Cost of Operating a Kiln.

Robert Cohen · Post by **Robert Cohen** » Thu Nov 20, 2003 7:29 pm

Just read an interesting letter from Skutt kilns about how to calculate kiln electrical costs. Baasically, you take the kiln's stated kilowattage, multiply that figure by the number of hours in a kiln firing cycle and then multiply that result by the cost per kilowatt hour in your area.
The next obvious cost question is the wear and tear on a kiln and how many firings can you get from a kiln before you have to start repacing elements,bricks etc. One article I read on the internet calculated that you could get between 400 - 450 firings before you entered a replacement cycle. Does anybody have any better information about kiln wear?

Bob

Kevin Midgley · Post by **Kevin Midgley** » Thu Nov 20, 2003 7:55 pm

Had a home built kiln with Kanthal elements for 15 years times 300 firing days times 2 firings per day. No replacements that I can recall until the frame finally rusted apart.

Brock · Post by **Brock** » Thu Nov 20, 2003 8:15 pm

3 LaLonde kilns, one has been fired almost daily for 20 years, the others less. Replaced elements, switches occasionaly, that's it. Brock

Tony Serviente · Post by **Tony Serviente** » Thu Nov 20, 2003 10:28 pm

I've got 5 kilns with an aggregate total of about 19,000 firings and the only recurrent problems I see in my studio are related to mechanical wear, and electrical connections. The elements don't wear out, and the insulation doesn't get used up. In my studio what happens is connections fail, and insulation gets damaged by people not being careful. Each kiln I build gets better, and my last two are almost trouble free. So if you do it right the first time, which I didn't, you can build, or buy, a kiln that will need less attention than the typical car. Wonder if the article you found related to industrial ceramic kilns. 450 firings until kiln replacement would be unacceptable for a glass kiln. As for electrical usage, check the archives and you'll find lots of ways to check that.

jim simmons · Post by **jim simmons** » Thu Nov 20, 2003 10:32 pm

A Paragon gl24 for 5 years, fire about every other day. Had to replace one infinit switch
jim

Wallace Venable · Post by **Wallace Venable** » Fri Nov 21, 2003 2:12 pm

"Just read an interesting letter from Skutt kilns about how to calculate kiln electrical costs. Basically, you take the kiln's stated kilowattage, multiply that figure by the number of hours in a kiln firing cycle and then multiply that result by the cost per kilowatt hour in your area."

That gives you the MAXIMUM cost per run. To find the ACTUAL cost, you have to know how much of the time it is ON, and how much it is OFF.

I use an external computer based controller, so I was able to plug a clock into the controller in parallel with the kiln. For my 8 inch 1400 watt AIM, as I recall, it was on about 40% of the time, and a run costs me about $0.35 at $0.07 per KWh. It was so reasonable I forgot the exact details.

Tony Serviente · Post by **Tony Serviente** » Fri Nov 21, 2003 5:26 pm

When I calculate the costs of firing even my big kilns, it always surprises me how little it is, and I live in the state with the highest electric costs in the country. In NY state, what can really hurt is what they call demand metering, which basically penalizes you heavily for using high current devices during normal business hours. Works for me though, I fire at night when rates are cheap.

Kitty · Post by **Kitty** » Fri Nov 21, 2003 5:58 pm

Tony Serviente - more than 22 cents per KwH? i thought we won the prize for expensive juice. and it's 25 cents on Kauai. but in any case, i've been surprised at how little it really does cost for me to run the kilns.

Bert Weiss · Post by **Bert Weiss** » Sat Nov 22, 2003 10:42 am

Tony Serviente wrote:When I calculate the costs of firing even my big kilns, it always surprises me how little it is, and I live in the state with the highest electric costs in the country. In NY state, what can really hurt is what they call demand metering, which basically penalizes you heavily for using high current devices during normal business hours. Works for me though, I fire at night when rates are cheap.

Tony

I have a demand meter. It is killer. Firing my kiln once in a billing cycle adds a charge of somewhere between $100 and $150. Firing at night is no help.

I had a residential meter when I got set up. Then I had a large commission and did 12 hour firings twice a day. After that month they slapped the demand meter on and I can't get it off.

Phil Hoppes · Post by **Phil Hoppes** » Sat Nov 22, 2003 11:22 am

Bummer Bert. I have a demand load meter up here in the sticks of Overgaard. Here it is a choice. They hit me $5 more per month on the minimum per month but if I fire between 9pm and 8am my electricity is 4.5cents/KWH. Haven't wired all of my kilns up yet to the new box. Should have that done by Jan so I will really see what happens but when I signed up I asked a lot of questions about hidden charges and there didn't appear to be any. We shall see.

I'm putting hour meters on my mongo kiln I'm finishing up and on my 24 inch paragon. If the 40% number is fairly indicitive (which I'm guessing it is as it sounds about right) my 26KW monster should only cost about $6/firing. Not bad.

Phil

Tony Serviente · Post by **Tony Serviente** » Sat Nov 22, 2003 3:19 pm

I stand corrected Kitty, should have said continental US, and my condolences. Now if you had about three football fields of solar panels, or maybe hooked up to little geothermal from one of those handy volcanoes... Bert and Phil, yes, demand metering is vexing. I once forgot to reset my timer when we had a time change, so the kilns kicked on an hour early. My electric bill went up $700 for the month. If that only happened for 1 second instead of an hour, it would have been the same. But like you Bert, the power company decides what kind of metering I get, and right now there are no viable options in NY state, so I just try to be very careful, and on weekends when there are no restrictions I get in double or triple firings if I can.

Tony Smith · Post by **Tony Smith** » Sun Nov 23, 2003 12:03 pm

Robert Cohen wrote:Just read an interesting letter from Skutt kilns about how to calculate kiln electrical costs. Baasically, you take the kiln's stated kilowattage, multiply that figure by the number of hours in a kiln firing cycle and then multiply that result by the cost per kilowatt hour in your area.
Bob

This statement is not exactly true. It would be accurate if you were heting the kiln on full power, then shutting it off, but that isn't the way most firings are done.

If you listen to the clicking of your relays, you'll notice that the relays are only powering the kiln a fraction of the time. The only time electricity is being used is when the relays are on (other than the small draw for the controller).

Wallace Venable's solution is a simple one. Attach an old-style analog clock (non-digital, with hands) to your kiln controller and measure how much time the elements are really on. My guess is that there is about a 10 - 20% duty cycle when you include time for cooling, so for a 10 hour firing, you might be using 2 hours of electricity... but that's just a guess.

Tony

Phil Hoppes · Post by **Phil Hoppes** » Sun Nov 23, 2003 6:00 pm

Tony Serviente wrote:I stand corrected Kitty, should have said continental US, and my condolences. Now if you had about three football fields of solar panels, or maybe hooked up to little geothermal from one of those handy volcanoes... Bert and Phil, yes, demand metering is vexing. I once forgot to reset my timer when we had a time change, so the kilns kicked on an hour early. My electric bill went up $700 for the month. If that only happened for 1 second instead of an hour, it would have been the same. But like you Bert, the power company decides what kind of metering I get, and right now there are no viable options in NY state, so I just try to be very careful, and on weekends when there are no restrictions I get in double or triple firings if I can.

Tony, I don't understand what kind of meter you could possibly have? On any given firing cycle your relays kick in and kick out. A meter can only register two possible inputs, the time of day and the instantaneous power that is being consumed. I assume that the meter is averaging over some periodic length but the point is, the meter can't possibly know that you have a 15 hour firing schedule and since you started it 1 hour before the cheaper rate you would get dinged for the higher rate for the whole firing cycle. At least that is what I believe you are trying to infer. Point is, if you started 1 second before the higher rate vs 1 hour you should certainly see a difference in the charge. I'm not disputing you saw a larger bill, you obviously had to pay it, but the way that electrical power is metered is not what I believe you are describing.

In my case, I pay 9 cents/KWH between the hours of 8am and 9pm M-F and 4.5 cents/KWH from 9pm and 8am and on weekends. The power I use in those respective windows is billed at the rates I described. If I have a cycle that starts say at 8pm, I'll pay 9cents/KWH for the first hour of the firing and 4.5cents/KWH for the rest of the cycle.

I don't know how big and how many kilns you have but it sounds like it must be a lot if the accumulated 1 hour delta on firings for one month amount to $700. That just doesn't smack right to me (like I think you are really getting hosed by possibly a miss reading of your meter) Anyway, I thought I'd at least pass that on.

Phil

Tony Serviente · Post by **Tony Serviente** » Sun Nov 23, 2003 11:05 pm

Here's how it works in NY Phil. Demand metering is based on the utility having available the maximum power their customers "demand" at any given time. Let's say that there were 10 customers in a service area and all used 1 kw, or less,but never any more. The utility would only have available the power to supply 10kw to them. If one day, the 10th customer used 10 kw, even instantaneously, the utility will now insure that they have the ability to supply 19 kw to that service area. They have a rate based on the maximum instantaneous demand for commercial customers, and it is billed out at say, $12 per kw. If you only demand 1 kw you pay $12. This is on top of the regular day and night kw/hr rates. In other words if I had a 1kw kiln and left it on all month I would pay $12, and the hourly cost to operate the kiln continuously for that time. If I turned that same kiln on for a second, and never again for the next month, I would pay $12, and virtually no hourly operating cost. The idea behind this sometimes frustrating scheme is to even out the demand for power, and minimize the need to build new plants that may only be needed for 2 hours a day. I may gripe about it, but it makes sense. So when I goof and fire outside the window, I'm penalized with the demand charge, and with the higher kw/hr rates. It sure teaches you to be careful though. PS-Hard to believe, but demand metering in NY used to be way worse than this!

Phil Hoppes · Post by **Phil Hoppes** » Mon Nov 24, 2003 8:52 am

Tony,

Ok, I understand. They are using peak demand meters in addition to RMS. My old house in Scottsdale had one of these. I my case, since I had a pool and ran the pool pump at night, it did save us money. I forgot all about those. I'm pretty sure the metering system I have up here in Overgaard does not work that way as the bill only reports KWH used for the month, not peak demand. Bummer. I understand now. They must have gotten the idea from cell phone companies that give you 300 min per month and then charge you a bazzilion dollars/min for every min above the 300 min.

Phil

Tony Serviente · Post by **Tony Serviente** » Mon Nov 24, 2003 10:32 am

Yeah Phil, I think their ability to sense peak goes to the millisecond. Wish it were only RMS, as at times I'd gladly pay the higher daytime rate.

Bert Weiss · Post by **Bert Weiss** » Mon Nov 24, 2003 10:41 am

My demand meter monitors 15 minute periods. The highest amount of power used in any 15 minute period is the amount that they base the demand charge on. I have no night discount available.

I found that if I ramp up to temps under control, that is less than full power, my bill is smaller. If I go whole hog for 15 minutes, I get the big bill. I am using SCR relays so I am using a percentage of power as opposed to relays that turn on and off.

We haven't discussed this much, but there are 4 kinds of relays: Mechanical relays that click click on and off, mercury relays that are quiet as they turn on and off, SSR relays that electronically turn on and off, and SCR relays that are capable of putting out a percentage of power.

My SCR's can be controlled to be on or off 120 times a second or every half cycle. This action is more forgiving to the elements than a hard on / off. It is also less disruptive to radio and TV signals in the area. In theory they should be more accurate, but the click clicks I have used seemed to do a pretty good job of following temps.

I researched relays recently and was told that SSR's were the inexpensive relay of choice today. While mercury works well, if there is a problem, companies have been required to jackhammer up their floors to deal with spilled mercury. This makes the SSR the favored choice. A mechanical relay is supposed too be good for 100,000 clicks. When you think about how many times it clicks during a firing, that number comes up fast. A 50 amp SSR costs less then $85 including the SSR, a heat pad, a heat sink, safety cover, and fuse setup. The fuse is also a manual on/off switch. This rig should be mounted on the wall not on the kiln. Excess heat can kill the SSR. It is designed to be used with ambient room temp, not side of kiln temp.

Tony Serviente · Post by **Tony Serviente** » Mon Nov 24, 2003 12:16 pm

Bert, years ago when I looked into SSR's the big issue was spikes. I was told that they were notorious for being trashed by nearby lightning strikes, as well as any kind of big inductive load kicking on or off nearby. Have they built in enough surge suppression that it is no longer an issue? Other thing I worried about was failing in an on state, which solid state devices can do, but mechanical relays are far less likely to. Have you experienced either of these problems?

Bert Weiss · Post by **Bert Weiss** » Mon Nov 24, 2003 2:20 pm

Tony Serviente wrote:Bert, years ago when I looked into SSR's the big issue was spikes. I was told that they were notorious for being trashed by nearby lightning strikes, as well as any kind of big inductive load kicking on or off nearby. Have they built in enough surge suppression that it is no longer an issue? Other thing I worried about was failing in an on state, which solid state devices can do, but mechanical relays are far less likely to. Have you experienced either of these problems?

I have been using SCR's since the mid 80's. Once I had a relay stick closed. A few times, stuck open. I have heard of mechanical relays sticking closed.

personally I don't have a hi temp cutoff setup, but I am designing one in to the unit I am building for the book. It requires a thermocouple, a setpoint controller, and a normally closed mechanical relay. The Bartlett RTC 1000 has a built in hi temp cut off so the extra circuit is redundant.

I never had any lightening issues with my kilns or controllers. I have lost a motherboard on my computer even though it was on a surge protector.

Phil Hoppes · Post by **Phil Hoppes** » Mon Nov 24, 2003 4:19 pm

Couple of points:

Surge protectors....most, if not all of these are totally worthless and provide little protection to AC surge voltages and no protection to lightening. What most of these do do is give you lots of AC outlets that you do need for a typical computer setup. Some power companies offer shunting relays in your breaker panel that are suppose to protect you from a lighening strike. If you really want or need power conditioning for your computer, they make units for this but be prepared to drop some substantial coin. What you want is called a Line Conditioner. These are typically bundled together with an uninterruptable power supply (UPS) and for a common PC will run you a couple of hundred dollars.

SSR's and mechanical relays will both fail either on or off depending on how they are used, who made them, etc. It is a little bothersome to me, as coming from an engineering background, the perception on failures of these devices now that I've dug into it a lot more. If the device is installed correctly and sized correctly they should have a quite reasonable lifetime without failure. There have been a number of failures mentioned on the board about controllers/relays/etc. but in IMHO there has not really been what I would call a root cause analysis of the failure. This leads to missconceptions about the actual reliability of these devices. That being said, I too have put an overtemp control on the kiln I'm building. Like Berts, it is simply a relay, a separate setpoint controller and a separate thermocouple. I splurged a little on the setpoint controller as for a few dollars more I could get one that could double as the main kiln controller should my other controller fail.

Phil

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