Clay vs vermiculite fire bricks

Hello, I am new to this forum.
I am wanting to replace the fire bricks in my wood burning stove. The stove is by Lopi and is probably as old as the house, (36 years). I have used this stove as my main source of heat here in Northern California, since the winter of 2005/6. It is quite likely that the fire bricks are the original and they have served well but are now quite badly cracked. I want to replace them and I'm wondering if I should use clay or vermiculite bricks?  From my reading I gather that vermiculite  would be easier to work with but my question has to do with the heat transfer rate of the two types.

I have read the thread here that discusses whether or not firebricks are necessary and while I know that without bricks, the heat transfer would be largely unimpeded, I agree with the posts that say that they help provide a more even temperature and protect the steel. My current bricks appear to be some type of lighter weight concrete but I'm not sure if they are vermiculite or some other material,  I have read that vermiculite  has insulating properties and I have seen videos of a blow torch being  applied to one side of a vermiculite  brick  which is resting on the palm of a hand and the heat not being transferred, (at least not initially), through to the hand. My specific question is: would clay bricks transfer heat to the steel sides of the firebox more readily than vermiculite bricks?

I ask this because if I use vermiculite bricks, I would hate to find that heat is transferred to the walls of the firebox at a rate  less than the with the current bricks... whatever they are made of.
Thanks!
Kevin

Hi Kevin and welcome. The main functions of refractory materials in wood fires are directing the flow of flue gases, and/or thermal impedance (flywheel effect) to keep heat from going straight out the metal and keep the active combustion zone at a higher temperature. The former is usually in the form of a baffle, such as low density CFB, and it's there because it can take high temperatures that would destroy steel in short order. It will wear out and it's considered a consumable.

The latter function can be either low density boards or high density bricks. Each has their advantages and drawbacks...CFB or vermiculite is nice because it's lightweight and doesn't take up much internal volume, and also doesn't soak up as much heat in the early stages of a fire, where you want to get that core hot in order to get efficiency up. Bricks are good for durability and longevity, and keeping the fire hotter later on in the cycle, but they take up more space in the firebox. Both types are usually part of a stove's design to make the fires easier to manage and cleaner burning, so any changes you make might have an impact on how well it performs.

In terms of the net effect on the heat output of the stove, it's little or none. Refractory slows the flow of radiant energy but it will come out somewhere. That means maybe more out the front, and almost certainly more from the flue.

Hi Kevin;
What size and thickness are the current bricks?
Standard dense clay firebricks are 9" x 4.5" x 2.5" thick, and split bricks at 1.25" thick are readily available as well.
Are your bricks flat with square edges? or are they curved, or have a tapered edge?
Vermiculite or insulated bricks are generally more fragile than dense bricks.
I suspect your stove uses dense clay for longevity.

Kevin McAuliffe wrote:Hello, I am new to this forum.
1)  It is quite likely that the fire bricks are the original and they have served well but are now quite badly cracked. I want to replace them and I'm wondering if I should use clay or vermiculite bricks?  From my reading I gather that vermiculite  would be easier to work with but my question has to do with the heat transfer rate of the two types.

2) I have read the thread here that discusses whether or not firebricks are necessary and while I know that without bricks, the heat transfer would be largely unimpeded, I agree with the posts that say that they help provide a more even temperature and protect the steel. My current bricks appear to be some type of lighter weight concrete but I'm not sure if they are vermiculite or some other material,  I have read that vermiculite  has insulating properties and I have seen videos of a blow torch being  applied to one side of a vermiculite  brick  which is resting on the palm of a hand and the heat not being transferred, (at least not initially), through to the hand. My specific question is: would clay bricks transfer heat to the steel sides of the firebox more readily than vermiculite bricks?

3)  I ask this because if I use vermiculite bricks, I would hate to find that heat is transferred to the walls of the firebox at a rate  less than the with the current bricks... whatever they are made of.
Thanks!
Kevin

Kevin, you have got some good answers, but I would like to break this down a bit further, and numbered your questions for more detail.

1) you say, a bit easier to work with?, in all likely hood when the stove was made it was made with standard size bricks, thus you would replace with standard size bricks, thus not harder or easier to work with.  If your bricks are that old, it would be nearly impossible for them to be of soft type.

2)The bricks are used to protect your steel, not to transfer heat to the steel, if you get your present bricks out and they are very light, then they could be of insulating type, but I would tend to agree with Tom, and say they are most likely standard fire bricks. There is quite a weight difference unless your go with a very high quality insulated fire brick  (IFB) but even these tend to be a fair bit lighter in weight.
2b) unless others feel completely different, nearly all of your heat transfer in your stove, is above or beyond the bricks.

3)  I think this part is mute, as most stoves  followed what is stated in #2,  and the longevity of your present bricks almost for sure indicated Hard Fire Bricks were used.

Best of success.

Thanks Phil, Tom and Scott for your replies!

The existing bricks are 9x4.25x1.25 and they are quite lightweight so I assume that they are vermiculite so with your reassurances regarding heat transfer qualities, I will replace them with the same.

Regarding being easier to work with,,, the vertical bricks on the back and both sides will take the standard size, But the base measures 18x by approx. 16,  so two bricks will need to be trimmed by about an inch on the long side.
Thanks for the help!
Kevin

If the firebricks in your wood stove are of a lighter material, they are likely pumice firebrick.  Pumice firebrick are used by most wood stove manufacturers in North America and come in the same size as the clay splits (4.5"x9"x1.25") .  The pumice firebricks insulate better than clay and thus promote higher firebox temps

Hi Rick,

Welcome to Permies.