Jotul 3 CB with granite slab behind -- rear heat shield?

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John Lehet

Member
Nov 9, 2013
12
Vermont
Greetings, my first post here. And thank you all, as I've been reading these forums over the last weeks as I shopped for a new stove. I've found these forums to be helpful.

(I've been burning wood for most of the last 30+ years: "airtight" Vermont Castings in the 70s, then a tempwood in the early 80s, a self-built Tulikivi Masonry stove, 3500 pounds of soapstone, from mid 80s to late nineties, a few other stoves, and a Waterford Leprechaun over the last decade, which now needs rebuilding)

Without going through the whole history, my life evolved so that the Waterford Leprechaun moved from the kitchen of the little farm hand's house I had in NH to this retrofitted one-room schoolhouse with addition here in Vermont. We've insulated it; it's OK. But I think we burned the Leprechaun a little too hard.

After noticing the baffle in the Leprechaun was really beat this fall, the season already starting, I ended up with a used Jotul 3CB, just installed. It seems to be in good shape, and it has a new baffle in it.

The stove has a 4' x 6' granite slab 10 " behind it with no wooden studs holding it. The granite slab is in front of the masonry chimney, held by a wooden frame. The slab heats up right behind the stove, and then that hot spot cools as it spreads across the stone. The stove also has some massive slabs of soapstone underneath as a hearth. (I scored some used soapstone science lab counters and sanded them down, very nice).

It seems like letting the stove-back blast back and heat up the granite, and letting that re-radiate to us is a nice and good thing. Not having a rear heat shield seems like a good way to get the heat into that stone. Obviously that heat shield would be stopping the stove from throwing heat back to the wall behind it; that's why they put it there.

I would just omit the shield and not even ask you stove experts, but I'm wondering if the stove might actually be more efficient with the shield in place. I say this because there is a little plenum in the cast iron for the secondary air, I think, in the back of the stove, normally under the shield. I'm guessing that this would be a bit hotter with the heat shield in place. Having the combustion air hot seems like it might be more likely to facilitate the burn. So maybe a better burn, but less of a hot slab with the heat shield? Or maybe a just-about-as-good burn and a nice hot hunk of stone? Anyone familiar with the purpose and engineering of that plenum on the Jotul 3 CB?
 
Along with preheating the secondary air the heat shield provides good hot convection air coming up out of the top of it. Were it me I would leave it. When I was burning in my F3 CB I left it on and the wall behind it was concrete block.
 
Well, it turns out this was pretty self-evident, when I got the stove really going. Yesterday was just a couple of small fires. Today we let it get hot for a while.

It turns out that without the rear shield, the granite behind the stove got frighteningly hot. Burn your hand hot. The IR thermometer said it was about 240 F. I was really afraid it might crack, since it's a solid slab with no expansion joint, with a concentrated hot spot. This wasn't even a cold winter day with the stove really cranking.

The rear shield is back on now, the granite is more like 150 in the hot spot.

On another note, this is a fantastic stove. It's very responsive, easy to light, has a nice range of output. For the relatively few logs I put in it today, we got a lot of heat. It also looks much better in the space than the old Leprechaun. I wish I had replaced the Leprechaun years ago, but I was somehow attached. Jotul rocks.

One question: It seems one side of the top is consistently at least 50 degrees hotter than the other side while burning. It vents from the top. I put the top on very carefully, tightened the allen nuts to hold it; it seems to be even and really on there. Shouldn't it be quite even? Something I can check?
 
Mine is always hotter on one side of the top than the other. I think it has something to so with how the secondary air comes into the firebox. But also take something like a burning candle and slowly pass it around where the underside of the top meets the stove body to check for leaks. Where a leak exists it will pull the flame toward the joint. Members have had problems with getting the top properly seated on the gasket.

ETA: Of course the leak test is done with the stove up to burning temp.
 
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