Jøtul F45 V2 burn times (short) vs. runaway fires

[firenightdance]

I discussed this privately with the righteous @Todd last year so maybe he can chime in, but figured I'd make this public in case anyone else has this issue. Have the Jøtul F45 V2 and quite like it. Operationally, however, I find we can't ever pack the firebox for a long burn. If I put more than half the box's capacity in - loosely packed - I feel like we get either runaway or super-hot fires. I can pack it and put it on a lowest air setting, and it'll behave for a half hour or so, then it's just start looking like Hades and we watch the heat rocketing up towards 800+ STT. At that point I open the air full, then open the door to cool it down, and nervously watch the roaring fire until the fuel burns down enough that we can bring it back under control and shut the door. Our wood is pretty dry, want to say 15-18% or less. I use decent-sized splits, obviously if I packed it with smaller splits I know it's going to runaway.

What am I doing wrong? Or is a long burn just not possible for me? It's a bummer having to just get 1-2.5-hour burns and have to put new logs in. I mean, I certainly don't mind it if we're sitting around the fire, but it makes overnight burns or low-maintenance burns pretty tough. Best I can hope for is some glowing embers in the morning but that doesn't help on really cold nights. Thanks.

 

[Highbeam]

What does your chimney look like? Is it perhaps overdrafting?

Regardless, if you can’t find a way to block more of the incoming air that is feeding that inferno, I would install a key damper in the flue to control the fire.

 

[Trevor Tahclep]

You need to analyze your complete system and not just the stove itself. There are many many variables to consider that will affect certain problems/situations. It normally is not only the stove !

What type of chimney ?
Chimney length ?
Chimney size ?
Chimney location ?
Etc., etc., etc.

 

[Todd]

How tall is your chimney? Mine is 22’ straight up and I found it was too much draft at times so I thought about installing a pipe damper to slow her down but I ended up tweaking the air a bit first and that has definitely helped. First thing I did was block the unregulated boost air from the two holes in the doghouse located in the front lower firebox. Then I also played around with blocking the secondary air with magnets. After many burns I found blocking that air by 25% was where I wanted it.

Now before you start going down that rabbit hole we need to find out more info about your install and how you are burning.

Reloading on a large coal bed will cause a lot of off gassing all at once and shorten your burn times.

Split size makes a difference as well. A full load in my F45 is 6 or 7 splits. If your filling in more than that your splits may be a bit small.

When reloading rake all the coals forward and leave about an inch of ash in your firebox as well. Place the largest splits on the bottom and fill in from there. Burn on high til everything gets going good then turn it down to medium air. This could take as little as 5 minutes and usually for me this is when my internal flue probe reaches 400. Once I’m up to 500 I turn it down to med/low air and continue to burn there until the temps starts to climb again. Once I’m up to about 550-600 I shut her down to low. She usually settles down into a nice lazy flame and good secondary burn. I easily have plenty of coals for relight after 12 hours following this routine.

 

[Wisco Shepherd]

Flue dampers, large splits packed tightly, and turning your air down sooner seem to be the best ways to stop a runaway and improve burn times.

 

[Highbeam]

A hot fire is a clean fire so the ever tightening emissions regulations have forced stove manufacturers, especially those of noncats, to prevent the operator from any fire less than blistering hot which is how they pass the test and are able to sell the stoves. They rig the intake system so you can't really close it very much. Even with poorer quality fuel, shorter chimneys, warmer outside temperatures, the stove needs to burn hot enough to be clean. So when a good operator with dry fuel and a tall stack uses the stove, the dang thing can run away and go nuclear!

Some of the new stoves have completely removed your control of the intake air. No throttle anymore. Totally lame, inefficient, and honestly unsafe.

 

[firenightdance]

Thanks people for the replies. I'm not sure I can answer some of the questions because I'm not sure how to measure the chimney accurately. The guys who put it in were recommended but ghosted shortly after and their receipt isn't the most detailed.

What type of chimney? You can see photos in this thread. Extant chimney is a large masonry chimney through the center of the house from basement, up living floor and attic and out through roof. There are 2 flues inside. The F45 V2 went into a fireplace on the living floor in the center of the room. SS liner exits out the top, but a very slight diagonal at first (no 15" adapter) to get into what I assume is the left-most flue. It's pretty annoying how they did it - cut through a Heatilator, stainless steel top plate with fiberglass insulation, but no plate at the bottom so tons of heat goes up the chimney. I'm not even sure how I'd put a bottom plate on the way the cut up the Heatilator.
Chimney length? I have to guess here: gonna say 15' max from exit of Jøtul to chimney cap. I'm talking about the SS liner.
Chimney size? The chimney is gigantic (3 x 6 feet) but it's standard SS liner. I'm burning now so can't measure diameter, but can update later.
Chimney location? Center of house.

How would a flue damper go into this setup? It's tight inside the fireplace. I also wonder if it's totally the draft. @Todd's tips might be a good place to start.

The split size is definitely on the small side this year, which doesn't help matters. I didn't think about that. But we used some decent-sized ones (6 or so) when we had that really hot fire. Turning the air down earlier seemed great until it wasn't...it behaved at first with nice secondary combustion and lazy flames, then picked up and got so that you had the "fire waterfall" at the glass in front. Hades!

 

[firenightdance]

I should note I'm watching nice, lazy flames right now. Blue and orange curls with secondary combustion. But most of the firebox is empty.

If it is a draft issue, would there not be a fast burn at all times - full load or not?

My issue seems specific to burns with 1/2 or more of the firebox full. Under 1/2, it behaves pretty predictably unless I do something idiotic (loads of small splits/kindling). With 1/2 or less, we get STT above 400, bring down the air, and it usually burns like this (450-600F STT) until the wood is gone then goes into coal mode. I should note we did have larger splits last year where I think we could only get 6-7 in, but we still had trouble with it burning too hot/crazy above 1/2 full firebox.

 

[Todd]

Ok 15’ liner should be fine so I think there’s something else going on here. Probably a combination of split size, loading on larger coal bed and not turning down the air soon enough?

Since you can’t install a flue thermometer you need to go by the looks of the flames and your stove top temps when adjusting your air down. The stove tops temps will be lagging behind compared to flue temps. I’m betting you just need to turn it down a little quicker.

Another thing I would do is block those boost air holes. This should limit your primary air and give you a little more control.

Looks like it would be very difficult to install a lower block off plate where they cut through your fireplace damper. I would at least shove some Rockwool insulation up in there to help keep the heat from going up your chimney.

 

[Todd]

Oh one other thing to check is your door gasket. Make sure it passes the dollar bill test.

 

[firenightdance]

Checked the gasket last year after the season but will do so again. Thanks for the info.

With the SS liner, is too short or too long the issue?

 

[Trevor Tahclep]

A 15 foot chimney is not very long so it should not be creating excessive draft if properly sized for your stove, my exterior 6'' chimney here is 28 feet high and the colder it gets the stronger the draft. Most manufacturers stove testing is done with 15 foot chimneys so no excess there.
You may need to lower your air intake quicker for better control which is what I do here after heating it up with kindling and small splits, I have plugged my 1/2'' boost air outlet at the bottom of my door with a piece of 5/8'' door gasket pulled through it which has also helped in my case. To obtain the best control of my fire I usually use large pieces/splits/chunks and can then pretty well control my fire any way I want and still do have lots and lots and lots of heat, some pieces just barely make it through the door sometimes.
I am anal about my door gasket however and pretty well change it every season or so ( even if it looks good and passes the dollar test ) I have found that this definitely does make a real difference in controlling my particular stove. I have also found that graphite impregnated door gasket stays supple, pliable, conforms longer and does not hardened and dry out as quickly as standard white door gasket off bulk rolls sold by stove shops.
NB: Same stove same house for 25 years, burning experience with changing climate however seems a bit different year after year, so continuously need to adjust and adapt every fall. The longer I use my Pacific Energy stove the more I love it, so SIMPLE !

 

[Todd]

Yes as said above 15’ should be good. I used to burn a Jotul 602 CB on my 22’ chimney before I replaced it with the F45, then moved it to my workshop on a 14’ chimney and the stove ran completely different and was much more controllable.

 

[EbS-P]

Do you measure flue gas temps? If not i suggest you do. I’m going to guess that on 15’ of liner which is close to minimum you are not turning down soon enough. Or you have a leaky gasket somewhere. Have you had the top off to check everything (baffle, deflector etc) are all correctly positioned?

 

[Wisco Shepherd]

The split size is definitely on the small side this year, which doesn't help matters. I didn't think about that. But we used some decent-sized ones (6 or so) when we had that really hot fire. Turning the air down earlier seemed great until it wasn't...it behaved at first with nice secondary combustion and lazy flames, then picked up and got so that you had the "fire waterfall" at the glass in front. Hades!

As weird as it sounds it may not have been early enough yet. If you are loading onto hot coals that turned down process is really fast and with really good wood it can be pretty darn quick on a top-down cold start as well. For reference my stove really took off this morning on a good load of dry Ash so I was turning it down in small increments every 2 to 3 minutes so within 10-15 minutes of engaging the bypass on my catalyst I had her turned down completely

 

[firenightdance]

Do you measure flue gas temps? If not i suggest you do. I’m going to guess that on 15’ of liner which is close to minimum you are not turning down soon enough. Or you have a leaky gasket somewhere. Have you had the top off to check everything (baffle, deflector etc) are all correctly positioned?

I do not measure and it would require a retrofit I'm not totally comfortable doing despite doing a decent amount of DIY. A problem here is it's super hard to find reliable chimney people, which is BONKERS considering every damn house up here has one. Or come to think of it...maybe that's why.

As far as checking I'll see what I can do. This thing is awkwardly placed and I have no strong friends locally to help me move the stove so it might be hard to do any of the above. Will look into it though, thanks.

 

[Trevor Tahclep]

The wood I am burning presently is 3+ years old, it is not only dry, some is too dry. This firewood flames up extremely quickly so I need to close my air intake down very quickly, after 90 minutes of slow burning I presently have a temp of 400 - 425°F on my Condor probe thermometer in my double wall stove pipe and 325°F on my stove top magnetic thermometer. I have a large chunk of elm in the stove presently, probably last 4-5 hours.

 

[firenightdance]

As weird as it sounds it may not have been early enough yet. If you are loading onto hot coals that turned down process is really fast and with really good wood it can be pretty darn quick on a top-down cold start as well. For reference my stove really took off this morning on a good load of dry Ash so I was turning it down in small increments every 2 to 3 minutes so within 10-15 minutes of engaging the bypass on my catalyst I had her turned down completely

You know, my issue is balancing the turn-down time with a) getting the internal temps necessary for secondary combustion vs. b) smoldering fire. If I turn down too early, the flames can die out completely, or they stay low/not spread for a really long time which causes also smokier environment outside. But clearly I need to experiment.

I feel like split sizes and primary airflow are two things I can focus on now without help, and maybe if I keep running into issues I'll have to get a pro here.

I still am curious about this question though if anyone can answer:
If it is a draft issue, would there not be a fast burn at all times - full load or not?
If it really is airflow, just seems strange that I really only have this when packing the firebox...hmm

 

[Wisco Shepherd]

I still am curious about this question though if anyone can answer:
If it is a draft issue, would there not be a fast burn at all times - full load or not?
It just seems strange that I really only have this when packing the firebox...hmm

This issue may be too much draft. More is not always better.

 

[Trevor Tahclep]

I still am curious about this question though if anyone can answer:
If it is a draft issue, would there not be a fast burn at all times - full load or not?
If it really is airflow, just seems strange that I really only have this when packing the firebox...hmm

I seriously doubt you have overdraft with only a 15 foot chimney, however the draft can easily be measured by a qualified tech for your peace of mind.
Larger wood pieces take longer to start off and make it easier to control the burn with your air intake. If my stove got carried away with a large load I would immediately start searching for excessive amounts of air entering my firebox, hence I would check all gaskets including glass, door(s), refractory cement if so equipped, and all gaskets, plates, screws on cast iron stoves..
For the present time concentrate on controlling the base fire itself and not as much on secondary burn as this will follow after if all works well.

 

[EbS-P]

I do not measure and it would require a retrofit I'm not totally comfortable doing despite doing a decent amount of DIY. A problem here is it's super hard to find reliable chimney people, which is BONKERS considering every damn house up here has one. Or come to think of it...maybe that's why.

As far as checking I'll see what I can do. This thing is awkwardly placed and I have no strong friends locally to help me move the stove so it might be hard to do any of the above. Will look into it though, thanks.

It just takes the correct size drill bit. I highly recommend adding one.

 

[stoveliker]

You know, my issue is balancing the turn-down time with a) getting the internal temps necessary for secondary combustion vs. b) smoldering fire. If I turn down too early, the flames can die out completely, or they stay low/not spread for a really long time which causes also smokier environment outside. But clearly I need to experiment.

I do think that 15 ft in current MA weather is not overdrafting. And indeed, your later stages should go faster too if it's overdrafting.
I think it's a turn down issue.

Turning down in one go will have to be tweaked to just the right time as you describe. That's very hard to do (I think it'll be impossible).

You need to turn down in 4 or so steps. And that first one should be quite early if your wood is indeed dry as e.g. Trevor's to avoid runaway.

 

[Todd]

It just takes the correct size drill bit. I highly recommend adding one.

I think his stove is inside a fireplace with little room for reading a flue probe? But an Auber AT200 installed in the liner may be an option.

 

[Trevor Tahclep]

I think his stove is inside a fireplace with little room for reading a flue probe? But an Auber AT200 installed in the liner may be an option.

Or a TempSure monitor, real simple as well, magnetic sensor therefore requires no hole to be drilled, can be attached to stove or somewhere on flue pipe.

Home -

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[stoveliker]

Lol, testimonial:
"I love the fact that I can monitor my wood burning stove temperature from my smartphone. The alarm has informed me several times that my stove was too hot because I forgot about the open vents."