New Barrel for Rocket Mass Heater/Forge

I got a new barrel for my Rocket Mass Heater/Forge (RMH/F) from my neighbor. The new barrel is over 10 times bigger then the old one. This should give me a performance improvement.

The current barrel is too small to work, really. There is no room for the insulated heat riser they need to work. I folded up a metal sign to use for my heat riser. I have to cheat it by putting a pair of inline-duct blowers at the far end of the heater. This forces the draft to work in the direction I want it. Works well for the forge, it sucks all the smoke out of the building, not so good for a hot fire, I have to keep the fire a bit held back or I overheat the blowers.

There is a bit of math that is required to build one of these. Not too much though. Area of a circle is pi time the square of the duct diameter, all divided by 4.

I am using 6″ ductwork as that’s what I got for free on freecycle. 8″ would probably work better – that extra 2 inches gives me twice the air volume, but, I don’t have it. This gives me a cross sectional area of 28.2 inches. I want to keep this 28 square inches minimum throughout the length of the system. Especially critical are the areas around the burn tunnel. You can get bigger than this in places, such as with the burn barrel, but everything up to the burn barrel had better be pretty close to 28 inches or this thing will smoke.

The space between the top of the heat riser and the burn barrel is one of the few spaces where you can ‘tweak’. I guess going smaller will actually make the heater sound more rockety. I am going to try to maintain my 28 square inches. The circumference of my 6″ burn tunnel is 18.8 inches. So I need to gap the top of the heat riser from the inside of the barrel an inch and a half.

This is primarily a forge, so that means I need to forgo the burn tunnel. However, I can lay a dozen bricks or so and make a quick, loose burn tunnel for the times I am wanting to heat the shop to do woodworking, or some other project.

I want to be able to ‘peel back’ a few of the ‘layers’ so to speak, so I can show people how these things work. The barrel I got is conducive to such abuse. It has a lid with a bolt-tightened band to hold it on. I will make it so I can take the outer barrel off so I can show people the insulated heat riser inside.

So, All I have to do is cut two 6 inch holes in the barrel lid, balance the lid upside dine precariously on some uneven brick, and then somehow sling the barrel up over the top of the heat riser which tops out near 6 foot off the floor.

I have a hunch I won’t be taking the barrel off for show and tell all that often..

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