* Page last updated March 12, 2020 *

What I did on my spring vacation, 1994

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.

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.

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? Well... 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.