The Firelight 12 is a downdraft catalytic stove, a design which has several legitimate advantages, and a few very well-known disadvantages. Also, this being such an early catalytic model, they made some mistakes in the implementation (IMO), they’ve learned a lot about how to design catalytic stoves since then.
First, on downdraft catalytic stoves, in general. In bypass mode, the exhaust goes straight out the top back face of the firebox, and up the flue. The combustor is isolated from the fire, and therefor has no chance to come up to operating temperature before closing the bypass. You are essentially always thermally-shocking the combustor from a nearly-cold state into active region, when you close the bypass. This makes them difficult for beginners to operate, as you have to find another method to judge when it’s time to go from bypass to active cat.
In newer stoves, the combustor is more often located at the top of the firebox, so it gets pre-warmed by the fire during bypass mode. In these newer stoves, the cat probe thermometer will often indicate the combustor is already at active temperature, telling the operator it’s time to close the bypass. The downdraft catalytic stove lacks this “feature”.
But, it’s not all bad. First, you can put that too-short probe thermometer you bought previously in the stovepipe about 18 inches above the stove. Use this to tell you the exhaust temperature of the stove, and as an indicator the fire is ready for you to engage the combustor. You will have to learn what minimum exhaust temperature will guarantee combustor light-off, but start with 500F as a guideline. The other thing we can do is to ensure the entire load is charred (no brown wood left to see thru the window), before you engage the combustor, at least until you get a better feel for things.
The advantages of the downdraft design is that they can be very efficient, and can avoid one of the common problems of some of the more modern catalytic designs: cat ash plugging. With the downdraft design, all ash swept up into the exhaust gasses will end up on the floor of the cat chamber, which is very good. In my newer stoves, with the cat at the top of the firebox, they sometimes get swept into the cat itself. This is bad news, and Jotul avoided that in the design of the Firelight 12.
Now, for where Jotul screwed up on this design, again IMO. First, the combustor is just way too small. This combustor has a cross-section of only 28 square inches, which is almost exactly half the size of the combustor in any modern Blaze King or Woodstock stove of similar size. I call out BK and Woodstock, because they’re the two current leaders in pushing catalytic technology, and they both do it very well. I had a lot of problems with my catalytic combustors running way too hot in the Firelight 12’s, and I belive their too-small size is the primary reason for this. I had a few scary nights, when I stayed up past 2am after a 10pm reload, because cat temperatures soared beyond 2000F degrees.
That said, my stoves were 20 years old. It’s possible my cat temperature problems were the result of a leak somewhere in these very complicated assemblies. The Firelight 12 is one of the most complex assemblies I’ve ever seen in a stove, with it’s dual bottom plates, top load door, air plenums down both sides and across the door sill, and sealed ash pan tray. Many others have run Firelight 12’s for a very long time, and only a few of us have reported constant cat temperature problems, but were we the only ones using the combustors and monitoring temperatures?
One other note, I don’t see how you can control the stove temperature with the combustor bypassed. It seems like a recipe for a run-away, to me, but you have more experience on that item than me.
My least favorite thing about the Firelight 12 was the cost and fragility (is that a word?) of the cat chamber. Jotul raised the price of that assembly from $380 to $620, and I went thru at least three full assemblies in just three years. The first was lost when a squirrel got into the stove thru the chimney, and just destroyed the whole cat chamber. The second was replaced just due to age, it was falling apart and going soft, to the point where the combustor wouldn’t stay in place. The other replacement parts were caused by combustors badly distorting due to the aforementioned 2000F problems, and the chamber being damaged in the process of trying to get those disintegrated combustors out.
Many of the cat combustor manufacturers wrap their ceramic in interam gasket, before pressing into the steel “can”, that holds the whole thing together. It seems this interam can expand, pressing the can outward. This can also damage the combustor chamber, and a few here have come up with ways to battle that. Search on it, and if I remember correctly, just last year someone recommended a new combustor that’s been working very well in the Jotul 12’s. It might have even been a Durafoil combustor, I don’t remember since I’m not running Jotuls anymore.
My advice? Go ahead and run it, if you have it, and can verify it’s in good working order. Until you get your feet under you, just load it half way (to prevent putting the combustor temperature into the stratosphere). Get your half load of wood going well, wait for exhaust temperature to hit 500F or the load to be completely charred over (15 - 20 minutes operating in bypass), then close the bypass damper. The fire will go from vigorous to slow and lazy. Set a timer for 5 minutes, and walk away. Come back in 5 minutes and verify combustor probe is reading over 500F, then turn down on the following marks:
5 minutes: 50%
10 minutes: 25%
15 minutes: final cruising positions (0% - 10% setting)
Combustor temp should spike up into the 1000 - 1500 F range around the 10 minute mark. If it doesn’t, then your wood is likely not dry enough to be running an EPA (whether catalytic or non-cat) stove.
Catalytic stoves do have one advantage for folks with less than ideal wood, I think. Operating in bypass drives off most of the initial moisture, and you can extend the bypass phase to get it down to where you can make it work. Do note, however, that during this phase you’re putting massive amounts of creosote-making compounds into the chimney. If your wood is dry (< 20%MC), this doesn’t amount to much over the course of 6 months regular burning, but it can be very bad if your wood is not dry.
