Part two of my three part kiln series.
Kilns can be heated in two different ways. The heat source can run all the time, so that the temperature of the kiln is relative to the temperature of the air in your shop. Or the heat source can be controlled by a thermostat – either one you add or one that comes with your heater – so that the temperature of the kiln is always the same regardless of the temperature of the air. If your shop is heated it probably doesn’t matter which way you go, but if your shop is down at 30 degrees and your kiln is at 130, you are wasting a lot of energy. See previous post for more info.
Kilns can have a number of different heat sources. I have used all of these at one time or another:
Incandescent Light Bulbs: These are 90% efficient at producing heat (and only 10% efficient at producing light), and they are great heat sources for a kiln. 200-300 watts worth of bulbs will heat a 2x2x3′ kiln, which is plenty big enough for anyone building one chair at a time. Wood contacting the bulbs will char (see photo), so a heat shield (flashing, etc.) is a absolute must.
Oil Filled Radiator: All of these radiators I have seen have a heat cut-off at 130 degrees, which makes them great for large kilns (up to 4x4x8′ or so). They are rather big and take up a lot of space in the kiln. They are really safe, since they never get very hot.
Radiator: Most baseboard radiators seem to have built-in thermostats that turn off at around 100 degrees; far too cold for our purposes. After a search, I found this one without a thermostat that has a safety cut-off high enough for our purposes. You have to wire it to a thermostat or it will run too hot. I have two 30″ radiators in my 3x3x4′ kiln, the equivalent of 1000 watts. Buy a longer length radiator if it will fit in your kiln – they come in sizes up to 72″ (and in 120 or 240 volt options).
Thermostat: Any heater can be wired through a thermostat. However, thermostats aren’t really needed for any of these heat sources, except the baseboard radiator. I bought this one. It works well and can be set to whatever temperature you like. But it wasn’t easy to wire (the terminals were small) and I had to build a box around it so there wouldn’t be exposed wires. Search ‘temperature controller’ and see if there’s a better one out there. [I have since bought this temp controller and it’s much more user-friendly.] A water-heater thermostat from your hardware store might work well too, I don’t know.
Safety: Most of the heaters here do not get hot enough to burn wood, except for light bulbs. That said, the combination of heat, electricity and dry wood is a dangerous one. Make your electrical connections inside an electrical box; use good stain-relief on the wires and make sure everything is properly grounded. Keep wood away from the heater. Don’t override the built in safety cut-offs that come with most heaters – they are there to help keep you from burning your shop down. Don’t use high-temp heat sources (like a hot-plate) or heat sources with an open flame.