Fire bricks: How to arrange bricks in wood stove?

Gentlemen, are there any particular principles to be followed when arranging fire bricks in a wood stove? Below are photos of my wood stove with my placement of the fire bricks. Note that the photos are taken from the open side door of the stove. The top photo's focus is on the bricks that line the back wall of the stove. The second photo's focus in on the bricks that line the front wall of the wood stove. And both photos show how I placed the bricks on the floor and on the side-wall opposite the open door from which I take the photo. Note that the stove will be loaded from this open door side. Also note that there are air intakes on the stove's front, so I have not placed bricks in the middle third of that side. As well, I have not placed bricks in front of the open door because I have to load the stove from this doorway. And I have not placed bricks across the top because I cannot find bricks long enough to span across the stove.





I have been unable to find any science online regarding the use of fire bricks in wood stoves. There is plenty of anecdotal evidence for the need to have fire bricks, but very little detailed explanation regarding the arrangement and purpose of such bricks. I only found some empirical information regarding the advantage of fire bricks on the floor of the stove: The Permies website published a study that established a reduction in temperature beneath a stove that has bricks covering the stove floor. Beyond that, the only understanding I have is that the bricks should extend the life of the metal of the stove. So I find myself curious about the following:
1. Should I somehow place a sacrificial steel plate in the stove to protect the top of the stove? I could place a steel plate such that it rests on the fire bricks that line the walls.
2. My stove is cast iron and somewhat 'curvy.' As the bricks sit now, there is an airspace between the bricks and the stove walls of up to an inch. What, if any, effect will this space have on the operation of the stove?
3. The stove came to me without any fire bricks and I am unable to find any manual for the stove. So I cannot know what the stove's designer had in mind as far as fire bricks are concerned. What I do know, is that the stove, though manufactured in the 1970's, was made from the original American molds from the 1840's. I do not know whether fire bricks were employed in stoves in the 1840's. Can I assume that my use of fire bricks can cause no harm?
4. The bricks currently in the stove are about 1.25" thick. And I have more of them if needed. I also have an ample supply of fire bricks that are full size, i.e., about 2.25" thick. How does one decide which thickness to use?
5. Finally, I find myself confused about whether a cracked fire brick must be replaced. I have read online here and there that they 'should' be replaced. But why is that? If the crack is very thin, and if the brick is still 'closed up,' why would it be mandatory to replace the brick? And what if one would need to cut a brick to fit; doesn't that contradict a rule that says cracked bricks must be replaced?

If anyone can help me answer these questions, I would be appreciative.

If it helps, I provide here the distances from the stove to combustibles:
-The stove's firebox floor is about 14" above a wood subfloor. Between the subfloor and the underside of the stove is 0.5" cement board on top of which is 3.5" perforated red brick. On top of the foregoing red brick is 7" of clay brick. The balance, some 3", is an air space between the clay brick and the underside of the stove.

-The stove's back wall is about 25" from a sheetrock wall. Between the sheetrock wall and the back of the stove is 6.5" airspace; 7.25" perforated red brick; and 11" airspace.




Thank you.

 

BrianK might be able to give you some insight help you out

 

In all my reading, the only purpose of firebrick in a traditional wood stove seems to be to protect the metal from coming in direct contact with the burning firewood, to prevent warping and otherwise degradation of the metal from the acid in the ash, etc. More would decrease the space in the firebox for a load of wood. It might help the stove hold heat longer but I doubt it's really necessary. (I'm certainly no expert and I'm open to correction. )

 

In my uneducated opinion your fire bricks are not arranged as good as they should be.

On my old stove that had firebrick, they were placed vertically, the back ones being installed first. In this way, when you start doing the sides, part of the back wall of fire bricks is covered up so there is no seam where fire could lick at the steel, or heat penetrates.

My old stove also did not have multiple levels of firebrick. Being laid so they were tall, (yours are sideways) one layer is tall enough as the heat is concentrated down low in a fire. Considering this is where the ash and coals are, this makes sense. This was how my factory stove had firebrick laid. The upper half of the fire box was just steel.

If you need to custom cut firebrick, if you go to a cheap tool store you can find diamond blades that fit on a hand held grinder for $5. They make quick work of cutting brick for very little money. I buy the notched ones so they can be used dry as trying to cut with water is super messy.

In no way am I trying to be a know-it-all on this, just wondering if there might be a better way and how I would do it, and how I saw it done.

 

LodgedTree, thank you for your reply. I played around with the arrangement until it got tedious. That is why I began to wonder whether there is any prescription regarding the use of firebricks and found almost nothing of real practical use about them online. The question of just how much the inside of the stove should be covered by brick is the question that I am attempting to answer. My stove is somewhat tall and that is why I have stacked the bricks. I did this based on many other stoves that I have seen where the bricks were stacked up. Regarding seams, I pretty much do not worry about them. The seams are relatively tight and I just cannot imagine that whatever heat goes through them is heat to worry about. My concern is more for the inside top, the load door, and the front side, which are not protected. As for the lower section of a lit stove being the hottest, I am just not sure. It seems to me that the upper portion of a fire box would be the hottest. Wouldn't a layer of ash serve as some kind of protection for the floor? And even if the hottest portion of a lit stove be the bottom half, the flames are moving vertically and hitting the inside ceiling. So it just seems to me that the inside top of the stove needs protection if any part should need it.

 

BrianK,
Thank you for your reply. Yes, it makes sense to me that the brick protects the metal. So I am wondering whether the inside ceiling of the fire box should have a spanning fire brick or other sacrificial plate. And, in the same vein, if the brick is necessary to protect the metal, what will be the fate of the load door and front side of the stove, both of which cannot be protected? These are the questions that I am trying to resolve.

 

The acid is in the ash, not so much in the smoke which is moving up and out through the chimney. That is why only the bottom of the firebox is typically covered with firebrick. If you line the entire thing with firebrick it will make the stove a lot less efficient because it will be so dense.

 

I think the hottest part of the stove will vary with the burn. Heat is transferred through convection, conduction and radiation. When you have flaming combustion the main source of the heat is above the fuel (from the flames). As the burn progresses and glowing combustion dominates, the source of heat is the fuel itself. In my humble opinion, the hottest part of the stove would be any part that is actually touching the burning fuel and receiving heat through conduction. Think of a forge used by a blacksmith. The iron is not put over the fire, but directly in the burning fuel itself. You are adding fire brick to protect your stove from NORMAL burning. As such, you should install the firebrick as high as you anticipate stacking fuel. The wood will never come in direct contact with the top of the stove so no brick required up there. As for the ash and the insulation properties, yes, it does insulate (both by preventing conduction of heat as well as radiation) but you can't have ash as high as you'd have fuel touching metal in your stove, so the firebrick is your protection. What ash does is protect your firebrick from cracking due to intense direct heat. What your firebrick will not do is protect your stove from OVERFIRING. That is what will cause your stove to warp and there's not much you can do if you let a fire get that intense.

In your situation, I would consider making my own fire bricks using refractory cement. Make a template with wood or cardboard. You can custom make the size you need and not be concerned about bricks toppling into the fire. Also, if you don't have an inline flue damper, I would recommend one to help control the airflow and burn.

 

Stuckinthemuck, I like the idea of castable refractory but in this instance, I will be sticking with the bricks. The cold weather is just about upon me in this region and I hesitate to start such a project. The bricks are at hand, free, and just plain easy, lol. Though just floating, I think they are stable enough so as not tumble down into the fire. I do have them installed so that they press each other wherever possible. I did not install the damper in my stove pipe because the damper I received with the stove pipe seemed to me to be very poorly designed. When I installed it, I noticed that the plate is smaller than the pipe by a rather wide margin and thus could not really damp much. The damper was also loose and did not stay in position. I know I could have worked out these kinks but I put that off for a future endeavor. I really do not do long burns, and I do not run the stove overnight. My house is relatively small and excellently insulated. I anticipate doing a burn in the morning and a burn in the evening, each one running about three hours in total. If I see any issue from lacking a damper, I could easily install one with little muss or fuss. I like to do a one-charge burn, and I prefer to not have damped, slow burns. I will do some burns with the stove filled as I have it now with the bricks and see how it goes. My hope is that the ample lining of the bricks will send some extra heat up the flue pipe so that the upstairs bedroom receives some of it. From experience, I am sure that the stove is going to give off plenty of heat downstairs even full of firebrick. These stoves tend to give off so much heat, and so quickly, that I am reckoning on the thermal battery surround and hearth to help with the comfort level as well as efficiency. I think that with less firebrick, I will end up with too much heat in the stove area. Plus, the thermal mass around the stove can only absorb that heat so fast. Nearly every wood stove that I have seen in operation is just too hot in the room in which it sits, making the room uncomfortably warm while other rooms are uncomfortably cool. My hope is that I will send that 'extra' heat to the surround and hearth (to have for later), as well as to the upstairs via the flue.

Scotty, I think I will just lay a sacrificial steel plate across the tops of the bricks. To my mind, so long as the stove gives enough heat to the room in which it sits, there is no downside to having it fully lined. I burn only hardwood that has been seasoned for at least three years.

LodgedTree, I understand what you mean about the bottom of the stove needing protection. But I am sure that I have seen as many stoves with brick lining the sides as I have seen stoves with just the floor lined. And I have never found any published science that explains the differing arrangements. Anyway, it is easy to remove or add firebricks and I suppose within a short time I will find what works here. What is the name of the acid in the ash that degrades the floor?

 

You might try going to a pawn shop. Purchase a cheap, used skil saw.
Get stone cutting blade for it and cut the bricks to fit.

Another thought, maybe a rental stone cutting saw.

Another thought, if you're patient, a chisel and hammer.

I would stand the side/back brick vertical.
Then cut the bottom ones to fit tight against the side bricks.
It will tend to hold them in place.
I would think one course of vertical side/back bricks would be all you need.

 

Interesting comment I just found:

Wood ash is definitely very alkaline. The alkalinity comes mainly from the potassium carbonate formed during burning. As noted by others, there is also a significant amount of calcium carbonate present in wood ash as well. However calcium carbonate is far less alkaline than potassium carbonate.

An interesting piece of trivia is that wood ash is the reason the element potassium got its name.

Before the advent of electricity, wood fires were used to heat cooking pots, the ash left was called pot ash or potash. This potash was used to clean pots and cooking utensils because it removed the grease far better than plain water. (the alkaline nature of the pot ash was actually producing a crude soap, in situ, by reacting with the grease and fat) Eventually potash was widely used commercially in the manufacture of soap from animal fats.

In time, early chemists isolated a new element contained in the potash and called it potassium.


Potash
​

 

Oh for sure, my Great Grandfather many moons ago (1830's) had a Potash Factory here in Maine. It was known then as Potash Fever, the settlers burning the wood to clear land for farming, taking the ash and sending it to England who had depleted soils. It still is called poor mans fertilizer today. In fact I still apply wood ash to my farm fields from the massive Wood Boilers that convert hog fuel to electricity throughout this state.

Wood ash has a fertility rating in NPK terms of 0-1-3 meaning it has zero nitrogen, 1 percent phosphorous, and 3 percent potash (potassium). It does not matter the volume, these numbers never change because they are percentages. Sadly it takes a lot of ash to make fertilizer. I have Eastern Hemlock that has no commercial value at this time, but to get a 15 acre field up to sniff that calls for 80-40-210 on the NPK recommendations, I would need 156 cord of wood burned and the ash spread over that 15 acre field! Even then I would need to buy Urea at 46-0-0 to get the nitrogen where it needs to be.

It is however a lime equivalent to, as noted from the calcium carbonate which is what quarried lime is. One trick a wood burner can use, or someone who plays in concrete; when the stuff gets on your hands and dries it out, do not wash with water, wash with vinegar and leave it on as long as you can stand the smell. Vinegar is an acid base so it neutralizes the alkaloid content of wood ash (or cement) which is just ground up limestone.

 

As already mentioned, typically older stoves had the brick about halfway up, or as far as the fuel was to be stacked...modern stoves often have brick clear up to the secondary combustion baffle plate.
As far as that damper not fitting correctly...sounds like it is correct to me. They are not made to block the pipe off completely...just about 75-80%...they don't want you to knock the draft down too far and then the flue cools, the draft drops, and then you get CO2 poisoning because of low draft. I've seen 'em two ways, either they fit loose, or have some good sized holes in the damper plate. I would think you will want a damper in place for that stove.

 

Brenndatomu, thank you for that info. I was not aware of that. I was already considering fabricating a damper that would completely close off the flue.

 

The only reason to do something like that would be to install it when you are not using the stove at all and want to prevent convective heat loss up the chimney.

 

Yes. That is why I noticed that the one I received with the stove pipe was small. I tend to not burn damped fires. And I like to close the flue when the stove is not fired, the house is warm, and it is freezing outside.

 

I use a damper similar to the one you received. I wouldn't necessarily call it a damped fire. The damper just gives me more control. Instead of reducing primary air way down to control the fire, I close the damper a little and the primary air a little. I find that much less heat goes up the chimney. To prevent heat loss when the stove is not in use, I shut down the primary air completely and close the damper... the only way to totally cut off the heat loss is to stuff something in the chimney pipe or to install a damper similar to what you were talking about. Every house and installation is different. You will find what works best for you and roll with it. By our second year burning our current stove, we had it dialed in...

 

Yeah, yeah, that's it!