Materials used to line fireboxes - a test

Woodrat · Feb 10, 2009

------I love to see threads like this-goes to show that not all good ideas come from the corporate think tanks. FWIW, I have been able to get very similar results with my old Dragon Nashua by double bricking the firebox and going another tier high on the sides-- also put some firebrick on top of the baffle
------Net result of this tinkering has been to increase overall stove operating temps by about 100 degrees-- extend the burn time-- leave me with very little in the way of an overabundance of coals (where I used to sometimes have 3-4 inches- now I still usually have 200 degrees or more in the AM, the stove restarts rapidly and the bricks are always clean!!
-----My uneducated “take” on this is that the increased mass of the stove due to all the extra firebrick helps hold the temps up in the stove (more by moderating the tempereture swings than by insulating the firebox). I am more than happy to be generating more heat, keeping it longer, & most importantly--doing it with 1/3--to ½ less wood than I burned in years past.
----I have other changes I plan as time and money allow (secondary air etc)
----Admittedly, changing the firebox size this much may not be a workable option for many-- the Nashua was a massive stove to begin with, and even with the extra brick, it’s probably still a much bigger firebox than most stoves. All I can say is that it WORKS and didn’t involve spending a ton of money (also would have been easy to “undo” in the event that it didn’t work out. These are just standard firebrick “splits” that I’m talking about.(under $2 each)

Best wishes, Woodrat

precaud · Feb 10, 2009

Glad to hear of your similar good results, Woodrat. Your stove is big enough to eat my F602 for breakfast! The only comment I'd make is your take about the increased mass. The concept of "holding heat" doesn't apply to what is needed in a firebox. High insulation and low mass is best. Doubling up the brick doubles the insulation and that's making a big difference. But all that added mass doesn't help. It just slows down the response time of the system.

Woodrat · Feb 10, 2009

precaud said:
It just slows down the response time of the system.

----- If by "response time" you mean the amount of time before I get meaningful heat from the stove-- The Nashua is so big and I'm moving so much air (about 600 CFM) through its internal passages- inside of 10 minutes in the AM, I'm getting 3/400 degree air coming out of the side vents of the stove-- I'm a happy camper! Additional benefit to the high volume of high temp air is that I can dry my freshly cut/split firewood in a matter of days. Has taken many years to get it down to a "science", but it works very well!

Best wishes, Woodrat

precaud · Feb 10, 2009

No, that's not what I mean, but that's ok. Enjoy your setup - it works.

precaud · Feb 11, 2009

Yesterday I did another round of three burns, using rounds instead of splits.

Zircar refractory liners
Load 1 : 620 Top...440 Side
Load 2 : 700 Top...500 Side
Load 3 : 720 Top...500 Side

As one would expect the rounds don't burn as fast so temps were down a bit. But the temp on the sides remained higher. Smoke output was quite a bit higher. The first two loads didn't burn all the way to the back. And the coals buildup came back, as rounds don't burn off as fast. So while the refractory liners do make a big difference, it doesn't solve all the problems of this stove. No surprises there.

Think twice before buying a stove with cast iron liners.

This experiment is closed for now. The X33 is reinstalled. It's getting cold here again.

begreen · Feb 11, 2009

The ceramic insulation is impressive. It gives some great results in this test. However,I can't completely agree with the final conclusion. Having owned the predecessor of that stove and a few other cast iron lined stoves, none had the issues reported with this 602CB. All exhibited very good, even burning and there were no coaling problems. They were all good heaters too. And smoke during the burn was not an issue. Quite the opposite, normally you wouldn't know the stove was burning by looking outside at the flue. Even the old 602 didn't smoke except on startup. In the newer EPA castiron stoves I've owned, the firebox definitely stayed hot enough and even enough to support great secondary combustion and even burning.

I'm wondering if this could partially be related to high altitude burning? My neighbors have a 2 yr old 602CB (at low altitude). I'll try to stop by on the weekend and will ask how their stove is burning, especially regarding the back of the box.

However, that's not to say that a good stove can't be improved upon? I wonder if one can get hold of used space shuttle tiles?

precaud · Feb 11, 2009

BG, you're reading me wrong. In no way am I suggesting that iron liners cause coaling. In fact, I said that burning rounds made it worse. Nor am I saying that stoves with cast iron liners are "bad." They're just lower in efficiency than they could be. And I am absolutely certain that the performance of ANY EPA stove with cast iron liners can be improved with refractory liners. If you disagree, set up an experiment that demonstrates it. Replace the nice ceramic bricks in your T6 with iron pieces and see what happens. You won't like it.

Look at the trends in the high efficiency heaters using other fuels. Low-mass, super-insulated combustion zones, followed by low-mass heat exchangers.

"Over the last 50 years many new high temperature insulation systems and materials have been developed and have provided significant benefits to the process industries... The economic benefits of low thermal mass systems, through energy saving in intermittent/cyclic temperature processing and lightweight constructions is now well documented. These factors have enabled industry in N America and Europe to make savings currently estimated at US$2.5B/annum, and to reduce emissions of greenhouse gases by-15M tonnes/annum." (WIPO)

The same principles hold true for woodstoves. You do not get "super-insulation" with cast iron liners, because iron is not an insulator. And it weighs too much. The only reason I see for choosing cast iron liners is if you own a foundry and are trying to make use of it. It's not state-of-the-art design engineering.

It's unlikely that high altitude had any effect on this test. The stack is a 25 ft rigid stainless liner in a central masonrychimney. Draft is excellent.

Look at the back of your friend's F602, I am certain it will be black. How can it be otherwise? There is no air back there.

Maybe someone with a different stove with cast iron liners will step up and replace them as I've suggested and report the results. Until disproved, I stand by my conclusions.

Peter B. · Feb 11, 2009

precaud:

I've read much of this thread at least once, but may have missed some relevant points, and have some questions for you.

Somewhile ago, I lined the lower firebox of my old cylinder stove with dense heavy firebrick (halves or 'splits'). The intent (or hope) was to protect the cast firebox from what I anticipated might be a routinely hotter fire (and possible coal burning). I also thought the bricks would help retain heat within the (brick) surround to help elevate firebox temperatures. I suspected I was trading off some of the direct radiating capability of the firebox, but figured the other (believed) benefits were worth it.

I'm now in doubt whether I achieved either goal (even in part) by the use of the dense brick.

Would you care to speculate on same? Do you think I'd be better off using lightweight brick instead? Does it stand up to routine mechanical shock as well as the dense brick? Is it very costly?

At this point, I'm torn between removing the brick altogether (to regain firebox volume), replacing the broken dense bricks next year, or replacing the dense brick altogether with the lightweight variety.

I'd be curious to hear your comments.

Thanks.

Peter B.

-----

precaud · Feb 11, 2009

Hey Peter,
First, it's not clear to me if better insulation is a good thing for such a stove. That's why I've said it would improve any EPA stove. Without an above-fire secondary air system, it may just make things worse.

I'm remembering the year or two back in the 80's when I had Godin stoves, ornate sheet-metal cylinders lined with dense clay firebrick. I burned wood in the shoulder seasons and coal in the winter. Good little coal stove but really bad for wood - burned very dirty.

karri0n · Feb 11, 2009

Peter, since you have the secondary mod, and based on where you put your baffle and secondary air inlet, I'd be willing to put my money on the lightweight insulating firebrick vs. the hard clay you have now. I'd imagine the hard clay is indeed helping, but holding as much heat in to help with your secondary as possible seems like the best move in your case. This helps the EPA stoves by increasing firebox temps, but since yours is self engineered, I would imagine that it might not hold as high a firebox temp as an EPA stove.(not bashing your design, in fact I love it) Knowing this, I would imagine the lightweight firebrick which has now been proven effective at holding higher temps within the firebox would greatly improve your combustion and efficiency. As far as know, the prices are pretty comparable between the two types. According to firebricks.com, there are quite a large number of low density and high density types to choose from, and I would imagine one with a good balance of insulating value and price is the best for your application.

EddyKilowatt · Feb 11, 2009

precaud said:
Cast iron : 150+ (from 55 at 200Fº)
Firebrick (heavy, yellow) : 14 (5.5 for a typical 2.5" thick brick)
Firebrick (light, insulating) : 1.1 (0.88 for a typical 1.25" thick brick)
Skamol V-1100 Vermiculite slab : 1.10 (slabs are typically 1" thick)

Precaud, mind if I ask your source for this data? I'm curious to see the units and testing methodology, so I can compare these solid refractories with the fiber blanket I'm using.

I'm not here to mount some great defense of cast iron, but I will again point out that I'm burning a stove with cast iron firebox liners backed with refractory fiber insulation, and it is no effort at all to get both the firebox and glass up to self-cleaning (i.e. hot) temperature on every burn... with a flue that stays clean and no visible smoke out the top.

I have to doubt that you'd find any EPA stove these days with cast iron firebox liners that didn't have some kind of supplemental insulation behind the liners. The overall goal, as you point out, is to reduce heat flow out of the firebox in order to maintain high combustion temperatures... while meeting various secondary goals like ruggedness, durability, structural design, and cost. There are several ways to meet those goals, and advanced refractory materials are one of them... but iron with supplemental insulation is certainly another (especially for manufacturers already tooled for iron). I think you'd need a pretty sharp pencil and a lot of data to conclusively prove that one approach beats the other in the wood stove firebox. (And note that the answer may well be very different for the industrial furnace, which has a different set of requirements.)

Eddy

precaud · Feb 11, 2009

EddyKilowatt said:
Precaud, mind if I ask your source for this data? I'm curious to see the units and testing methodology, so I can compare these solid refractories with the fiber blanket I'm using.

Hey Eddy, I'm at work now, I don't have that info. I'll have to do some backtracking to find the sources I found.
All by itself, the fiber blanket is superior to more dense ceramics, but obviously that changes when compressed between two pieces of iron.

I'm not here to mount some great defense of cast iron, but I will again point out that I'm burning a stove with cast iron firebox liners backed with refractory fiber insulation, and it is no effort at all to get both the firebox and glass up to self-cleaning (i.e. hot) temperature on every burn... with a flue that stays clean and no visible smoke out the top.

I don't doubt that. But it would be even better with ceramics.

I have to doubt that you'd find any EPA stove these days with cast iron firebox liners that didn't have some kind of supplemental insulation behind the liners.

Agreed. And they all would benefit from what I've suggested.

The overall goal, as you point out, is to reduce heat flow out of the firebox in order to maintain high combustion temperatures... while meeting various secondary goals like ruggedness, durability, structural design, and cost. There are several ways to meet those goals, and advanced refractory materials are one of them... but iron with supplemental insulation is certainly another (especially for manufacturers already tooled for iron). I think you'd need a pretty sharp pencil and a lot of data to conclusively prove that one approach beats the other in the wood stove firebox. (And note that the answer may well be very different for the industrial furnace, which has a different set of requirements.)Eddy

Sounds to me like you're the guy we've been waiting for. Replace your cast liners with equal size ceramic and test your conviction. I'll bet against the iron/ceramic wool sandwich any day.

karri0n · Feb 11, 2009

I agree that if you left the ceramic wool in there and replaced the iron with fiberboard you would probably at least slightly raise temps inside the firebox, but you would certainly be sacrificing durability in a system that is already tested working using the current configuration. I've been known to occasionally be a bit rough with loading, and with nothing rigid behind the fiberboard, I would be really scared of damaging it.

Peter B. · Feb 11, 2009

precaud:

As karri0n mentioned, I do in fact have a rudimentary secondary air supply that feeds the fire near the coal bed... and the coal bed / lower firebox is the area where I want the highest possible temps to prevail.

So again I might ask whether you think the lightweight firebrick surround would do a better job of maintaining elevated temperatures... at the same time being as durable - mechanical shock (i.e. repeated loading) wise?

I tend to think of the lightweight brick as being fragile... maybe I'm plain wrong. I don't know offhand how to make a ready comparison of the physical characteristics of the two bricks.

Thanks.

Peter B.

-----

precaud · Feb 11, 2009

Peter,
I didn't know about your added 2ndary. Sounds like a fun project.

Yes, there is no question the lightweight firebricks will keep temps higher. As to durability, my Quad has them and I've not broken one in 2+ seasons of use. Perhaps you will have to be a hair more careful, and not do things like slam tools into them and such. I assume you keep some ash on top of them, which protects them some.

The difference between the twp types of bricks: One resists heat, the other absorbs and stores it. You want a firebox that is lightweight and highly insulated, not one that stores heat.

The F602 that I modded also has a massive cast iron piece with ceramic wool underneath that the fire is built on. I make take another step and replace that with IFB (Insulating FireBrick). karri0n is right, IFB is the same price as the heavy stuff in standard sizes. I haven't found a good source for the half-thick IFBs yet, have you?

begreen · Feb 11, 2009

precaud said:
BG, you're reading me wrong. In no way am I suggesting that iron liners cause coaling. In fact, I said that burning rounds made it worse. Nor am I saying that stoves with cast iron liners are "bad."

Maybe someone with a different stove with cast iron liners will step up and replace them as I've suggested and report the results. Until disproved, I stand by my conclusions.

Maybe I misstated the case. The results are good. I don't disagree with the findings, but do take exception the statement to "think twice before buying stoves with cast iron liners". That's like saying don't buy a Chevy Cobalt because it can be improved as a hybrid. Sure this mod can be added (at great expense) but that doesn't make the Cobalt a bad car. At 37mpg it still a nice vehicle, right? There are other factors in buying a stove besides ultimate efficiency. I have no problem recommending the Jotuls, Isle Royale or Hearthstone cast iron stoves, regardless of liners. Actually with a top loader I'd prefer the toughest, most durable liner I could get.

karri0n · Feb 11, 2009

My Isle Royale is equipped with the low density firebrick, not cast iron liners. On the sides of the firebox near the front, there is a sheet of cast iron, but behind it is still the low density firebrick.

precaud · Feb 11, 2009

BeGreen said:
Maybe I misstated the case. The results are good. I don't disagree with the findings, but do take exception the statement to "think twice before buying stoves with cast iron liners".

That's my opinion. You have different priorities.

Sure this mod can be added (at great expense) but that doesn't make the Cobalt a bad car... There are other factors in buying a stove besides ultimate efficiency. I have no problem recommending the Jotuls, Isle Royale or Hearthstone cast iron stoves, regardless of liners. Actually with a top loader I'd prefer the toughest, most durable liner I could get.

Gawd guys, the pessimism and inability to keep things in context is amazing.

I AM NOT SAYING WHAT I AM NOT SAYING! Sheesh. If you love cast liners, knock yourself out. Start a thread extolling their virtues. Recommend whatever stove and brand and design you want to. There are hundreds, maybe thousands of threads on this website doing just that - do you want to turn this thread into another one of those? Gimme a break. That has nothing at all to do with this experiment and further ones to come in this direction. And they will follow - this is just the start, folks. Now's the time to do it, before wood heating season goes away. And in spite of the negativity I encounter here, some from completely unexpected sources, I will continue to post the details.

About the expense: I have never recommended that anyone go out and buy RS-1200 for their stove. In fact, I said don't do it, it's not cost effective. This is just stage one of this process, done to one stove. Going out and finding lower cost materials is a totally separate job from making a hypothesis, setting up an experiment, and testing it. Lower cost, and maybe even superior, alternatives will be found. Some of them already exist.

Maybe you're not, but I AM learning alot from this. It's given me insights and shattered some myths.

begreen · Feb 12, 2009

Precaud, this is not negativity, more practicality. Most folks buy and use -one- stove for their heating needs for many years. They are not experimental in nature. After the large expense of the stove the last thing they want to deal with is tweaking their stove towards it's ultimate efficiency. Don't get me wrong, I love your tests and experiments. Please don't stop! This is good information. But in the big picture, folks buying stoves have to deal with what is available, tested and warranted with currently available stoves.

You are doing good research. If anything, your results should be communicated towards the stove companies making these stoves. No doubt, there's always room for improvement. Yes there is room for improvement. Case in point, how many stove employ what your testing shows so far? What stoves are out there employing high grade ceramics for liners?

My point is that if this dialog is still from a purchasing standpoint hypothetical, then it doesn't seem fair to make owners of current technology stoves feel like they have purchased their stoves in error.

karri0n · Feb 12, 2009

Precaud,

Please don't take offense at BG's post. He's going for discussion, not conflict. I'm quite certain BG is not attempting to dispute your findings or the relevance of them. It seems to me he's providing reassurance to those who may have stoves with cast iron liners that might see your findings and think that they have purchased the wrong stove. Frankly, I'm quite impressed with your findings and would be quite interested in your next update.

kksalm · Feb 12, 2009

This is a fascinating subject. One of my first inquires to this forum was questioning the logic of placing a more efficient wood stove inside my mammoth Hearthstone I. The needed replacement parts (cast iron) are either no longer available or the cost is astronomical. After following the thread on adding secondary combustion and now this thread in particular I can actually rebuild my stove cheaper and have a more efficient product as well. Bt all means, please continue with the experiments and continue to post your thoughts. It is much appreciated.
Best regards, kksalm

begreen · Feb 12, 2009

Yep, I do love this kind of research and hope to see more interesting data.

Ken brings up one thing that I was wondering about. Would the greater heat transference of the Zircar help in the case of an old soapstone stove or would it risk cracking the stones due to the higher temp.

Dexter · Feb 12, 2009

For what it's worth, I changed the iron top burn-plate(s) of my Jotul Firelight cb with home-cut white/grey split firebrick. I've only done heating curves with the brick. 'Never did it with the Iron. Anectdotally, however, my typical fires with the original top plates seemed to top-out at about 600 f. with the thermometer on the top right rear of the stove. Now it's hard to get it over 525, and it takes a long time to get it higher than that. Curiously, the heating of the house is perhaps better: 'Lots of radiant heat out the front, sides and back, and extremely clean glass for many weeks. (Burning 24-7 during winter would warp the iron over 2 or 3 seasons.)

Best Regards

johnn · Feb 12, 2009

"precaud" Been A nice thread with a good learning curve for myself, thanks to you and all those sharing.I`ve followed this from the beginning, and intended on tinkering with my old steel box,,still thinking? Considering input from this thread and realizing the construction of my burner, Ive developed many questions as to the probability of success,,,most of these issues I feel would not be considered favorable to the content of discussion, and will try to find answers through the "navigation history"

However in your earlier thread you had two objectives: ALTERNATIVE MATERIALS and MODIFICATION.
What thoughts might you entertain on how to modify, the thinner "Fire Brick" (space and economics may leave many in this scenario)?

johnn · Feb 12, 2009

OH yes, I forgot to mention how much I envy your SUNSETS and SUNRISES111

Materials used to line fireboxes - a test

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