Temperature Drop Through Fire Brick

4.5" system size.

I measured about 500℉ on the outside surface of a fire brick right beside the raked coals in the throat of the fire chamber. Another thermometer about 3" inches away measured 130℉.

The first two courses of the fire brick were buried in about 6" of sand. These are full size fire brick, 2.25 x 4.5 x 9" and rated to 2500℉.

There was no smoke during the burn, so I'm assuming it burned in excess of 1200℉.

Does a 700℉ + degree drop through the 2.25" thickness of the fire brick seem about right?

If I were to double wall the core, would you anticipate a similar drop through the second fire brick? Would you lay these face-to-face, or space them apart with cardboard or something?

My experience with half-bricks (1.5 x 4.5 x 9) give temperature differentials similar to the ones you've seen. It will also depend on much firewood you are burning:

You can research Fourier's law:

Q = k * A * (T2- T1)
where Q is the amount of heat (firewood being burned),
k is the thermal conductivity of the firebrick
A is the area of heat dissipation (area of the firebrick)
T2 is the outside brick temperature
T1 is the temperature of the fire (1200 F)

[edit] added wording to T2 definition for clarity.

Thank you. I had forgotten that equation, if I had run across it before.

I've just about finished re-building my dry stack test bricks. I've made the fire box out of fire brick slips this time ("half" size) roughly 1.25" thick, all the way along the bottom (first course) and sides (second course). Then along one side I immediately stacked a full size fire brick right along side the slips (but not the bottom). This will allow me to slide a thermometer along each side and compare the temps, one just the slip width (about 1.25") and the other slip + full brick (roughly 1.25 + 2.25 = 3.50" total wall thickness). I hope to get that test burn done tomorrow.

I've been too busy at work to check some diagrams, but it seems to me that the double walled fire brick at set up tight against one another. The card board, fiberglass, rockwool, etc, used to provide expansion gaps is placed between differing materials, not materials with the same expansion (as I recall).

A gap in this situation would be less for thermal expansion clearance and more for insulation. Conduction of heat from brick to brick will be interrupted by a space where the heat has to radiate; even a small space should help.

Erik Weaver wrote:Thank you. I had forgotten that equation, if I had run across it before.

I've just about finished re-building my dry stack test bricks. I've made the fire box out of fire brick slips this time ("half" size) roughly 1.25" thick, all the way along the bottom (first course) and sides (second course). Then along one side I immediately stacked a full size fire brick right along side the slips (but not the bottom). This will allow me to slide a thermometer along each side and compare the temps, one just the slip width (about 1.25") and the other slip + full brick (roughly 1.25 + 2.25 = 3.50" total wall thickness). I hope to get that test burn done tomorrow.

The half size brick (1.25x4.5x9.0 inches) are commonly referred to as "splits" or split brick by the refractory suppliers, at least those here in the SE USA. Slip is slang for a slurry made of fire clay. Just so that you're on the same page as the majority of experienced RMH and masonry stove builders;->

Erik Weaver wrote:I've been too busy at work to check some diagrams, but it seems to me that the double walled fire brick at set up tight against one another. The card board, fiberglass, rockwool, etc, used to provide expansion gaps is placed between differing materials, not materials with the same expansion (as I recall).

Those expansion joints are common practice in the construction of double skinned masonry heaters. The core will develop cracks during its service life. The expansion joint prevents those fractures from making their way into the heaters "skin" and compromising its otherwise airtight gas seal, as I understand it.

HTH, Byron

Thank you Byron, I appreciate the correction to terminology. I too believe that is useful to keep to the terms in general use. Here in SW MO I believe they also use the terms split for the half fire brick. I just messed up my terms the other day.

I'll see if I can get slip in my mind right too. Now that you mention it, I do recall hearing that term. I don't know if I recalled it was specifically a wet mix of fire clay as I heard it used or not; it is too dim of a memory for me, as I sip on my first cup of coffee this morning.

When it warms up in a few hours I am planning to get this next dry-stack tested. I'll be interested to measure the different sides. I'm also interested in the temp under the fire box, as this is a little larger than the previous test build (both 4.5" tall burn chamber, but having increased the horizontal interior measurement from 4.5" to approx. 6.25 inches (CSA= 28.125)).

"Slip" is a universal (at least in America) potter's term for a liquid mix of clay and water.

OK, here's my update from my 5th burn test. I took the following temperature measurements at four locations:

1. Far Left-side of the Burn Chamber, with the thermometer probe touching the outside
of the fire brick 3.50 inches away from the coals (the combined width of one-split and one-full
fire brick).

2. Left-side of the Burn Chamber, 3-inches farther to the left of the fire brick, with the
probe sticking into the sand (to measure the heat conduction through the sand).

3. Right-side of the Burn Chamber, with the thermometer probe touching the outside of
the fire brick 1.25 inches away from the coals (the width of one-split fire brick).

4. Far Right-side of the Burn Chamber, 3-inches farther to the right of the fire brick,
with the probe sticking into the sand (to measure the heat conduction through the sand).

Hopefully the chart image will load. If it does, there are a couple other temperature measurements: top of barrel (immediately above fire riser) and the bottom of the stack, where the double-elbow forms a U-shaped connection with the bottom of the barrel to the exhaust 5-foot stack (which was required before it drafted properly).

Basically, the results are what I think one would expect. The energy source is obviously the same, so the thinner the fire brick the more rapidly the heat conducts through the material and radiates/conducts from the face; and the thinner the fire brick, the hotter it is (I fully expect there is a direct relationship to mass of the brick).

Thus, with regard to the measurements I took of fire brick forming the sides of the burn chamber, 1.25 inches splits get very hot, pretty quickly; 2.25 inches full fire brick will also get pretty hot, but it takes longer, and may even extend into the coal burning stage before reaching their maximum temperature. Whether the fire brick's maximum temperature occurs during the flaming burn or the coal burning stage, appears to depend where they are located - nearer the flames, or nearer the coals. And where I had the double-walled fire brick (one split and one full, for a combined thickness of 3.5) inches the temperature reached its maximum much later, and at a much lower maximum temp.

My initial thoughts are:

1. 2.25 inches thick fire brick ought to be sufficient for most builds (I'm using a fire brick rated at 2,500 degrees F). Of course, this is not a surprise; most builds appear to use full size fire brick.

2. 1.25 inches thick fire brick may be used if lower weight is important, but I would want to add sufficient insulation to mitigate the faster conduction of heat; I suspect providing air/expansion gaps around the splits would be wise. And providing a means of allowing the heat to escape would expect to be quite beneficial.

3. 3.5 inches thick fire brick seemed to significantly delay the conduction of heat to the face of the fire brick. Temperatures were also much lower on the face of the brick. I suspect this is because the majority of the heat was flowing out of the system, as opposed to saturating the brick.

4. Heat rises It is not shown in this burn test, but my first burn test (in a 4.5 inch system, as opposed to this 6-inch system) showed moderate temperatures just 1/2-inch below the floor of the fire chamber (on the order of 180*F, as measured by a thermometer probed stuck into an inch of sand upon which the fore box was built).

Another interesting observation is that the temperatures of the fire brick could approach the temperature measured at the top of the barrel, immediately above the fire riser. Now, I'm only using a $10 magnetic "flue" thermometer to measure the temperature at the top of the barrel, so this observation may be in error. But right now, it appears that whatever temperature I measure at the top of the barrel, I can anticipate temperatures of the fire brick to not quite get that hot. Another variable, is that I am running these burn tests outdoors, and the ambient air temperatures were in the 25F - 40F range, which I suspect promoted a more rapid cooling of the barrel; indoors the maximum temperature of the barrel may rise higher (how much higher, I do not know; perhaps 100-200 degrees F as a guess?).

So that's where I'm at.

Trying the chart again...