Kiln taking longer and longer to fire

[cadenrank]

Hey there everyone. I'm noticing an issue with one of my older manual kilns, that I don't really understand. 

Just for reference, the elements were replaced about a month ago, and I also changed out the element feeder wires, connectors, the sitter tube, and the plugs and sectional connection cables (as I've posted about here before). So, pretty much everything inside of this kiln is new, except for the switches (4 position switches), and the bricks and stainless steel. (The elements were replaced first, and then two weeks or so after that I did wires and everything else) 

I've had the kiln for about 2 years, and haven't really had any issues with it. It gets fired to cone 6 pretty much weekly. I was getting firing times of about 14 hours (to cone 6) about 2 months ago, which was why I put new elements into it. I did this once before when I bought it, and the original elements that it came with were giving me about 16 hour firings, and replacing them took me down to about an 8 hour firing. Which lasted for about 7 months of firing, before it started to slowly add more time to each firing.  This time, after replacement I was dropped down to about a 10 hour firing  (from a 14hr)  to cone 6, which really isn't a big deal for me. But it's seemed like over the past couple weeks, that time keeps increasing by an hour or so. For instance, last week, it took almost exactly 11 hours to end, this week, I'm surpassing 13 hours as I type this, and the witness cone is in the process of bending. The only thing that's changing is the ambient room temperature during start up (kiln is in my attached garage, so only a few degrees above outside temps, but I have records from last year that didn't show this being much of an effect as the outside temperatures dropped. So I don't understand the difference between my last element change, and this one, nor do I understand the lengthening of the firings.

The firing time usually isn't an issue for me, and my glazes generally are still how I expect them, but I'm just not sure why there's so much difference between this element replacement, and last years, and why it seems like after only a month, I'm already back to 13+  hours to cone 6, following the same schedule I did last year. The initial lengthening of the firing time started prior to the wires being replaced, so I don't believe that was an issue.

(Update, pretty much ended exactly at 13 hours tonight. Cones through the peep are perfect, but still, 2 hours longer than last week.)

Thank you guys in advance!  
-Caden

[neilestrick]

What brand and model is it? Do all the elements glow when it's on high?

[cadenrank]

 

1 minute ago, neilestrick said:

What brand and model is it? Do all the elements glow when it's on high?

This kiln is a Blue Diamond model 180. When the elements were installed, as well as when all of my wires and stuff were replaced, all of the elements glow when on high. I don't know about currently, but I would guess they do, but will verify it once I unload this firing. 

During bisque fire (04) on Sunday, timing was normal, for reference. 

[cadenrank]

22 hours ago, neilestrick said:

Do all the elements glow when it's on high?

Yes, all of the elements still glow on high. 

[neilestrick]

What turn-up schedule are you doing?

Where are you getting the elements?

[cadenrank]

1 hour ago, neilestrick said:

What turn-up schedule are you doing?

For glaze firings I go on low for an hour, medium for an hour, then on high until it shuts off. 
 

1 hour ago, neilestrick said:

Where are you getting the elements?

Both times I've replaced I've gotten them from Duralite. But I plan to go to Euclid's next time I need to switch them. Mostly because Euclid's told me that theirs come prestretched, just not pre crimped, and Duralite's need stretched about 2 feet (for this model of kiln, at least) to fit which is a pain.

[neilestrick]

Have you measured the resistance of the elements? That would tell us if it's an element problem or a switch problem.

[cadenrank]

5 minutes ago, neilestrick said:

Have you measured the resistance of the elements? That would tell us if it's an element problem or a switch problem.

I did measure them when I installed the new wires, (so, at that time the firing time was still an hour or two longer than I got last year with new elements) and they measured all around 10ohms. I don't have a reference what the elements were originally, nor do I know what they were when I replaced them the first time, or when they were brand new, but I imagine around 10 ohms. 

Is there a way I could test the switches to find out if there's an issue there? I believe they're functioning properly, just based on the fact that I can hear the changes between all of the positions, and can see them in regards to firing, but I'm guessing there's more to it than just functioning properly. Resistance, etc. 

[neilestrick]

1 minute ago, cadenrank said:

I did measure them when I installed the new wires, (so, at that time the firing time was still an hour or two longer than I got last year with new elements) and they measured all around 10ohms. I don't have a reference what the elements were originally, nor do I know what they were when I replaced them the first time, or when they were brand new, but I imagine around 10 ohms. 

Is there a way I could test the switches to find out if there's an issue there? I believe they're functioning properly, just based on the fact that I can hear the changes between all of the positions, and can see them in regards to firing, but I'm guessing there's more to it than just functioning properly. Resistance, etc. 

If it still has the original switches, those are timer switches, which cycle on and off. It could be they're not cycling properly, although I wouldn't expect that to change when the elements are replaced.

Just to be sure that I'm talking about the right kiln, this model is 18 inches wide, 4 elements, 24 amps?

Does your power cord have 3 or 4 prongs?

Are the elements wired in series or parallel? 10 ohms makes sense if they're wired in series on 240 volts.

Measure the resistance again and see where they're at now that they're firing slower.

Since everything changes when you get new elements, the problem is most likely related to the elements or how they're wired up, not the switches or wiring or anything else that causes firings to slow down.  Are you sure you got the correct elements for your kiln? I see Duralite has 3 different element options for that kiln.

 

[cadenrank]

 

13 minutes ago, neilestrick said:

If it still has the original switches, those are timer switches, which cycle on and off. It could be they're not cycling properly, although I wouldn't expect that to change when the elements are replaced.

I  could be wrong, or maybe the switches were replaced, but I don't believe they're timer switches. I'm not exactly sure how you would tell, but they don't sound like they cycle. 

13 minutes ago, neilestrick said:

Just to be sure that I'm talking about the right kiln, this model is 18 inches wide, 4 elements, 24 amps?

Does your power cord have 3 or 4 prongs?

Yes, 18 inches sounds right, and 4 elements, and 24amps is what it's rated for. 
and there are 3 prongs. 

13 minutes ago, neilestrick said:

Are the elements wired in series or parallel? 10 ohms makes sense if they're wired in series on 240 volts.

I have a wiring diagram that I drew up for it, that I'll attach here. This is how it was wired when I received it, and is how it's wired at the moment. It may help some others down the line too maybe. Again, the wiring that is in that diagram did work well during the last element replacement, and it is still wired the same now.  (note, I'm not a professional at wiring diagrams, this was purely just so I had a better understanding of where the wires were and something to reference in case of future projects lol)

13 minutes ago, neilestrick said:

Measure the resistance again and see where they're at now that they're firing slower.

Since everything changes when you get new elements, the problem is most likely related to the elements or how they're wired up, not the switches or wiring or anything else that causes firings to slow down.  Are you sure you got the correct elements for your kiln? I see Duralite has 3 different element options for that kiln.

 

I will take a measurement again, and report back. 

I guess I can't really say for certain that this issue wasn't starting back before I replaced elements, since I did replace them because it was taking longer, but replacing the elements did drop the firing time by about 4 hours, but I'm almost back up to that again after only about a month or so. 

The 180 model only has one element option on Duralite. I believe the 180D was a different model, and had two different resistances. It was the same serial number that I purchased during the initial replacement which seemed to work well for a while.


 

kilnwiringdiagram FINAL.pdf

[cadenrank]

32 minutes ago, neilestrick said:

Measure the resistance again and see where they're at now that they're firing slower

All elements still measure 10.2-10.3 ohms. All connections inside still looked good as well. 

[Bill Kielb]

Just an observation
So 240 volts, 24 amps means  total resistance of 10 ohms which once they wear about 10% it’s time for a change for most kilns. 10% of 10 ohms is only one ohm so at 11 ohms they are basically done. These are in parallel which means each element on its on ought to be 20 ohms and once connected in parallel the combination is 10:ohms). So accurate measurement is important here.

your drawing indicates these very likely are three way switches.

[neilestrick]

2 minutes ago, Bill Kielb said:

So 240 volts, 24 amps means  total resistance of 10 ohms which once they wear about 10% it’s time for a change for most kilns. 10% of 10 ohms is only one ohm so at 11 ohms they are basically done. These are in parallel which means each element on its on ought to be 20 ohms and once connected in parallel the combination is 10:ohms. So accurate measurement is important here.

Good point there. @cadenrank are you measuring them while connected?

That does look like a standard wiring setup with 3 heat switches (lo-med-hi).

Check that the switches are working in all 3 positions. Maybe one position is dropping out?

Have you checked the power cord and outlet to see if they're heating up during firing?

Also check the contact plates on your sitter. It could be that they're corroded and not making good contact.

Have you check the voltage under load (can be difficult/dangerous to do on a manual kiln).

[cadenrank]

3 minutes ago, Bill Kielb said:

These are in parallel which means each element on its on ought to be 20 ohms and once connected in parallel the combination is 10:ohms. So accurate measurement is important here.

I don't really know if I understand this concept of resistance very well, so I may be measuring them in parallel, assuming that would just mean that they're connected to the switches. I know for sure that during the measurement I took today, everything was still connected to the switches in the manner depicted in the drawings. And it's very possible I did the same thing the first time when I measured, but I'm not certain. 

[cadenrank]

19 minutes ago, neilestrick said:

Check that the switches are working in all 3 positions. Maybe one position is dropping out?

Just based on sound alone, each position sounds different. That's not very definitive, but at the very least they're all doing something in each position. 

19 minutes ago, neilestrick said:

Have you checked the power cord and outlet to see if they're heating up during firing?

The plug and wire themselves don't physically feel too hot during operation. The cable runs pretty close to the kiln for a few feet, but it's never excessively hot. 

19 minutes ago, neilestrick said:

Also check the contact plates on your sitter. It could be that they're corroded and not making good contact.

I'm not exactly sure how to check that part, unless that's referring to the terminals on the back of the sitter. If that's the case, then those were all undone and replaced when I gutted the old wiring, and the terminals themselves were very clean. 

19 minutes ago, neilestrick said:

Have you check the voltage under load (can be difficult/dangerous to do on a manual kiln).

I had checked the voltage at the plug (kiln on high, plug very slightly pulled out of the socket to where I could touch with the test leads) when I first tested the kiln under load (this was almost 2 years ago at this point though) and the voltage didn't drop by much, but I don't remember the exact numbers. I also don't know if that's a good enough place to read that voltage. If I remember correctly I think it only dropped down to 238 or 239 volts with all the switches to high. and if I'm remembering correctly again, I believe it went back to 240 after a second or two. And on second thought, I don't think testing it there is really telling to anything other than the power supply from the wire and the receptacle doesn't drop much, but I could be wrong on that secondary thought too. 

That being said, I would expect that I would have had issues this whole time if it was a power supply issue. I was content with 8 hour glaze firings, haha. I mean, I was even content with the 10 hour glaze firing I was getting a month ago, I just didn't understand why it was different from the last time I switched elements. 

[neilestrick]

4 minutes ago, cadenrank said:

I'm not exactly sure how to check that part, unless that's referring to the terminals on the back of the sitter. If that's the case, then those were all undone and replaced when I gutted the old wiring, and the terminals themselves were very clean. 

The contact plates are in the guts of the sitter, behind the porcelain block. You may have to take the sitter out of the control box to clean it.

5 minutes ago, cadenrank said:

The plug and wire themselves don't physically feel too hot during operation. The cable runs pretty close to the kiln for a few feet, but it's never excessively hot. 

Have you checked this when it's been on high for a while?

Is there any correlation between the time of day that you're firing and the slower firings? Like maybe you're getting a voltage drop at a certain time of day?

[neilestrick]

24 minutes ago, cadenrank said:

I don't really know if I understand this concept of resistance very well, so I may be measuring them in parallel, assuming that would just mean that they're connected to the switches. I know for sure that during the measurement I took today, everything was still connected to the switches in the manner depicted in the drawings. And it's very possible I did the same thing the first time when I measured, but I'm not certain. 

Unplug the kiln, push in the power button, turn one switch on high, and measure the resistance at the two prongs of the plug.

[cadenrank]

Just now, neilestrick said:

The contact plates are in the guts of the sitter, behind the porcelain block. You may have to take the sitter out of the control box to clean it.

That's what I thought. That might be something I'd have to look into. 

1 minute ago, neilestrick said:

Have you checked this when it's been on high for a while?

Yes. I checked this after the wire replacement, when I was almost at cone 6. There was a difference from room temperature, but not by much at the plug. I also had the receptacle apart during that wiring replacement too, and checked for any discoloration or anything, and made sure everything was still tight, which it was. 
 

4 minutes ago, neilestrick said:

Is there any correlation between the time of day that you're firing and the slower firings? Like maybe you're getting a voltage drop at a certain time of day?

This was my original thought, but pretty much over the last two years, I keep a pretty strict firing schedule. They might vary by an hour or so when I start it, but I generally start it early in the morning, and it usually finishes in the late afternoon/early evening. This has been the same thing the whole time, and has been within 10-20 minutes of each other the past three weeks, on the same day. 

[cadenrank]

5 minutes ago, neilestrick said:

Unplug the kiln, push in the power button, turn one switch on high, and measure the resistance at the two prongs of the plug.

I just used the top sections, since the plug is easier to get to.  On high I measure 20.2ohms from just one section of the kiln. (2 elements) 

Out of curiosity, what should low and med read in this test? I expected to see some sort of resistance on low and med, but I only get 0L on low, med, but get 20.2ohms on high. 

[Bill Kielb]

I suggest:
If you just measure the elements, each should measure 20 ohms from your drawing. Disconnect one side of the element and then measure just that element as precisely as your meter would allow. Your switch will influence the interconnection of these so first, just measure the elements independently to see how worn they are. From there we can show how a three way switch works and you can check.

[neilestrick]

7 minutes ago, Bill Kielb said:

I suggest:
If you just measure the elements, each should measure 20 ohms from your drawing.

Wired in parallel shouldn't they be 40 ohms each? That would be 6 amps per element, 12 amps per section. Typically kilns of this size are wired in series, with infinite switches and 10ish ohm elements.

[cadenrank]

If it's of any usage, I measured between element pigtail to element pigtail, with the switch in the off position. Which was when I got 10.2

And then between the two hot prongs of the plug on Neil's test with the switch on high. Which was when I got 20.2.

I don' t know when I'll be able to get into it again to measure the elements unconnected from the switch. But will try to get that measurement

[PeterH]

 

4 minutes ago, neilestrick said:

Wired in parallel shouldn't they be 40 ohms each? That would be 6 amps per element, 12 amps per section. Typically kilns of this size are wired in series, with infinite switches and 10ish ohm elements.

My reading of Duralite's site is that the kiln has four elements
https://duralite.com/shop/kiln-replacement-elements/replacement-heating-elements-for-blue-diamond-kilns/

... and I was assuming that the two  "elements" on the posted kilnwiringdiagram each consisted of two elements in series.

Duralite's additional information seems to give the resistance of the elements as 17 ohms, which I find confusing.

[cadenrank]

1 minute ago, PeterH said:

 

... and I was assuming that the two  "elements" on the posted kilnwiringdiagram each consisted of two elements in series.

Duralite's additional information seems to give the resistance of the elements as 17 ohms, which I find confusing.

The 180, and 180D are different models. They don't list the resistance for the 180 model on the website (I just looked for it too). The 180D model had a 17ohm model, and a 35ohm model, which is what they're giving there. 

The elements on the diagram are a single section of elements, each one is pigtail to pigtail. Two elements in a section. 

4 elements in the whole kiln.

[Bill Kielb]

I would agree, the math doesn’t work 24 X 240 = 5760 watts not 5520, so something is up. so I still say measure each element and make sure one side is disconnected. Let’s figure out what they are and should be. Take a picture of the element posts so we can see how many and how they are wired. The total wattage of the kiln should be listed on the equipment tag, how many watts does it say it should be?