Rocket Mass Heater

It's that time of year again... Hope you have plenty of wood split for the winter, I know I do. At least some of you must burn wood for heat in the winter. If so, good for you! Heating with wood is a lot of work and mess, but it's much cheaper than your other options, namely gas, oil, or electric heat.

It does seem to take a tremendous amount of wood. Unfortunately a lot of that wood goes right up the chimney, as heat and smoke. Extra wood to burn means more cost, extra splitting, extra storage, and possibly running out. What if there were a small cheap stove that got better "mileage" and was DIY friendly?



Let me introduce you to a true econobox, the Geo Metro of the stove world: the Rocket Mass Heater. It's dirt-cheap and super efficient.

A Rocket Mass Heater (RMH) uses 80% less wood than a standard stove - AND - you only need to fire it once or twice a day. The large heatsink stores the heat from an small intense fire in the morning, and radiates this heat all day. The exhaust is only about 100 deg F. No smoke, just steam and CO2.

Here's what you're seeing in the picture: The wood and air go in through that little "bucket" in the foreground. The heat radiates from the 55 gallon drum, enough to heat a kettle of water on the top. And the large masonry bench is warmed by the exhaust, capturing all the energy before it goes outside. Comfy bench, hot barrel, wood in a hole. Got it.

The wood actually sticks out of the hole, but only the inserted end is burning. Flames from the end are sucked sideways into the heat riser. The heat riser is an insulated pipe where all the smoke burns at high temperature - this is the "rocket" part. The extreme heat creates a powerful draft which drives the system. This flows into a barrel which surrounds the heat riser, and back down into the base. The barrel serves to radiate some of the heat, and cools the gases, improving the draft. The exhaust gas enters a large thermal mass which stores all the heat remaining in the gas, and slowly releases it to the room.


Video tour of several installations:

One being built at a workshop:

The RMH is by far the cheapest stove you can build. You will need a 55 gallon drum, stovepipe, perlite, some firebrick and plenty of "cob" (clay + sand + straw). This is a triumph of DIY design over mass-production. Anyone can build one of these, operate it and get "extreme MPGs", so to speak. Read more about these at Permies.com, get the book at Rocketstoves.com, or just watch the videos on Youtube.

I plan to build one, it looks fun and very easy. Stay tuned...

The moment of truth

At long last I have gotten all the pieces of the gasifier in place. I haven't gone through the leakproofing yet, but everything is hooked up. Here's the plumbing under the hood:

The air cleaner still fits just fine:

The final filter for the woodgas, and the bilge blower:

The plumbing from under the truck:

Here you see the same pipe, going back to the hay filter. Further under there you can see the pipe from the condensate tank coming forward to the hay filter.

The pipe from the condensate tank, ready to go into the hay filter.

Again my oven-heating PVC trick, made a nice flange for this pipe. Metal window screen, this needs filling with hay.

Some hay for the filter. Used about a quarter of this.

All hooked up.

And I added a jar to catch soot and condensate from the cyclone.

So I decided to flare some gas, and see what happened... It flared beautifully, although I could tell there was air getting past the leaks. Still, I went ahead and flared it under the hood - perfect flame, pointed right at the wires on the firewall...OK, enough of that! So the next step....I tried starting it: And it started. It ran a few seconds, and quit. I have to get the air leaks fixed, and advance the timing. But it ran, I heard it! It lives!!

You can hear it too - I have a video.  It's a little dark, shot in the evening, but you can hear it run (starts at 3:35). Enjoy!

Plumbing the gasifier - Part 2

It's been a while since I posted. I haven't had as much time to put in as usual; but I still manage.

I have run 3" PVC pipe from the condensate tank forward to the vicinity of the hay filter; this part isn't finished yet.  I went ahead and started work under the hood, and I am facing several challenges. I would like to run as much 3" pipe as possible throughout the system, to avoid restrictions which will reduce performance later. Unfortunately, 3" PVC is big, and fittings are expensive and clunky. So I had to transition to 2" for the last bit.

I wanted to get a check valve for the front blower, to keep air from backing into the woodgas line and causing problems. I found a 2" check valve at Lowes; but the internal spring was too much for my bilge blower. I decided to make my own:

I got the idea from a friend who made the same thing, and this is the critical part: you need a disc of flat PVC, but where do you get one? You have to make one. Turn your oven to 250 degrees, and cut the PVC pipe to a sufficient length. Slit one side. Place it on the rack (the tinfoil was unneeded) and close the door.

Keep an eyey on things, when it begins to droop, wait another minute and pull it out. For me this happened just as the oven got to temperature. I tried stretching it against the rack; this wasn't necessary, but you can see how flexible it is:

Immediately flatten it with a board; it will take whatever shape you press right then (I later made one to fit the side of a paint can). Let cool and it hardens up again. Voila! Flat PVC. No nasty smells, either.

It was a good idea - unfortunately, I found that even without a flapper in place, the PVC screen was too restrictive for my wimpy blower, and cut the airflow drastically. So I just am using a pipe cap for now, in lieu of a proper check valve.

I have to have a final filter for the gas, before it goes into the engine. This can be paper, or better - foam. I took a sheet and blew through it - it works fine. So how to get that surface area into a compact space? This is what I came up with:

A pipe in the middle of this paint can is drilled full of holes, and wrapped with foam. The gas flows into the pipe, through the foam and into the can (and on to the engine). Here's the pipe - note the flat PVC top:

And the foam:

Sealed with duct tape (low temperatures here):

Now I need the outlet pipe. I made another "oven piece" and wrapped it around the can. Then I cut a piece of pipe to fit the profile. This is tricky! I was going for a tangential entry, to help swirl the gas flow and spread out the suction. Here it is glued up:

It fits so:

That's all for today. Like I have said, it's a bunch little things that aren't very impressive, but put together it's a lot of learning for me. Baby steps...I really want to get this done, so please be patient - it will happen!