thomas rubino wrote:.....7.5 "x 7.5" square with a 5 gal pail as a feed tube extension... end of season the feed tube was the size of a 5 gal pail. !!! I had to keep the feed tube partly covered to draft. In our greenhouse during the winter the fire stays going all day, many days my wife is in charge .... seems that if you force wood into a feed tube made of a soft substance it crumbles... therefore making more room for wood. My next core I used soft insulated firebrick ,thinking that they would hold up better than perlite... nope, seems that if you force wood into a soft firebrick you can make it wear also. My next core was my first with hard firebrick , but I did not bring the retaining walls up past the top of the feed tube... seems that you can break the bond of fireclay if you force wood into a 7.5" X 7.5" square hole without proper support ... my current core is hard firebrick and the clay brick surround is cemented and is above the level of the feed tube... winter is almost over and guess what seems that you can't force wood into hard firebrick feed tube that is surrounded by concreted brick ! Bottom line , the cast core was by far the Superior material to build with but... it is fragile ! In an insulated home where you are sitting near your rmh and your fire does not need to stay going 12 + hours a day it would work perfectly. If you search my posts you will find pictures of all rebuilds.
you said you used a 5 gal pal but then at the end of the season the feed tube was the size of a 5 gal pail? or did you mean the burn tunnel ended up as big as the feed chamber?
this whole cast core fragile thing was one of my concerns and the reason i went with hard fire brick with a thick steel propane tank. the propane tank can take the beating but is only holding the wood and at times may burn up in side of it, the air flow is positioned at the bottom where the refractory cement was poured and the hole to the burn tunnel entrance even if it did deteriorate the steel it can only get as big as the hard fire brick burn tunnel. i have shown since the air tight wood magazine does not have air flow down through it and only through the bottom front air port the propane tank stays intact. believe me... i beat the snot out of my Rocket Mag Stove feed magazine. i use other logs to pound pieces down and jab it with the poker and slam the lid down and you would never be able to do that with a hard firebrick feed tube let alone a fireclay feed tube so propane tank with air tight lid is a must.
Also, splitting fuel to very small sizes is a persistent notion that is not actually necessary. Small wood burns faster and hotter, but Ernie Wisner recommends wood of a size that only two or three can fit in the feed tube. For a standard house-sized 8" system, 3" to 5" pieces can work. There is no particular reason to cut big trees for firewood if you have a choice. Thinning smaller trees in a woodlot gives convenient-sized fuel with minimal splitting and lets you use all of the wood, some as kindling and some as long-burn fuel.