Once the kiln is properly calibrated after the test firing, it will fire very reliably for many firings. The digital controller achieves this reliability by constantly comparing the thermocouple (TC) readings to the program that it is following. Having multiple TCs in the kiln makes this a redundant operation as well. While calibration issues like firing a little too hot or cool are common, it is very rare to hear that someone seriously overfired a digital L&L kiln.
That said, there are still a few ways to seriously overfire the kiln that the controller cannot prevent. Here are some examples:
Stuck relay
- The relay is told by the controller when to turn the power on and off to the elements. It makes the clicking sound in the control box
- If the relay sticks closed- fails and stays on all the time- it will not turn off from the lack of a signal from the controller like it should. Even when the controller says the program is complete or shuts off with an error code because the kiln is climbing so quickly, the temperature in the kiln will continue to climb. This is only the case in small kilns that have just one relay.
- Larger kilns and kilns with more than one TC have multiple relays. If one relay sticks closed (on) then the elements will be on all the time in part of the kiln, not the whole kiln. Even having half the elements on all the time is not enough to overfire the kiln.
- In theory, more than one relay could fail stuck closed (on) at the same time. But the likelihood of it happening would be incredibly small. Most relays fail open (off) when they fail.
Thermocouples (TCs)
- If there was just one TC and it was not all the way in the kiln, but in far enough to feel some of the heat- that could overfire the kiln.
- Most L&L kilns have more than one TC, bolted solidly to the side of the kiln, and the controller will shut the kiln off with an error code if the TC temperature is too different from the program.
- Another way would be if you had the wrong type of TC connected to the controller. The controller can be set to read a Type K or Type S TC. As a safety, a little jumper on the circuit board itself must be moved to correspond with the K or S setting. An E-9 error code will result if the jumper and setting do not match. But if the jumper and setting do match say for Type K and you hook up a Type S TC, the kiln will overfire.
Wrong Cone Number or Hold Time
- While the controller will not allow you to program any cone over Cone 10, you could accidentally program Cone 6 when you are meant to fire to Cone 06.
- Or you could put a 10-hour hold at the top temp when you only meant to put a 10-minute hold there. The kiln would be ruined if you held it for 10 hours, even for just 1 hour, on a cone 10 firing.
- Minimally the clay or glass in the kiln would be ruined with too long a hold at a lower temperature.
- With all this in mind- one can see it is possible, but unlikely a digital L&L will overfire.
- We highly recommend that the operator monitors the firing and is present when the kiln is to shut off. That way any chance that it might overfire is removed. Overfiring for any reason is not covered by the 3-year factory warranty.
