The first thing you’ll notice is that the burner is running far hotter than before - nearly 200C hotter, as a result of the pot insulating the top and reflecting heat down. The outside of the pot is showing a toasty 240C, which more or less makes sense with an internal temperature reporting of 280C. But it’s also far from as hot as I want things to be. This would work well for drying wood and starting the biochar process (which would probably release quite a bit of heat), but it’s not as hot as I was hoping to get things.

Fortunately, I have some left over rock wool insulation from my office - and it’s been sitting out in the weather for eight years, so isn’t exactly the nicest stuff to deal with. But it’s a good high temperature insulation and there’s not much to catch fire in it - so I wrapped a bunch around the pot and stuffed the top (leaving an opening for the thermometer). The results were instant - the thermometer started going up, in a hurry!

Thermal imaging shows clearly that the insulation is effective - the top part, exposed for the thermometer, is hot, but the rest of the insulation is keeping the heat in. There’s some leakage, which doesn’t surprise me with how beat up this insulation is, but the concept of “Rock wool insulating the pot nicely and keeping the heat in” seems to work very well! This is about 3.5” of the stuff - and while I expect more heat would leak through eventually, the stuff works!

After a while of heating while insulated, I pulled the shroud open. Yeow! That’s cooking! And, importantly, nothing except the thermometer has come apart! I saw somewhere around 425C (800F) reported up top, which is the sort of temperature I need for my processing. Just, the BBQ thermometer didn’t quite like the heat…

After about 10 minutes at temperature, I shut down. The burner has some surface flaking, but I’m not sure how much of that is a problem versus “debris from the pot.” The coil seems fine, and even at the 750C I saw above, I’m nowhere near the 1200C melting point I expect. The heat reflector is rather blued from the heat, but I don’t care, and will probably insulate that better.

The “1000F BBQ Thermometer” failure

While the core of the experiment worked fine, I can’t say as many nice things about the “1000F” thermometer. It handles normal temperature ranges just fine - 550F is pretty darn toasty, well above oven temperatures, and there are no problems at all.

The problems started after I insulated the pot and temperatures went up in a hurry. Somewhere around 750F, I noticed that the thermometer wasn’t looking so good. It was not burning off moisture, it was burning off… something far more important, like the paint.

A bit later, it was almost entirely unreadable. The needle still pointed in the right general direction, but the inside had apparently burned off and deposited on the glass. Also, the stainless steel surround is darkening noticeably.

But, after things cooled down, the needle was pointing in the “Low” direction again. So I’m not actually sure if this is fried, or if I just can’t read the numbers anymore. Maybe I’ll scratch some markings on the outside ring. In any case, if you’re curious about how this thermometer handles the rated temperature range, it doesn’t! Don’t let it get over maybe 700F on your BBQ! Though what you’d have a BBQ at that temperature for is beyond me…

Final Thoughts and Conclusions

Well, I’ve toasted a thermometer, and I’ve learned a lot of useful things in an hour or two outside.

First, it seem as though I can get a retort hot enough to process charcoal with pure electrical heating. I sort of assumed I could, but the devil is in the details, and until I actually saw these temperatures, it was far from a given. The process may still lead to some failures - the coils are running HOT down there, but they’re still far enough from melting point that even a well insulated setup shouldn’t cause failures. I hope.

But I also have the problem of generated gasses - they’re not going to get burned if I use this method, and they’re generally on the “nasty” side. Wood alcohol vapors, tars, etc. I also don’t want to load up the rock wool with a ton of flammable condensates, because “The insulation was loaded with a range of flammable condensates…” is the sort of passage you read in industrial disaster reports. I’m inclined to think that this process may be more useful for post-processing of other charcoal, once most of the gases are driven off. My propane pot, right now, doesn’t get things hot enough throughout to be usable as a single stage process.

So, I shall carry on the experiments.

I’m also rather interested in what happens if I move the burner inside the pot. Right now, a lot of the energy is wasted. What if I just toss the burner inside, and insulate the whole thing? I’ll probably have to learn something about thermocouples in order to get good data there - and, oh no. Something else to learn about!

This is a companion discussion topic for the original entry at https://www.sevarg.net/2024/05/04/hot-pot-experiments-with-stove-burners
2

Random additional data points:

Courtesy of clueless renters, I’ve learned that stovetop elements are wrapped in ceramic. If you first heat it up to glowing red and then put a cold pot on it, it will eventually crack and fail. Inside you find a coiled heating wire. They were destroying half a dozen coils a year (!) before I figured this out. After cracking, they’re electrically conductive, so you may want secondary protection against shorting out against a metal container.

Thermocouples and PID controllers on ebay are stupidly cheap. No idea how rugged they actually are, I’m using it to heat my elderly cat’s bed sitting on top of an insulated wood box containing a (old, used) 40W incandescent bulb. That said, the SSRs are generally questionable, but for me, the bulb being on continuously isn’t a safety issue. OTOH, getting a thermocouple meter doesn’t seem terribly expensive either. If you want to build your own controller (i.e. computer / microcontroller driven wrt time/temp), adafruit has boards as low as $15.

Calibration is easiest done with ice and boiling water (as I do with kitchen tools) but I don’t know how well that extrapolates up to 1000C.

I hadn’t thought about unusual uses for excess power before, but you’ve given me some ideas (aside from running more servers for “free”)

Heating regular steel to glowing means you’re oxidizing the top layer of steel into rust. That black layer will flake off and means it’s getting thinner and thinner with every heat cycle. Ask any blacksmith! OTOH, if you’re in a low oxygen environment, it should reduce this effect. I had to point that out to folks who were building “rocket stoves” and didn’t understand why their steel burn chambers were eroding so fast. I don’t know how this applies to stainless, but stay away from galvanized! The galvanizing metals will outgas and can kill.

Good luck!

3

I’ve learned this too! In another experiment, I was baking some charcoal with a coil, and I overheated the coil enough that it melted the inner wire and shorted out. Fortunately, I was running it on the solar trailer, which has an inverter that will put a good oomph in, and then quit, instead of running into a dead short.

I had no idea you could damage a stove like that, though. I generally don’t turn the burner on full until I’ve got the pot on, and I’ve yet to burn out a stove coil in the kitchen in the 8 years or so we’ve been out here.

I honestly don’t care about the difference between 950C and 1000C. It doesn’t matter.

I’m trying to figure out how to better integrate my power systems next winter. Having an extension cord between my office and the house, with the solar trailer buffered in the middle, would let me do some house heating with office power - which would offset some of the rather excessive power use we have in the winter. I’m also planning to finally vent the dryer inside in the winter next year, because that’s a lot of heat I’m sending outdoors that could be recycled (and the humidity would be rather welcome - it’s an electric dryer, so no exhaust fumes to worry about).

Not a problem I’m aware of with stainless. But in some of my firepit charcoal experiments, I’m quite careful when I’ve added new galvanized hardware. I’m quite aware of the zinc poisoning, as are a bunch of people who tend to be around, so I do the initial burn with new galvanized hardware when nobody is around.

Though I’ve found better methods that don’t require as much of that sort of hardware - I’ll document them over time here.

4

Dryers will throw out way more moisture than you think.

TLDR: moisture dripping from the ceiling in a 3200 cu ft garage
When we moved into this house, the dryer was in the garage with a flex tube running to one of those foundation wire mesh vents, but it restricted the air enough that the ceiling dripped with moisture. After punching a hole in the wire mesh and sticking the vent tube through it, the moisture problems went away. As part of a remodel, our contractor put in nice solid sheet metal (smooth) pipe exhaust for low friction. Slight caveat is that it was a gas dryer, so there was extra moisture coming from the fuel itself. Also, dryer was in an unheated garage in the california bay area (i.e. mild) with insulated living space walls on two sides plus ceiling.

Anyways, I really wanted to point out that people have built cross/counter-flow heat exchangers out of readily available PVC, ABS, metal piping, plastic “twin wall” sheets, etc. Heck, even two sheets of corrugated roofing material would be a quick afternoon build (if you have the space!) If you try, just slope it towards the outside so the condensation runs out. Or halvsies and build a small heat exchanger specifically to extract (and drain/reject) some of the moisture. I presume you’ll be buying more sensors to track humidity now!

Dryers should be 150-200 cfm @ 120-160F. Arguably that’s too hot for pvc, but cpvc is fine (but not like you can find this for free in scrap piles). Or an old radiator. polycarb (twinwall) is fine. metal of course is fine and probably transfers heat better. HDPE is ok (4" corrugated drain pipe) as is ldpe (some “poly” sheeting products).

I mention sheeting because I’ve used it in the past to create impromptu tubes (i.e. vent from the back of an SGI “deskside” server in my office up and out the false ceiling) and adapters by cutting it exactly to shape, then taping it into a cone shape, then attaching it either to a tube or flange. (printed) paper template works great for this, or just draw it by hand. For small adapters (i.e. 3-4" vent for lower temperature), I’ve 3d printed a design using hull() in openscad to create the cone between automatically.

I will write a blog post detailing the tube-from-sheet process. Cutting it exactly (don’t overlap the plastic sheeting) and using packing tape on both sides works amazingly well. Maybe in June when I have more free time…

yes, I’ve already thought about this (too much!), but never built even a prototype, because our dryer is gas, and I recently encountered a near CO poisoning incident. I got an instant headache walking into a bathroom with gas dryer running with a disconnected dryer vent. I ought to build one for our barn owner who has an electric dryer…

5

So I hear. However, it’s something I’m willing to experiment with. I can get away with humidity things that most people can’t, given how bone dry our area is. And it may involve having to keep the house blowers on while the dryer is running. Not sure. But I’m pumping a gallon or two of water a day into the house in the winter with a humidifier, and it doesn’t make much of a difference outside our bedroom. So I think I can sink a lot more water without too much trouble.

Based on my experience in my office, with propane heat, I think a LOT of your water was coming from combustion results. Remember, natural gas is methane - CH4. The reaction is CH4 + 2 O2 = CO2 + 2 H20. You get a lot of moisture from that, and I certainly wouldn’t be inclined to vent a gas dryer inside. Electric? Well, I’ll see and find out!

Probably not. My (now disconnected, dumb) Nest thermostat will show me humidity, the Temptop air quality monitor I use in the house will show it, and in the winter, my windows do a solid impression of a humidity monitor based on how much water is on them in the morning (almost never anything, but if I have the bedroom closed and the humidifier is purring away at full blast, sometimes I’ll get some condensation around the edges).

I’m really getting away from “all the sensors” and such. Even locally logged. I just monitor stuff manually, and it’s simpler than dealing with the computers, servers, and automation. If I have humidity issues from the dryer venting inside, it’ll be obvious in a hurry and I’ll rethink the project. But as I see it, all I really need is a dust capture system and vent into the utility room. I can always run the external vent fan and dump some of the humid air outside that way, but I think the house will easily sink the heat and moisture from a load or two of laundry.

… which “deskside” server, out of curiosity? I briefly owned an Onyx, but didn’t have space for it, and it was having some fan rack problems. I only recently got rid of my fully loaded Indigo 2, but college for me involved a lot of SGIs as desktops.

CO2 sensors are useful enough, though the cross sensitivity to hydrogen drove me nuts in my office. I’ve just got a portable unit that I move between the house and my office depending on where I’m burning kerosene (or, on rare occasions, propane in my office - but I’m several winters into a tank, and at this point almost never use it).

6

I hadn’t considered that it was generated by combustion! At 20,000 BTU/hr, assuming it ran for 30 minutes, that’s 0.1 therms, or 10 cu ft = 0.28 cu m. that apparently is about 12 moles of gaseous methane. so 24 moles of H20 comes to 432.5 grams or 0.95 lbs of water.

wet laundry weighs… ? vs dry? probably more like 10 lbs. so while not the majority, still that’s not ignorable.

Well I fixed this in the first week we owned the house. They tried to vent it outside, but the wire mesh restricted airflow enough to prevent the flex duct pipe from actually exhausting all the exhaust; a lot got shunted back in.

I was a (student) sysadmin for a “special projects” Power Series in college for ~3 years before graduating (8 CPUs, more disk than we had on all the other systems put together… in 1 GB 5.25"full- height disks, lol).

Then I worked at SGI for 4 years, so I had various desksides, but that was was probably a Challenge L with R4400’s. I didn’t want it for the cpu, I wanted it for the 4 scsi busses! I scavenged 2 old “Power Series” (previous generation) disk trays and filled it with 5 (I think?) CDROMs (caddies!) and ripped the audio from hundreds of CDs with it. Because the trays used differential scsi externally and single-ended internally, I never had any serious problems like bus resets or cable too long.

I haven’t had any SGI hardware for years, I ended up giving everything away.

7

I rocked a R10k purple Indigo 2 as my desktop for several years, and had an Octane for a while, plus a whole pile of Indys I was slowly refurbing and scrapping out on eBay (someone’s company had a pile of SGIs they’d replaced, and they were going to have to pay to scrap them, so they ended up in my station wagon in a few trips of “If these end up discarded somewhere and it’s traced to me, it’s your head!” arrangements).

SCSI CD burners, though… man. My favorite trick in the early 2000s, when I had a dual Pentium III 866 box on Win2k and dual monitors, back when CD burning was a “Press burn, back away, and cross your fingers…” affair, was to start a CD burning, then go play Quake 3. The reactions I got were priceless, and I never had trouble - SCSI burners rocked like that.

8

Looks like a fun project. On the question of venting a dryer into the house. I lived in a very dry location with very expensive fuel and electricity… We dried out clothes by hanging them on a line in the house and never had a problem with the moisture even in an 800 sq. ft. house. It seems to that would release same amount of water into the air as a dryer, just more slowly. Two caveats: it was only two people’s laundry and our primary heat was from a wood stove without a dedicated outside air source.