Electric Furnace Control System

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* Updated February 11, 2011 *

My electric glassmaking glass furnace.
Born: May 1994; Laid to rest (before her time): May 2008

For those concerned with proper English grammar, please forgive the mixed tenses used in the page below. This page was originally written a few years after building my furnace. In May of 2008, the furnace was dismantled when I moved out of NY. Some parts of this page were updated after that to reflect the latest information I had on the furnace. Other parts were left as-is. Because of that some of the text on this page refers to the furnace while it was in use, while other text refers to the furnace after it was dismantled. So past and present tense tends to be mixed at times. When time allows I will try to make things a little more coherent.

How my electric furnace was wired


What I did on my spring vacation, 1994

by Brad Shute

Over the last 10 or 15 years I've had many inquiries from people seeking information about my electric glassmaking furnace and the low-budget control system I built for it. The early inquiries prompted me to put together this page around 1999 or 2000 because I simply didn't (and still don't) have the time necessary to answer a bunch of detailed individual questions about this. (But try searching the website discussion board, and if you don't find what you need, post a question there.)

Not surprisingly, the number of people interested in electric furnaces jumped noticeably as natural gas prices headed through the roof in 2002/2004 and then proceded to bounce between the roof and the moon over the next few years. During that time more and more people realized that an electric glass furnace could be a practical alternative to gas. My furnace was a freestanding crucible type with 6 SiC elements, three on each side of the 200lb crucible. (It could actually hold up to about a 350lb crucible, but that was more glass than I needed.)

One thing to be aware of is that this control system requires a careful choice of heating elements to realize its full potential. (Translation - choose the wrong elements and they won't last very long.) Of course, that also applies to any furnace heated with SiC elements, and is more a function of the particular elements used than the particular control system. The key is surface watt-loading, and while some sizes of SiC elements will have satisfactory watt-loading for use on line voltage, others will not. That's why most (all?) SiC element manufacturers typically recommend a phase angle SCR and/or transformer to drive SiC elements. For many of their elements it is necessary. But if you choose your elements wisely you can get away without these expensive controls. I don't claim this is the absolute best way to build an electric furnace, but it is comparatively inexpensive and in the 14 years of personal experience I had with the SiC furnace described on this page, element life was good and the control system proved to be virtually trouble free (*see below). Had the impending sale of the building I was renting for a glass studio not forced me to move, I am confident that both the furnace and control system could have gone many years longer without any trouble at all.

Here you go...

(Sorry about the crummy text reproduction, I couldn't find a way to export this from MacDraw and keep it looking good.)

After looking at the diagram above, the more electrically astute may say to themselves, "That 's pretty much the same way to wire a 3 phase annealer", and they'd be right. That is the nice thing about this control system - simple and cheap, especially relative to other types of electric furnace control systems requiring a transformer. There's no big mystery involved in doing this, but done this way - as I mentioned above - you DO need to choose your elements wisely. Out of the many sizes and resistances available in SiC elements, only a small percentage will work well (i.e., last a long time) when using this system. This DOESN'T mean you need to buy the fancy-schmancy, high priced elements to make this work. My elements are the standard type and composition - the cheapest model available. You just need to choose the right element size (which translates to the right resistance and surface area). Something else to keep in mind about the diagram above is that a 240Vac, 3 phase supply connected this way actually delivers roughly 140 Volts to the elements. I won't try to explain why here - it's just one of those weird electrical things. If you really feel the need to know, Google it.

This isn't the only way to do this. It just happens to be the way I did it when I was short of cash. I wanted to build an electric glass furnace that worked well and was relatively cheap to build. I did have some doubts when I built it - due primarily to an electric furnace manufacturer who told me, "It will NEVER work. You should buy one of my high-priced units instead!" (Okay, not in those EXACT words - but pretty close.) In spite of his warning, it worked great and didn't cost an arm and a leg to build. Not surprisingly, although he knew of my furnace through this website, 7 or 8 years later that same furnace maker told a friend of mine much the same thing when my friend mentioned he was thinking about building an electric furnace using my type of control system - "That can't possibly work. Buy one of my obscenely expensive furnaces instead." Yeah, right...

Not including the temperature controller (which you'd probably want no matter what type of furnace you built), or the manual disconnect switch (which was given to me), the original control system on this furnace cost me less than $500 at retail prices. That doesn't count the elements, or the heat sinks that I scrounged for the SSR's. If you are a good scrounger it could cost even less.

*(The "see below" part): The only trouble I encountered with the control system was with the original mechanical door switch, which had been scrounged from a surplus yard. It only lasted about 6 or 8 months before dying, at which point I replaced it with an Omron photo-electric door switch. The Omron was still working perfectly when I dismantled the furnace. Because I used an old mechanical shut-off switch on the door when the furnace was built, instead of the Omron photoelectric switch that replaced it, this control system actually cost me less than $350 when I built it. But as mentioned above, the mechanical switch proved to be unreliable (my only problem with the system over the years) so I was forced to spend $165 for the Omron (it was the only thing immediately available when the mechanical switch failed). While the Omron is a very nice switch, less expensive photoswitches are available that will still do the job. Decent ones usually cost in the $100 - $120 neighborhood. Based on my experience, they are well worth it. I strongly recommend a photoelectric door switch instead of a mechanical one.

My approximate parts cost
(2) 125 Amp Solid State Relays $65 each - Allied Electronics
(2) 75 Amp definite purpose contactors $100 each - Grainger
(1) Omron E3K-R10K4 Photoelectric switch $165 - Grainger


There are some modifications that could be made to the system (and might well be advisable) like fuses and such. Wiring a controller alarm output or a limit controller into the circuit to shut down power in case an SSR fails and locks on is also a good idea, and something I eventually did on my own furnace. My disconnect switch was fused, but there were no fuses protecting the SSR's, which require a special - and expensive - type of fusing. My reasoning here was that this type of fuse may or may not protect the SSR, and they cost just as much as a new SSR anyway. However, proper fusing to protect personnel is a must.

A year or two after building the furnace, I added overtemp alarm shut-off wiring to my system, but it isn't shown on this drawing. An overtemp alarm and cut-off switch is a very good idea no matter what, but especially if you forego the semiconductor fuses for the SSR's. The reason for that is that if an SSR fails it usually fails "on" and without an overtemp shutdown circuit the furnace will just keep heating at full power until someone shuts it down, or it self-destructs when it melts down. (SiC elements will heat to approximately 3000F.) For maximum protection a separate overtemp controller is best, but that's an extra expense and I didn't have any problems using the alarm circuit of my controller. Either one is far better than nothing at all, which is true on annealers too. All in all, this proved to be a very reliable system which was reasonably cheap and easy to assemble. My elements typically lasted 2+ years in nearly continuous use.

SiC elements do have some drawbacks. The main problem with them is an increase in resistance over time (known as aging) which either requires an increase in voltage to maintain furnace power, or slowly decreases the maximum available power. My elements - which were standard Norton Hot Rods (Kanthal subsequently bought out Norton's element business quite a while back) - were directly exposed to the furnace atmosphere and never seemed to be adversely affected by batch dust or popping batch pellets or cullet. Over 14 years of melting in this furnace, I used Spruce Pine, Glasma, Gaffer, and several other batch mixes. I also used cullet a few times. My last set of elements was in use for over two years before I changed the wiring to increase the voltage from 140Vac to 240Vac to give the elements more power. They were over 4 years old when they were finally replaced.

One VERY IMPORTANT thing about electric furnaces - most refractories leak electricity at glass melting temperatures and molten glass itself is a good conductor. If you build an electric furnace, you MUST have a door switch to shut off the power when you gather. This also makes the choice of materials for your thermocouple tube important. Mullite should NOT be used because of electrical leakage. At best, this leakage would cause faulty temperature readings. At worst, it would toast your temperature controller and possibly you as well. I killed a temp controller thermocouple circuit this way on another electric furnace when I didn't have an alumina tube available and thought I could get away with using mullite for a little while. Whoops... Use a pure alumina protective tube for your thermocouple instead of mullite and if it ever cracks, replace it immediately.

One other thing - Solid State Relays (and SCR's) leak a small amount of electricity even when they are turned off. It isn't much, but depending on the situation could be enough to put you in a world of hurt. So just turning off the SSR's isn't good enough, you need to disconnect the power to the elements with a contactor, either electro-mechanical or mercury. Otherwise, you are tempting fate in a BIG way. YOU WERE WARNED!

My electric furnace design recommendations based on personal experience are:

1) Don't underpower your furnace. One of the most common complaints I have heard about electric furnaces is that they take too long to heat. Mine heated just fine. And not just with new elements, but after they have been in use for one or two years as well. The initial element sizing should provide at least 50 percent more element output at full power than necessary. This will allow a power "cushion" for the elements as they age and the resistance changes. Keeping the watt loading as low as possible will also help them last. I recommend a watt loading of 35 Watts per square inch, or less. Some "store bought" electric furnaces use Watt loadings of 60 Watts per square inch, which I believe is a large part of why their users complain of short element life. That, and the fact they were underpowered to begin with.

2) Don't necessarily believe the "conventional wisdom". The higher priced spiral cut, single ended SiC elements may be better for some applications, but they have some fatal flaws in a studio glass furnace, one of which is their tendency to short out between the element windings. Their main advantage is that the power connection is on one end only, making them easier to install and the furnace easier to insulate.

Nor is an expensive, phase angle fired SCR control system necessarily the best way to go for power control either, judged on a cost effectiveness basis. As shown above, my furnace worked flawlessly for 14 years using relatively inexpensive solid state relays. While SCR's can have some advantages, unless one finds a great deal on surplus equipment, they are usually quite a bit more expensive than the control system above.

3) Listen to the people who use electric furnaces (or, possibly better yet, the people who had problems and STOPPED using them). If you hear the same complaints over and over, you had better pay attention. I listened and was able to design around some common problems in my furnace. It not only saved me the aggravation of having the same problems as many others, but in addition to that the furnace design I came up with was cheaper to build than more "conventional" ones. I consider that a win-win situation.
Also, be aware that someone who has a stake in selling a particular brand or type of furnace may not be an entirely reliable source of information. (Trust me, I know.) Nor is someone who has been using a furnace for only a few months - or even a couple of years. Unless they are grossly maldesigned, it can take a little time for flaws in a design to become apparent. Someone who has recently bought a new electric furnace is often caught up in the the manufacturer's hype and/or the joy of having a shiny new piece of equipment. In addition, even if they have encountered problems they are sometimes reluctant to admit it out of pride/embarassment. Due to this he/she will often praise the furnace no matter what. My advice on this is to discount the opinion of anyone who hasn't had their furnace running for at least 5 years. Over that time period any hidden problems are likely to surface and what may have been a glowing opinion initially will tend to get a lot more realistic.

This also assumes the person giving advice isn't selling furnaces, parts for them, or getting kickbacks from someone who does. When profit is involved, all bets are off on the accuracy of information. So buyer beware... 4) Be prepared if some of your ideas don't work. Not everything I tried when I originally built my furnace worked the way I hoped it would. I had to make some changes to the materials I used when I changed my first crucible. Replacing ALL of my homemade insulation WASN'T fun!

5) You don't need to be an electrical engineer to build or use an electric furnace, but the more you know about electricity, the better off you are when something goes wrong. Learn about electricity and power control BEFORE trying to build one. It will save headaches later. If you don't know Watts from Volts, either learn beforehand or stick to a gas furnace. A glassmaking furnace is NOT the equivalent of a kitchen appliance. Don't make the mistake of thinking it is.

My feelings after using my furnace for over 14 years? I loved my electric furnace! It melted good glass, was totally silent, easy to control, and I never had to worry about it blowing up if something went wrong (my landlord was quite fond of that feature). I won't go back to a gas furnace if I have any choice at all. In fact, in spite of the fact that I now live where electric rates are 2 1/2 times what they were where I was in upstate NY, I just built myself another electric furnace, rather than going to gas.

So that's it, for what it's worth. Although my old furnace ran on 3 phase power, the general principles are similar for single phase. Anyone with a bit of experience and common sense should be able to adapt this information to a single phase furnace, as I did with my new one. If this information helps you out and you save a pile of money, feel free to send me a small chunk of the pile, or even a piece of your glass. :-)


(A special thank you to Michael Hansen. Even though we had never met before, it was a conversation with him over lunch at Boomer's during the 1991 GAS conference in Corning that convinced me to pursue the design and construction of my own electric glass furnace.)

And, of course, a page like this wouldn't be complete without a weasel clause:

The above is for informational purposes only. It is NOT a recommendation to build your own furnace control system this way. Improper installation or use of the equipment mentioned can be extremely dangerous. Seek professional advice on safety procedures and equipment before beginning. I am NOT a trained electrician or engineer, I'm just a guy who built his own electric furnace in order to both save a few bucks, and to have an electric furnace that actually worked well. So, if you do something stupid (like building your own) and get hurt, don't sue me. I don't have enough money to make it worth your while, anyway.

All information and images on this page, copyright 1994 - 2008 by Brad Shute. This page may not be reproduced in part or in whole without my prior express written permission.

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the URL of this page is: handmade-glass.com/subscribers/efurnace.html


So the owner of a brand new furnace is NOT someone you want to seek out for reliable information. Someone who has 5 or more years using their furnace is much more likely to be able to provide you with an accurate assessment.