We recently posted an article detailing the concept behind rocket heaters and how you could build one yourself. There’s plenty of info about that around the net. But how about a detailed review of an actual rocket heater in action? I’ve gotten several requests for that. Here you go.

First, let’s clarify some terminology.

Q: What’s the difference between a rocket mass heater, and a rocket stove?

A: A rocket mass heater IS a rocket stove, with the addition of stone around the metal surfaces to store heat for slower release. If your setup is wrapped in stone mass, then it is a rocket mass heater. If it is bare metal, then it is a rocket stove.

Does this difference matter? Do you really need the stone mass?

Think of it like a solar power setup with or without an additional battery bank. Adding stone mass to your rocket stove is like adding a battery bank to your solar panels. The technical term of interest in thermodynamics is “specific heat capacity.”

Basically, you’re not going to get heat out of a material if heat wasn’t put into it in the first place. Stone and metal can both absorb a lot of heat, but metal does it very quickly while stone does it slowly. Stone takes a while to warm up, but also takes a while to cool down. Metal will warm up fast, but also cool down fast as soon as the heat source is removed. But your choice of material WILL NOT release any more BTU’s then you put into it in the first place. This is the physics law of conservation of energy.

In the solar analogy, without a battery bank you get the power the panels are producing, but only while they are producing. Add a battery bank in between and it will take some time to charge the battery bank as you divert power from direct use to storage in the batteries, but the battery bank will keep emitting that stored power even after the sun goes down. Likewise, stone mass will take longer to heat, but it will store a lot of heat and continue to emit it long after the flame has gone out.

With that cleared up, I’m offering a review of a rocket STOVE. You can add stone mass after the fact, if you like.

The Liberator

The biggest problem with do-it-yourself rocket stoves is that they often don’t look pretty. If it’s heating an outbuilding, who cares? But you or the spouse might not want a 55 gallon steel drum surrounded by bricks sitting in your living room when company comes over.

Further, do you have the welding skills to build one yourself?

Also, your home insurance company might also have some objections to your choice of heat, depending on the underwriter and your specific setup.

An excellent solution is the Liberator Rocket Heater, made by Liberator LLC, a American small family business the likes of which many of us might aspire to.

Pros:

  • Looks as good as any other heating stove, minus the view of flickering flames.
  • Can retrofit into almost any chimney or fireplace arrangement.
  • Has UL-1482 fire safety standard approval.
  • Can add an optional air intake and pellet hopper to auto-feed the stove for hours.

Check out the manufacturer website for photos of various installation options.

My Setup

Here’s what you can see in the setup. The drum is in the center, with the fuel chute on the left. That horizontal line near the top of the fuel chute is the optional air intake accessory, and sitting on top of that is the pellet hopper since I was using pellets. Double-wall stove pipe exits on the right. There is a surface temperature thermometer sitting on top of the drum, and a chimney thermometer visible near the top of the stove pipe in this picture. The chimney thermometer pierces both walls and takes readings from the center of the stove pipe. Also on top of the drum I have a specialty steamer teapot, which holds water to raise humidity in the room but without a standard teapot whistle. Also, a fan with a motor powered by heat which spins when the drum gets hot and pushes more heat out into the room. A brick underneath the stove-pipe supports the weight of a very tall chimney stack. To the left of the stove is the single poker which comes with the liberator, designed for dragging ash out of the burn chamber.

15 feet of double-wall stove pipe below the sliding segment hanging from the triple-wall segment penetrating the roof.

With that much pipe above, a brick carries the weight of the pipe stack.

Drum thermometer at room temperature. Not highly accurate, as a digital sensor in the room read much lower. But consider that it’s meant for high heat and is only calibrated to the nearest 50 degrees F.

Chimney thermometer at room temperature. This would be my first time using it.

The air intake along with a plate which can cut off the supply of pellets, to extinguish the fire before the whole hopper is used up. I could connect a pipe from here to the house exterior to further improve efficiency.

A 40 pound bag of wood pellets.

The pellet hopper, empty. Note the square surface at the bottom and the round hole. The round pellet pipe is much smaller and inserted inside the larger, square fuel chute. When burning material other than pellets, the pipe is removed and the size for fuel is the square column seen in other shots.

40 pounds of pellets in the hopper comes up to here.

 

Lighting It Up

The burn chamber, with a removable plate on the side for ignition and cleanout.

Inside the burn chamber. That screen catches pellets but allows them to feed via gravity as they burn up. If not using pellets, the screen assembly is removable.

I rip up about half a square foot of cardboard for easy lighting. The first piece is shoved here so that airflow and flames will favor pathways through the pellets as it lights up.

The rest of that half square foot of cardboard. Far less than I was using in a normal stove.

The manufacturer videos show them igniting with a blow torch. I’m using an ordinary lighter. The air begins rushing in immediately. Notice how far it’s pulling the flame.

Within seconds, the cardboard is blazing and the cover plate can be put back in place. Notice again that the flames aren’t rising, but are being sucked horizontally into the stove by the amount of air rushing past.

Results

The drum thermometer comes up to this temperature and stays there. No adjustments required anywhere on the stove.

The chimney thermometer. It was the first time and I wasn’t sure I trusted it, since the surface of the pipe felt hotter. I moved over the drum thermometer and it confirmed the reading. If it’s 200 on the surface of the double wall, then it could be about 200 inside as well. I don’t agree with the markings as most chimney sites recommend a 350F exhaust, which would be the border between yellow and orange on this device. At least I know I’m losing minimal heat out the chimney.

The Cleanup

Scraping ash out of the combustion chamber using the provided fire tool.

Amount of ash left from a 40 pound bag of pellets, using this stove.

Biggest negative: the flat portion at the bottom (see empty hopper photo above) allows some pellets to pile on the sides and not fall into the pipe. There’s enough there to fill the pipe to the top, which is about two hours of burning. If I’m awake, I occasionally push this bit into the pipe while the fire is still burning, since the hopper is quite cool to the touch.

Conclusion

I’ve not tested the normal fuel burning yet (cord wood, brush, twigs, etc.) since all my cordwood was cut to a larger size and I didn’t feel like splitting it again small enough to fit the rocket stove. But I love the pellet attachment because it auto-feeds for many hours. Sure, I won’t be able to make pellets in a true grid-down scenario, but pellets are something to stockpile just like propane. It will carry you through short grid down situations, but after your stockpile is gone this stove can switch over to normal wood.

The normal stove holds a lot more fuel in the belly, and can get a lot hotter depending how I adjust the airflows. That means I can bring a cold room up to cozy temperature in about an hour flat. The rocket stove with pellets on the other hand only adds about a degree or two an hour, so it requires more planning ahead. The huge advantage is that I can light it and forget about it. No fiddling with dampers, no adding more and more logs. I just have to push the pellets piled on the edges of the hopper into the center as it gets low.

The manufacturer says the hopper allows it to burn for about 10 hours. I’m getting more like 16.

Temperature and performance might vary depending on your specific situation. My first light-off was outdoors in case the paint curing made a terrible smell. Outdoors the drum only got up to about 250F. I’m not sure if that was because of outdoor temperature, or more external air movement, or less internal air resistance since it wasn’t coming in through a house and out through a chimney.

Negatives: only two so far. I showed a picture of the pellet pile-up. Surely the design could be improved to add some more sloped surface and not give the pellets a place to cling to. Maybe I can make an insert to correct that. Second, if you look back at my first shot of light-up, showing the pellets in the wire mesh, you can see that the mesh is damaged and pellets are dropping through a little too easy. Seems like the grade of wire isn’t up to the heat it typically sees and I’ll need a replacement within a season. Other than that, there’s a lot to love.

Eventually, I’ll be putting a wood cook stove in this spot and moving the rocket stove to the basement where I currently have an oil heater. Even still, I love adding the option of pellets as a heat source, the no-maintenance required during 16 hours of heat when using pellets, and the flexibility to switch back to wood as needed.

The unit weighs about 200 pounds, far less than a normal wood stove. I paid $1500 to purchase, plus $300 for freight costs. I got it during a sale in which the air intake and pellet hopper accessories were added in for free. $1800 total is extremely affordable compared to many other wood stoves out there, so I feel like I definitely got my money’s worth on this.

In all, I recommend this to any readers considering a rocket stove as a heat source upgrade in their home. I didn’t have to learn welding to put this in my home, and when guests come over I don’t have to worry that they’ll expect me to tell personal stories which would have Jeff Foxworthy taking notes.