Electric Furnace Control System

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* Updated March 12, 2020 *

What I did on my spring vacation, 1994

by Brad Shute

[For those concerned with proper English grammar (as one should be), please forgive the mixed tenses used in the page below. This page was originally written a few years after building my first electric glass furnace in 1994. In May of 2008, that furnace was shut down and dismantled when I moved from NY to GA. Some parts of this page were updated after the move 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. I will try to make things a little more coherent when I have a chance.]

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

Over the 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 lots 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 proceeded 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 24 inch diameter crucible. (It could actually hold an even larger crucible, but that was more glass than I needed.)
One thing to consider is that this control system requires a careful choice of SiC heating elements in order to realize its full potential. (Translation - choose the wrong elements and they won't last very long.) Of course, this applies to any furnace heated with SiC elements, and is a function of both the specific elements used and the particular control system. As far as elements, the key is surface watt-loading. 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. A phase-angle firing SCR can vary the voltage to the load and decrease the watt loading as needed. So unless the element size is chosen very carefully, a phase-angle SCR is necessary to achieve adequate element life.

However, if one chooses the elements wisely, an expensive SCR power controller won't be required. 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 where my glass studio was located not caused 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 poor 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 him/herself, "That's pretty much the same way to wire a 3 phase annealer", and that’s right. That is the nice thing about this control system - simple and cheap, especially relative to types of electric furnace control systems that require 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 spiral cut elements to make this work. (In fact, I would recommend against spiral cut SiC elements in any glass furnace.) My elements are the standard double ended, solid/tubular type. Specifically, they are Norton Hot-Rod brand that were made sometime before Kanthal bought Norton’s heating element business back in the distant past. You just need to choose the right element size (which translates to the right resistance and surface area). With that said, for best element life I do recommended high density SiC. They will almost always improve element life over standard elements. Something else to keep in mind about the diagram above is that a 240Vac, 3 phase supply connected this way ("wye" or "star" pattern) 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 long before eBay was invented and 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 a maker of studio electric glass furnaces 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 and elsewhere, 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. [Added many years later: Now that the internet and eBay exist it could be done even cheaper if you know what to look for.]

*(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 the old mechanical shut-off switch on the door when the furnace was originally built, instead of the Omron photo-electric 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 spent $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 photo-switches are available that will still do the job. Decent ones usually cost in the $100 - $120 neighborhood, new. Based on my experience, they are well worth it. Overall, I would generally recommend a photo-electric door switch instead of a mechanical one.

My approximate parts cost (pre-eBay days)
(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. I was able to stretch that out to 3-4 years using some fairly simple rewiring tricks.

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. (In fact, don't use a mullite protection tube with a platinum thermocouple in any type of glass furnace.) 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 good 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 connections are on one end only, making them easier to install and the furnace easier to build and insulate.

Nor is an expensive, phase-angle fired SCR control system necessarily the best way to go for power control, 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. Although electric glass furnaces were very rare when I designed and built mine, I researched and was able to avoid 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've seen it firsthand.) 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 and/or embarrassment. 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 after the first 6 months will tend to get a lot more realistic. 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 provide you with an accurate assessment.

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. Some people are honest, but others... 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'll be if 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. And buy a clamp-on ammeter. It will prove invaluable when troubleshooting.

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've built two more electric furnaces since then, rather than going to gas. First a small wire heated one, and then another SiC furnace.

Last, some history...

When I designed and built the glass furnace on this page, the internet was still in it’s infancy and Google hadn’t even been dreamt of yet. Nor had eBay. To get online, one typically used a 2400 baud modem and AOL. Websites, as we know them now, were largely nonexistent. Most equipment shopping was done using a handheld magazine or catalog and a land-line telephone. If you were lucky, you might live someplace where you could buy a few things in person, although that was generally limited to refractories, since APGreen had locations everywhere. Unless you were a student at one of the few universities with a hot glass program, classes were generally limited to Pilchuck, Haystack, or Penland. The Studio at Corning wouldn't exist for another two years.

Obviously, things have changed since then. Glass classes are widely available (although quality definitely varies.) Specialized equipment is much easier to find due to the internet, and can often be had much cheaper than before due to eBay and China. (Although either of those can still be risky for the inexperienced.) The refractory industry, like many others, has undergone a massive consolidation, often making things harder to find locally and more expensive overall. Energy costs have bounced all over the map. As I write this, the coronavirus pandemic is taking hold in the US and natural gas prices have dropped to nearly their lowest level in the last 25 years. Only 12 years ago they spiked to 6-7 times higher than they are now.  Between that price spike and the financial crisis that immediately followed it, more than a few small glass studios closed their doors.

Another big difference these days is that molybdenum disilicide heating elements are now available out of China for 1/10th of what they cost were when I built this furnace. In fact, I had originally planned on using moly elements when designing it - until I checked the price. At that time, a single set of moly elements would have cost me as much as the entire SiC furnace that I wound up building - control system and all. These days, unless I found a good deal on SiC, I would probably opt to go with moly elements. Although they still have the drawback of needing a transformer, and are quite fragile, they last considerably longer than SiC (if you don’t break any while changing a pot) and their resistance doesn’t increase as they get older. 

So a lot has changed since I built the furnace above and originally wrote this page. But in spite of that, the information on it still remains valid.

(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, NY 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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