Ash as an Insulator

soupy1957 said:

I'll play dumb here, for the sake of clarification.........

When it's said that Ash acts as an insulator (in reference to leaving a layer of it in the bottom of your firebox),.........what's it "insulating" against?

Is it that the bottom of the firebox is the part of the box that gets the hottest, and it NEEDS protection? Wouldn't stove manufacturers do things to the steel on the bottom to protect it more (make it thicker, for example), BECAUSE it is the most vulnerable?

Why do we "need" to insulate the firebox with Ash?

I do it, I'll continue to do it, but just thought that it was worth asking the question.

-Soupy1957

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x2

My understanding is you're insulating the coals so they last longer. Has nothing to do with protecting/insulating the firebox.

Soupy, you must have been an engineer in your early life

Hanko said:

Soupy, you must have been an engineer in your early life

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He's definitely a woodstove philosopher in this one!

Some woodstoves, like Hearthstone models, require a very beefy hearth per their spec,
and it does get somewhat hotter underneath. On my model, I've found a big difference
in surface temps. when I leave a good layer of ash in the stove (my unit doesn't have an ash tray).

Insulting the bottom of the stove will prevent it from burning out. If it gets hot enough the metal can scale, and years of tiny scaling break down the metal. Also having an insulated layer will 'ease' the heat changes by preventing the metal from heating or cooling to rapidly. This is probably more important on cast iron stoves as they are much susceptible to cracking than sheet steel stoves. The insulating effect also directs the heat back into the firebox creating a hotter coal base. Sort of like refractory does. In a coal forge, putting a clay liner (often made form wood ash) in the firepan helps to get the fire a lot hotter.

nojo said:

Insulting the bottom of the stove will prevent it from burning out. If it gets hot enough the metal can scale, and years of tiny scaling break down the metal. Also having an insulated layer will 'ease' the heat changes by preventing the metal from heating or cooling to rapidly. This is probably more important on cast iron stoves as they are much susceptible to cracking than sheet steel stoves.

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I don't have a steel bottom on my stove, it's cast iron. If it cracks, the stove goes to the recyclers because that part is no longer available. I will say that, for many years burning in ignorance, I painstakingly scraped out all the ash from the bottom of my Scandia 118 clone. The stove was given to me by an old friend when he got a real job and bought a real Jotul, and it didn't come with an instruction manual. I looked at that ash as taking up valuable firebox space. Eventually, I found out I was supposed to leave ash in the bottom, so I stopped scraping it out. Still, all those years of abuse seemed to have no ill effect, nor was the ash layer ever needed to get firebox temps up. That stove was an energetic little beast to say the least.

I have to say that for a much maligned copy of a great stove, my personal copy served me well for 18 years of hard use without cracking anywhere. I did burn parts out, but only at about the same rate my buddy's genuine Jotul 118 did. I have to say, I sometimes miss that old gal.

soupy1957 said:

When it's said that Ash acts as an insulator (in reference to leaving a layer of it in the bottom of your firebox),.........what's it "insulating" against?

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Work.

It will accelerate heating up the firebox, but too much hash will prevent fresh air coming in eventually chocking your stove.

rdust said:

My understanding is you're insulating the coals so they last longer. Has nothing to do with protecting/insulating the firebox.

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+1

nojo said:

Insulting the bottom of the stove will prevent it from burning out. If it gets hot enough the metal can scale, and years of tiny scaling break down the metal. Also having an insulated layer will 'ease' the heat changes by preventing the metal from heating or cooling to rapidly. This is probably more important on cast iron stoves as they are much susceptible to cracking than sheet steel stoves. The insulating effect also directs the heat back into the firebox creating a hotter coal base. Sort of like refractory does. In a coal forge, putting a clay liner (often made form wood ash) in the firepan helps to get the fire a lot hotter.

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I didnt hear any one insult your stove

10-cc said:

It will accelerate heating up the firebox, but too much hash will prevent fresh air coming in eventually chocking your stove.

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To much hash will get you in trouble

Hanko said:

10-cc said:

It will accelerate heating up the firebox, but too much hash will prevent fresh air coming in eventually chocking your stove.

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To much hash will get you in trouble

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Uh huh . . . I think I had too much corn beef hash this weekend . . . didn't mess up the fire in the Oslo, but it did bad things to my bowels.

I've operated with two thoughts on ashes in my stove.

1. My stove box is too small to have several inches of ash.

2. The only reason I'll leave ashes in there are because I don't want to scoop them out (I get lazy sometimes...er...alot, whatever)

I, do, however, leave my ash tray full. I like to think it keeps the stove sealed a little tighter by preventing one more place for the stove to pull in air.

soupy1957 said:

Nojo: You said, This is probably more important on cast iron stoves as they are much susceptible to cracking than sheet steel stoves.

Since my Avalon Rainier 90 is a "Steel" stove.........does that mean I have a little more flexibility in terms of durability, and can get away with less Ash? I ask this because yesterday's visit to my local Wood Stove Dealer/Installer, yielded the comment that I can let my Ash go for a couple of weeks, (and probably should, in their opinion).

Personally, I like keeping a layer of Ash in the firebox, but I don't particularly care to let it fill up excessively (perhaps the obsessive/compulsive side of me, otherwise known as "anal.").

Based on what YOU have said, if I'm interpreting it right, Steel will resist cracking, more than Cast Iron will? Is that because Steel Alloys are stronger than Cast Iron Alloys? -Soupy1957

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I am no stoveologist and Im relatively new to wood stoves. My 2nd year owning and operating one. But I work everyday in the fire forging and whatnot. I think the reason cast iron is more susceptible to cracks is becuase its much more brittle (due to its carbon content?). If you drop a cast iron plate on the ground it will probably shatter. Plate steel will not (tool steel can as well if not heat treated properly). The The expansion and contraction of metal as the stove goes through the heat cycle can cause cracks in cast iron becuase of the properties of the metal, it is in nature less flexable. Its not the heat itself, its the un-even distribution I suspect. When one area is much hotter than another they will expand/contract at different rates causing possible failure. The casting process itself can cause problems like air pockets inside the metal. Sheet steel will crack along the welds becuase of the different rates of expansion between the weld material and sheet material. How much protection will ash provide? Who knows, but as pointed out in another post, the prevention of hotspots is important I think. Cast iron seems to handle heat better than steel though. Cast iron tends to 'burn out' and scale MUCH MUCH less than steel. Door frames on good gas forges are generally made of cast material for this reason. But they do crack often. Im basing this all on my experience. Im no expert.

Hanko said:

nojo said:

Insulting the bottom of the stove .

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I didnt hear any one insult your stove

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Actually someone called my Englander a soulless cube once. ...elitist PE owners. sheesh.

Soupy,
I posted a thread yesterday about trying to get longer burn times out of my VC encore cat. Coincidentaly a friend told me to try more ash in the firebox because it acts as an insulator for the coals. I had been raking most of the ash through the grooves in the floor plate into the ashpan and emtying them, a newbie mistake I have been told. Hopefully more ash can get me longer burn times. Thanks for the timely posting.

Simply put, the fire will burn better if you leave some ashes in the stove. It will also help insulate the bottom of the stove, thereby making it cooler, or not so hot underneath the stove.

Why are firebrick used inside a stove? Would we be better off removing the brick? After all, some remove all the ash.

I would the firebrick in a modern EPA stove serve two main purposes: 1.) insulating the firebox and thus reflecting heat back towards the fire, which helps it to achieve the very high temps required for sustained secondary combustion; and 2.) prolonging the longevity of the stove by preventing the fire from coming into direct contact with most of the stove body itself.

Also, never insult free hash.