Materials used to line fireboxes - a test

[precaud]

From some discussion in another thread, I thought it might be useful to take a look at the various materials that are used to line woodstove fireboxes. There are basically four that I know of: Cast iron, dense firebrick (the heavy, yellow ones used in most pre-EPA stoves), porous firebrick (the lightweight ones used in some modern stoves), and Vermiculite panels (such as Skamol, used in many European stoves).

What these materials have in common is that they are, more or less, rugged enough to withstand the temperature swings (called thermal shock) and survive having wood jammed into them from time to time (mechanical shock).

What differs greatly is how readily heat passes through them. The measure of this is called Thermal Coefficient, K for short. A high K says the material readily collects and stores heat. A low K says the material is a good insulator, and resists the flow of heat through it.

Here is the thermal coefficient of these materials at 1000ºF per inch of thickness:

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)

As you can see, cast iron and the heavy firebricks are poor insulators. It's no coincidence that they were widely used in pre-EPA stoves, when designers were trying to pull as much heat as they could OUT of the firebox. The lightweight firebrick is the better insulator by far, with the vermiculite a close second. And while all of them lose some of their insulating value as temps increase, cast iron is the worst, increasing by three times. No wonder it warps.

So if you want clean combustion AND high efficiency, you want a stove that uses the light firebricks.

Here's an excerpt from an article titled "The Straight Facts Concerning Refractories" by a company who designs and builds gas-fired forges:

Ceramic Blanket, Ceramic Fiberboard and Lightweight Insulating Firebrick are classified as Insulation, while Dense Ceramic Firebrick is NOT.

The insulating refractories have Low Thermal Conductivity, which means heat does not readily pass through them. The heat is reflected back into the chamber instead, creating a super efficient firebox.

Dense Ceramic Firebrick on the other hand, has a High Thermal Conductivity, which means heat readily passes through it, instead of "insulating" the chamber, it acts as a heat sink, absorbing heat and radiating it off through the shell. Dense Ceramic Firebrick, by it's very nature, ROBS a Forge of valuable heat. It's cheap and a very poor choice as a liner material.

 

[begreen]

Durability is also a factor in fireboxes. It took almost 20 years for my Jotul 602's inner burn plates to warp and crack. That's pretty durable and the servicing of them was fairly easy. The outside castings of the stove still look like new.

 

[precaud]

That would come under thermal and mechanical shock, methinks.

 

[precaud]

Here's a paper which deals in detail with the characteristics of refractory materials:
(broken link removed to http://www.wipo.int/pctdb/en/wo.jsp?IA=GB2001000470&DISPLAY=DESC)

Sure, reading this stuff makes your eyes bleed and head spin, but this is true of much scientific analysis.

Here's why this topic interests me so much. Higher efficiency and cleaner combustion have become the focus in all heating appliances, not just woodstoves. Do we want cleaner and more efficient woodstoves? Sure we do. If there was a simple way to make your stove better, would you do it? Sure you would. Maybe not immediately, but eventually.

If you look across the whole field, ceramic composite materials are at the heart of the innovations leading to improvements in nearly every respect. So it seems to me that, rather than go retro and go back to cast iron or heavy firebrick liners, we need to make the porous ceramic materials more rugged so they can last longer. I see two possible angles that could be simple enough to be done by a typical stove owner:

1. Improve early EPA stoves by upgrading the liners.
2. Modify lightweight firebricks to make them more durable AND increase their reflectance at the same time.

What I'm looking for are others who see the value in this and might want to take it up too. If you want to champion the value of iron and fireclay, please do it in another thread. If all you want to do is throw another split on the fire and pop another Bud, this thread is not for you. Let's keep this clean and positive.

 

[precaud]

I got the sheets of Zircar RS-1200 today.
http://www.zrci.com/rs1200.htm
This is very cool stuff. Much denser than I had imagined - much denser than the ceramic fiber boards used for baffles. A 24x24x1/2" sheet weighs 22.5 lbs. It is easily rugged enough for use in a stove. And the stuff is pure white - exactly what I think is best for a liner, as so much of the energy inside the firebox is radiant not convective.

My first experiment with it will be to make replacement liners for the cast iron ones in my Jotul F602 and see what difference it makes. I'm trying to dig up another magnetic thermometer so I can monitor temps on the side and the top of the stove. The local stores are all out of them.

 

[johnn]

I`ll keep your thread handy to see how it goes and sponge off your learning curve. Don`t expect much from this end , as the material you read and understand may very well need an interpretor for me. once I decide what to replace my insert with, I will have an old steel body to tinker with for the garage,,,just don`t try to talk me into a "home-made secondary"

 

[precaud]

Once I decide what to replace my insert with, I will have an old steel body to tinker with for the garage,,,

Ah, the perfect specimen for such a project!

just don`t try to talk me into a "home-made secondary"

Somebody else might, but not me

It turns out the RS-1200 is really expensive stuff, about $63 per sq ft. Ouch. But it's perfect stuff to test my theory, and if it works we can look around for more cost-effective material.

 

[johnn]

Once I decide what to replace my insert with, I will have an old steel body to tinker with for the garage,,,

Ah, the perfect specimen for such a project!

just don`t try to talk me into a "home-made secondary"

Somebody else might, but not me

It turns out the RS-1200 is really expensive stuff, about $63 per sq ft. Ouch. But it's perfect stuff to test my theory, and if it works we can look around for more cost-effective material.

Call 911, we`ll need a large compression bandage! Keep looking. Read somewhere in forum where cement board was being tested or used for a firebox??
Yesterday I came across a thread by ..Peter B.. with pic`s of home-made secondary`s, very impressive job. Might rethink my position on trying it! Materials and tooling very accesible if not free..even stainless..just cost me time and possible pride.

 

[Peter B.]

Once I decide what to replace my insert with, I will have an old steel body to tinker with for the garage,,,

<snip>

Yesterday I came across a thread by ..Peter B.. with pic`s of home-made secondary`s, very impressive job. Might rethink my position on trying it! Materials and tooling very accesible if not free..even stainless..just cost me time and possible pride.

Actually, this is THE thread on addition of secondaries to steel box stoves:

https://www.hearth.com/talk/threads/31420/

It's worth the effort, sez I.

Peter B.

-----

 

[johnn]

>
Yesterday I came across a thread by ..Peter B.. with pic`s of home-made secondary`s, very impressive job. Might rethink my position on trying it! Materials and tooling very accesible if not free..even stainless..just cost me time and possible pride.[/quote]

Actually, this is THE thread on addition of secondaries to steel box stoves:

https://www.hearth.com/talk/threads/31420/

It's worth the effort, sez I.

Peter B.

-about:Tabs----[/quote]

Trust me I got that one handy! Maybe we`ll have a well priced and suitable liner material mid year.

 

[EddyKilowatt]

I see two possible angles that could be simple enough to be done by a typical stove owner:

1. Improve early EPA stoves by upgrading the liners.
2. Modify lightweight firebricks to make them more durable AND increase their reflectance at the same time.

How about insulating behind cast iron liners?

The 3/8" airgap between the stove wall and the cast iron liner plates on my Jotul 8 (pre-EPA) probably wasn't too bad an insulator, right from the factory.

Never one to just leave things alone, I took the plates out and filled that air space with ceramic wool (leftover stainless chimney liner wrap), on all 3 sides of the firebox.

I did this pretty early in my experience with the stove, so it's a little hard to compare the before-and-after... but I would say the stove definitely holds a fire better (especially a small one) and seems to burn cleaner and hotter.

Time will tell what those liner plates think about having the heat-flow path behind them cut off. So far I'm not seeing any cracking or warping (knock wood).

Eddy

 

[precaud]

Eddy,
It's great that you're experimenting. What you've done is similar to Jotul's approach; cast iron liners with a thin layer of ceramic wool between the liner and the shell. It is an improvement over an uninsulated iron liner, no question, but I think there is a much better way. That's the first comparison I'll be making, using my Jotul F602 as the guinea pig. Stay tuned.

 

[precaud]

Here are some photos you might find interesting.

The first two are the left-side liner from my Jotul F602, which is sidelined this heating season, with an injury, you might say

The liner is cast iron, approx. 8" x 15", with a 1/4" thick piece of ceramic wool on the inside. Weight is just under 8.5 lbs.

Jotul uses this type of liner on the sides of many of their stoves.

As you can see, this stove has problems burning cleanly in the back half of the firebox. The design of the air supplies has something to do with that. But the less-than-optimal insulation of the firebox plays a big role.

The third photo is a piece of Zircar RS-1200 material shaped and ready to install. Weight is 4 lbs.

This material is a joy to work with (apart from the dust...). Imagine gypsum board but much more rigid and dense. It works easily with standard woodworking tools.

 

[Doctuh]

Oddly enough just took this pic of the inside of my firebox for another thread. I believe this is light firebrick.

 

[begreen]

cmon precaud, show us how she burned with the gold plated innards.

 

[precaud]

Oddly enough just took this pic of the inside of my firebox for another thread. I believe this is light firebrick.

I can't tell - your glass is too dirty

 

[precaud]

cmon precaud, show us how she burned with the gold plated innards.

At $63 a sq ft, it better be gold!

It may be a few days before I install it, we have a cold front heading in tonight and the little F602 can't handle 1000 sq ft, even with Zircar the Magnificent inside.

 

[begreen]

This is an interesting project. Can't wait to see how it turns out.

I also thought it was interesting to see the burn pattern in the F602CB. Our older 602 didn't show that. It burned pretty cleanly in the back

 

[precaud]

I also thought it was interesting to see the burn pattern in the F602CB. Our older 602 didn't show that. It burned pretty cleanly in the back

Yeah, they burn completely differently. I think with this new material and adding a few holes to the secondary manifold in the far back, it will burn hotter and cleaner.

 

[precaud]

Well the longterm weather forecast isn't that bad so last night I took down the X33 (BTW, the pipe looked nice and clean, the X33 lives up to its promise) and installed the F602 with new liners in it. As recommended by Zircar, I did a couple small burns to drive the moisture out of the material. Today I'll do the first full burns.

But first I gotta put this F602 liner test in context. The main problem with the F602 is not the liners. It is the air systems, particularly the adjustable one on the door. This has to be the most stupid, ineffective design I have ever seen in any EPA stove. Besides being too hot to touch when the stove is in use (doh!), it directs air into the stove at a 35º angle to the left. The only way around this and get an even burn is to either keep it fully closed (which you can only do with extremely high draft) or full open (almost no draft, the Florida bungalow syndrome). All Jotul needs to do to fix this is redesign the small manifold on the inside of the door behind the slide control. It's not something us mortals without foundries at our disposal can fix. Add to this that there is NO secondary air introduced into the back HALF of the firebox, and it's easy to understand the dirty burn pattern, and why it always burns from bottom left to upper right, and always leaves unburned wood in the right rear corner. Changing the liner material is not going to change this.

Is this what we're paying $900 for? (That's what F602's sell for around here.) As a former designer/manufacturer (not of stoves), it really frustrates me to see the performance of otherwise nice units totally compromised by such simple things. I think it is important to shine a bright light on such things in public forums. Otherwise those manufacturers will continue to get away with delivering incomplete designs to us with refuse like this in them. The fact is, this design should have never gotten out of the door at Jotul. And whoever OK'ed it should be forced to heat their home with one for the rest of their days.

Enough ranting. On with the test.

 

[precaud]

After placing thermometers on the top plate and side panel, I burned three loads today with the new liner, monitored temps, watched it burn, and went outside to view the stack exhaust. Yes, some improvement was noted, but nothing night-and-day. Tomorrow I'm going to put the iron liners back in, do the same thing, and compare numbers. I can't give it more time right now, because the F602 simply does not keep my 1,000 sq ft basement warm enough, and I work down there. More tomorrow.

 

[precaud]

Well perhaps my assessment was a bit too cautious yesterday... there is absolutely no question that this stove burns cooler and dirtier with the cast iron liners back in. It's gone back to being an underwhelming heater. Details to follow tonight.

 

[precaud]

Before I give the numbers, a few words about the process.
Each day's burn started from a cold stove, kindling fire to get started, then a layer of 1-2" pieces to get a coals bed and get the temps up. Then start the actual burn loads. Three were done each day over about 5-1/2 hours. Each load was three pieces, about 12" long and 4" diameter. As is normal, the supplementary air control (Jotul calls it the primary air but it isn't) was opened some until a brisk fire was burning, then slowly backed off until it burns with the control closed or just slightly open and all primary air is supplied by the airwash. (I've written in previous posts about the F602 primary air so I won't repeat it.) The load then burns without changes unless needed, see below. Today's run with the cast liners was 10-12º colder outside so the chimney draft was a little stronger, which favors the cast a bit.

About the numbers: I calibrated the two Condor thermometers at 300º, which is about the highest my Fluke contact probe goes. They track very well until above 500º, where one reads increasingly lower than the other. I put that one on the top plate since it gets hottest there. The other one was placed in the middle of the left side plate, right on top of the lion. So the absolute numbers above 500 aren't important; it's the relative difference that matters. The numbers are the maximum temps reached during that load.

Cast iron liners
Load 1 : 550 Top...370 Side
Load 2 : 670 Top...430 Side
Load 3 : 730 Top...450 Side

Zircar refractory liners
Load 1 : 650 Top...440 Side
Load 2 : 750 Top...500 Side
Load 3 : 770 Top...510 Side

So there it is. There is no question that with the ceramic liners, the stove warms up more quickly, burns hotter, and puts out more heat. And you can feel the difference in the room, and smell it too. Cast iron puts out a peculiar smell when temps reach a certain point. The real shocker is how much hotter the stove sides were. So much for the theory that the cast iron liner has better heat transfer.

At least four times during each load, I went out and observed the stack. With the cast iron, there was visible smoke all but one time. With the ceramic, there was smoke at the beginning and at the end of each load, but none or barely visible for most of the burn. The hotter temps come from a cleaner burn.

See the pics below. The first one is the right-side iron liner. It's dirtier than the left side iron liner (see it's pic a few posts above) because of the air system design. Compare it to the second pic of the ceramic right side liner. As you can see, it burns cleaner, and further back into the stove. You can clearly see the arc of the primary air wash circulation, and the secondary burn path at the top, neither of which are visible on the iron liner. The third pic is the left-side ceramic liner. Compared to the cast of that side , it too burns cleaner and further back. And the one super-clean spot shows how the air channels toward the left, as I've said. It's all in the burn pattern.

Totally unexpected was the change in how the wood burned. With the iron liner, after the temps peaked, the fire intensity would drop off and go towards a smoulder condition, due to little air and lower temps in the back of the stove. I had to rake everything forward and open the air control a little to get them to burn fully. But with the ceramic liner, to my great surprise, all three loads burned all the way to the back without changing a thing. The higher temps in the firebox are responsible for this.

Not only that, coaling was a problem with the iron liners. I had to burn the coals down after the second and third load before reloading. With the ceramic liners, I did not have to burn off coals. This was a big surprise.

Conclusions from this experiment:
: As suspected, cast iron is a poor choice for liner material, even if it is insulated.
: Ceramic refractory is a superior lining material, giving better insulation AND better heat transfer through the walls.
: In every way that matters, a hotter firebox makes for a better stove, giving cleaner burning and less smoke, more heat output, higher efficiency, and less coaling.

I'm going to put the ceramic liners in again in the morning and rerun the test with the cooler outside temps, like we had today.

Pretty solid results, I'd say.

 

[markleyh]

Wow!

I say, old boy, you've done it, you've really, really done it!

Bravo!!

Am I nuts to think about what King Zircar could do for my Fireview???!

Herbert

 

[precaud]

At $63/sq ft., King Zircar extracts quite a price. I'd bet there are more cost-effective choices. Is it firebrick? What are the dimensions and color? A high-alumina firebrick (white color) should do as well as the good King.

Added: The point of this experiment is not to show that the RS-1200 is the best material for a liner. It's probably the best stuff in 1/2" thickness. And it's clearly better than cast iron, which is as bad as it gets. (Heck, the glass door on your stove is a much better insulator than iron is.) But most stoves use thicker lining material, 1" to 1-1/4". If you replace the liner, you want to use the same thickness so the firebox geometry isn't changed.

There are firebricks that have characteristics as good as or better than the RS-1200, for alot less money. It would be great if someone who has a stove with the darker-colored brick would experiment and replace it with the whiter brick. If I had such a stove, I'd do it. But I don't. Anyone game? I'll help with finding the right brick.