Ashpan over-firing vs design defect on Jotul 500 causing the bottom to crack?
I recently joined this group. I have been burning wood in various stoves for the past 40+ years. I bought a new Jotul F 500 in Sept 1999 a few months after I bought my present house.
It was my first high efficiency stove. I installed it myself using an existing fireplace. The hearth of the fireplace extended far enough that using the short legs I could get 90% of the stove exposed outside of the fireplace. Hence I am using it as a free standing stove without any blowers. I used all stainless products including a special stainless adapter that is oval shaped and can turn up at a 90 upward into the chimney flue.
I then installed full length 6” stainless flue inside a masonry chimney 15’ tall with a stainless cap.
Last Nov. I heard a very loud pop coming from my stove about 3 hours after starting a fire. It was not running very hot as the weather was mild. I quickly left my Netflix movie and checked the stove. From the sound of it I was expecting a cracked window or cracked stove. But I found nothing after spending 30 minutes looking for a crack in the outer walls. I kept looking and finally after washing the inside of the stove I came across 2 hairline cracks in the bottom of the fire chamber. Part # 8 in the schematic. I noticed the cracks are identical shaped on each side. Whether these cracks happened when I heard it last Nov. or have been there for years I do not know.
When I Googled it I found going back a decade people having issues with bottom plate cracks on the F 500. I noticed these other cracks were all shaped identical to mine.
https://www.hearth.com/talk/threads/cracked-jotul-oslo-safe-to-keep-using.133374/
https://www.hearth.com/talk/threads/jotul-oslo-with-a-cracked-bottom-can-it-be-fixed.115237/
Some people claimed they were using the ashpan to fire the stove and other said they did not. I haven’t fired this stove using the ashpan in 19 years. When I inspected the inside of my stove it does not show signs of any overheating. It has the original bricks. My regular maintenance has been replacing the door and ashpan gaskets and cleaning the chimney.
After finding the finding and cleaning the cracks I drew lines along the cracks making it easier to see on the photos. I then spent some time studying the design of the bottom and how the bottom plate attaches near the back panel that runs behind the bricks. I noticed the cracks are convex in a very distinct curve going from the corner where the bricks are held in place. The curve of the cracks meet at the corners where the grate fits into the bottom.
Looking closely where the cracks appear in the back of the stove is where I believe is the design defect. Along the bottom plate the bricks are held in place by a retainer wall that is part of the casting. This wall at a 90º angle increases the strength of the rear bottom plate but Jotul did not cast this wall to wrap around the rear corners where the back panel attaches to the bottom. I put a detailed photo of what I think is the problem. I ran this by a friend who is a heats with wood and is a mechanical engineer. He agreed with me about it being a design flaw.
For the moment I put in a temporary fix. How well it holds up remains to be seen.
I ground the surface of the back half of the bottom plate exposing it to the clean shiny cast iron. I took 1/16” black fiberglass and impregnated it with Rutland stove cement (not refractory cement as that is a whole different animal). I then wet the surface and laid down thin layer of stove cement on the bottom plate. I then covered the bottom with the fiberglass and then covered the fiberglass with more stove cement sealing in all the fibers with cement. I ran the fiberglass around the corners and also up the small retaining wall that holds the bricks. I also closed the back right and left corners that Jotul had left open. My hope is this will hold for at least this year. I do not know the differences how cast iron and stove cement contract and expand. But I needed something now.
I know this runs contrary to what some believe on this site. In fact I expect that my belief of it being a design flaw will be considered blasphemy by some. But I believe I laid out the flaw as I see it.
I do not let ashes to pile up behind my ashpan and my gaskets are always in good shape and I do not fire it from the ashpan. I empty my ashpan almost daily as I heat a 1500 Sq. Ft. house that has 14’ cathedral ceiling. About 60% of this area is floor to ceiling glass facing south over a river. I use low E glass. I only need to fire the stove during the day when there is no sun. So on any given winter no matter how cold it gets I have enough solar gain to heat the house during a sunny day. I would guess I only use wood heat in the daytime about 1/4 of the winter days. So this stove cools down and gets re-fired a lot in about 4+ months in this part of N.C. Over 19 years you can imagine how many fires I have started in this stove. Like I said I have been heating with wood for 40+ years. I am no newbie to wood heat and wood stoves. I would notice if I had a leak in the ashpan door. If you have a leak in the door or elsewhere your primary air lever would not be responsive when you turn the air down completely. When shutting the air down I can see the difference in the flame at the base of the front window. Also if you listen carefully you can hear the change in air moving into the chamber where the lever rotates in the front of then stove.
