Core dimension details

[Prescott]

Hello all.

There seems to be some gray area with inner core dimensions. I would like to cast my core with as clean numbers as possible.

The below graphic is what works from what I both think and have learned here and beyond. I just can't see including the feed tube as part of the burntunnel as it full of wood most of the time. Correct me if I'm wrong!

Oh crud, I can't seem to post the photo and have to go to work, maybe my words make sense enough?

FT= feedtube
BT= burntunnel
HR= heat riser

FT= 1/3 HR
BT= 1/2 HR

BT= measuring from width of the FT to the back wall, back edge of the HR (the entire length of the bottom of the core MINUS the width of the FT)

Thanks all, gotta scoot! Keep it rockety~p

[matt walker]

I would say don't get too hung up on the ratios Prescott. In practice, you want both the FT and BT to be as short as possible. That FT 1/3 of the riser is max. It will run better with a shorter feed, but once cob starts getting added they tend to grow upwards. You will notice in my cast core video my feed is 12" high. The 31" riser on top of the 12" riser stub makes a 43" riser, so my FT is less than that 1/3. Once you put bricks around the rim or cob over it, it grows a bit. Keep it short is my opinion.

Same with the burn tunnel. It needs to be long enough that it can seat both the riser w/insulation canister, barrel gap, and barrel. Those things pretty much dictate the length, and is why when scaling down my 8" core to a 6" I don't recommend shortening the tunnel. You will end up with your barrel sitting over your feed if you scale it down too much. There's some wiggle room in there for sure, but those are the dictating factors in my core dimensions.

One thing I highly recommend, if you haven't done it, is mock up your inner mold with some scrap wood, or thick cardboard, and cast a rough one outside with mud. It sounds nuts, but do it once and you'll learn a ton. Should only take and hour or so, and will surprise you how well it can work for a short while. If you've got really good mud, they can last a surprising amount of time.

[Prescott]

Methinks I should be relaxing and having a brew at home! Thanks Matt, so much of my design is being dictated by materials and pre existing space details that this follows suit! I'll take the FT measurements to heart, it sounded deep!

I need to find some good clay mud for a mock up, I am starting to get itchy to build and burn as the days shorten, the fall colors explode around me and nights cool. Haven't had a frost yet, but it's coming!

Do you know of anyone that's incorporated fire bricks into the FT throat? I might not even need it with my higher ratio of castables, but it sounds very practical. I am hoping to cut the half bricks at 45* (with the seams radiating out from the center) so they key together and wrap a 1/4 of Roxul/rock wool around them for a little breathing room for expansion/contraction. I saw the red brick addition around the top of the FT and that seemed a bit rocky after dry out...

Thanks for everything~p

[matt walker]

Prescott, I've said it before, but I am of the mind that regular firebrick in the feed area is a negative in almost every sense except durability. They have a lot of mass, so they pull heat from the burn, and then, once they get hot, they tend to induce burning up high on the sides of the fuel and promote fire creep. I've built some brick models to existing plans, as well as just adding them into the feed of cast cores, and in every case I was bummed that they were there. My neighbor has a firebrick feed, and for sure that feed gets so hot you can barely hold your hand near it, and when you put new fuel in, it explodes into flames as it enters that hot oven. That sounds good, but in my opinion it's the opposite of what you want. That little hot feed is acting like a chimney opposing your desired draft. Mostly it will still go the right direction, but I think it's significant drag on the system, not to mention HOT in the wrong spot.


I'm totally open to the idea that I'm wrong about this, and it sounds like you might be committed to doing it. If so, I'd totally love to hear your results and thoughts once you get it running. Please, correct me if I'm wrong.

Manny, care to chime in? You have bricks in/near the feed. Regular bricks though, right?

[mannytheseacow]

Yeah, I like the way you're thinking, Matt. My bricks are regular... I thought I read somewhere.... maybe in the RMH book.... that regular bricks are more durable than firebrick, though can't handle the high temps from the core..... I dunno, anyway....

Yes, I really like your point about drawing air away. You know, my RMH seems to be working very well now that it's warming up and drying out but I still just feel like it's not that rockety. Also, I still have some smoking issues if I don't choke it back a little. What you're saying about the bricks in the feed is really making sense to me. Let me include a question I've been meaning to ask that might better help me determine my stove performance- what should I be expecting my thermometer on the barrel to be reading?

[matt walker]

Manny, you are running a 6" system, with a 55 gallon barrel if I'm not mistaken. Keep in mind I am running an 8" system, and while I have built quite a few 6" systems at this point, I haven't lived with one. Another note, depending on my fuel, I have a pretty broad range of temperatures I consider "normal". So, my magnetic stove thermometer is about 3"-4" down from the top on the barrel face. I feel like a normal burn it hangs out around 400°F to 500°. Closer to the low number is average. When I'm burning really dry hardwood, I will let it get to around 600°F, much past that and I start getting a bit nervous as it's roaring, and that's about where the top of the barrel starts exceeding 1000°F.

I will say that I'm a big fan of the 8" system. They just seem to be right in that sweet spot. Unfortunately, most modern chimney plumbing is 6", so unless you are installing from scratch, a lot of folks are stuck with 6".

Oh, also there can be over 100°F difference from one side of the barrel to another, so make sure you move your thermometer around a bit when you measure.