Jotul F3cb too hot ?

Where did you take the 850F temperature reading on the stove? It makes a big difference if you were reading 850F in the center of the top, versus on one of the corners, assuming the manufacturer calls for readings to be taken on a corner. Your son needs to get hold (or go online) and read where the manufacturer specifies the temperature readings should occur. I know on my Jotul F600 the readings are taken on a top corner and when I check with an IR thermometer the corner will read over 200 F lower than a reading on the center of the top. So, if something similar applies to your son's stove an 850F could be way too hot if recorded on a corner, or just plenty hot if taken on the center of the top.

I know there are a fair number of forum members who like to burn their stoves really hot. I often read of temperatures that would make me very anxious if I had them coming off one of my stoves. I chalk these high readings up to placing their magnetic thermometers in the wrong place on their stoves or a little bit of exaggeration on the part of the poster. If your son consistently gets readings of 850F when the thermometer is located in the proper place, then I would say he's burning the stove too hot. Some others will likely tell him to load the stove to the gills and sit back and enjoy the heat. The best advice I think is for him to read the stove manual and follow the manufacturer's advice on burning temperatures to avoid damage to the stove.

x2 with BB. That sounds a bit high, the 3 is a free breathing capable heater. The ash door, the ash housing and the glass should be checked. The air start switch at the bottom of the door is sometimes overlooked... but if the flames indeed do slow down when the air control is closed, sounds like the stove is slowing down.

Yep, as others have said, 850 is too hot. The manual describes its ideal range as 400-600. I hit close to 900 once due to absent mindedness and wouldn't want to do it again. The stove suffered no harm though... just my nerves!

It does sound like you've got something going on. My F3 is quite an easy burner and I get a strong pull up my chimney, but I would be hard pushed to get it up to even 600 after shutting it right down at 300. On top of checking the gaskets, the only other thing I can suggest is that you check if the cover is in place on the secondary air intake on the back? Mine fell off once (it turned out not to be fully screwed in place) and when it wasn't there the secondaries were much bigger and way more billowy somehow and I had to control the burn much more carefully as it had a tendency the climb very quickly.

Some other thoughts.... Are your splits quite small and Is your wood a really low MC... Like, well under 15% at the core? And is your secondary burn plate gasketted on the sides and the back? Mine wasn't; even though it was brand new and came that way direct from Jotul - seems they forgot but I went through one heck of a lot of wood til that was spotted and fixed.

Sometimes wood placement and amount of coals makes a big difference as well. If I took three 4" rounds, and dumped them on a coal bed, I'd have an unremarkable fire, with decent temps and would take awhile, then there'd be decent lazy flames. Those same three rounds, split in half and loosely stacked/airy would catch quick and rage along pretty good.

I don't think that's your issue though, it sounds like too much air getting in somehow.

The only time I ever had anything close to a runaway fire was when I put all ironwood or apple wood in the stove. Got north of 700*, but with this stove's small firebox, it didn't take long for the temp to start to drop. Burning the usual stuff, however, and she behaves herself quite well.
Good advice FionaD. I'll have to check that secondary cover when I go over the stove next spring as I have a warped baffle that I want to inspect (my own fault for burning too much cardboard, doh!)

colin.p said:

I'll have to check that secondary cover when I go over the stove next spring as I have a warped baffle that I want to inspect (my own fault for burning too much cardboard, doh!)

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If you unbolt the secondary baffle you will automatically remove the secondary cover too, as the left hand bolt on the baffle also holds the secondary cover in place.. Just mentioning that becuase it's an easy thing for someone to do without wanting to. To get the secondary cover back in place requires the removal, of the rear heat shield and a degree of access to the back of the stove... A bit of a pain if your stove is close to the wall and with a rear exit flue.

Mainely Saws said:

The three splits had a fair amount of space between them & they were on top of a hot coal bed even though the starting temp was at about 300 degrees . Maybe with a closely stacked load of larger pieces of wood in the stove there would be more control . The three pieces loaded into the stove when the temps got to high were about 4" thick at their widest point ...........
Do folks with the F3CB cut short lengths of about 10 " & stack N/S ? ......

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I stack E/W. 10" splits would be an inch too long to stack N/S in the F3.. but, as the general consensus seems to be that N/S loading burns hotter and faster, you probably want to avoid that if you're already getting really hot burns.

I think you're nailing it when you say yu've been loosely stacking pretty small (4") splits. I had to burn that way when I only had wood with too high MC, but if your wood is nice and dry it will burn fast like that.

Try tightly loading some thicker splits E/W and start to reduce the air as soon as the visible surface of the splits are charred all over and see what difference that makes.

You also need to experiment with how your whole set up and your wood behaves in relation to the F3.... For instance, in my case, I discovered with my set up that the wood at the front of the load flames quickest once I've turned the air down by 1/4, because the draft in my stove is so strong that it effectively blows the flames out at the front when the air is 100% open... At that setting, the wood at the rear flames up first and quite vigorously for quite some time before the front wood has even charred, which all leads to a faster hotter burn than I want. So when my wood is really dry, I aim to dial down the air by 1/4 almost instantly on colder days, or other strong draft situations. You may have something similar with your F3, or something else that you come to discover through trial and error... But much of what you've said above points you in a good direction to start with, I think.

BrotherBart said:

If my F3CB did that I would dollar bill check the ash pan door gasket. And also make sure that start up air dial on the front under the glass is closed, not open.

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yup, BB beat me to it. i have a F3CB its pretty rock solid for me over 5 years or so. most if the time if i get an active burn, i forgot to close something.