Ask Uncle Mud: Is a Basement Rocket Mass Heater Advisable?

MDK asks: Is a basement install for a Rocket Mass Heater advisable?

Uncle Mud says Yes and No. A basement install is great for ease of installing mass, and allows you to warm up the underside of the floor of the house which is very comfortable. A warm basement is also a lot more fun to use as living space. Bonus points if your basement has easy access to outside and a place out of the way to store firewood. It is nice to keep the firewood and ash mess out of the main area while limiting the number of cold wet trips to the wood pile out back.

On the down side rocket heaters require feeding every 20 minutes or so and if you aren't in the basement already for another reason you could find running up and down the stairs quite inconvenient. You might even keep forgetting to feed the fire with it out of sight and have to restart it over and over.

The Liberator rocket heater has the benefit of a non electric pellet feeder that will run without babysitting for 8 to 20 hours, but running a rocket with scrap wood requires almost constant attention. The Liberator also produces only about 30k BTU per hour, which is on the small side.

The above picture is a much larger RMH we built in a basement here in NE Ohio. The client's large farmhouse with walkout basement is an ideal candidate for a basement rocket. The wood stove on the main story tended to overheat the second story while leaving most of the main floor cold and uncomfortable. The large basement was almost never used except to store junk.

With a rocket heater in the basement it became the new hangout space. The warm bench always seems to attract people who are willing to sit around and feed the fire. The workshop in the basement also seems to get more use now that it is warmer down there.

The person paying the fuel bills notes that the RMH uses very little fuel compared to the upstairs woodstove and does a better job of warming the place. The gas furnace almost never comes on now. The hardwood flooring scraps that fuel the rocket mass heater are also about $30 per cord compared to about $300 per cord for the white oak burned in the other woodstove. The favorite benefit though is that the floor of the main story is warm and comfortable without overheating the second story.

Great response Mud.
Another possible problem with a basement RMH install is proper achieving reliable draft. Because RMHs are known for their efficient low exhaust temps, the draft in the chimney may compete with the house itself as a chimney. Each situation of course is different but something to keep in mind. A good bypass could help with this especially during the shoulder seasons.

Gerry,
In this situation the 40' tall insulated chimney draws so hard even in summer that its suction can hold a piece of plywood against the open bottom of the chimney pipe. I wouldn't install any wood burner even a RMH without a proper insulated chimney or stainless steel liner that runs out at least 2' higher than the house ridge.

Many years ago, I read Hugh Howard's House-Dreams https://www.kirkusreviews.com/book-reviews/hugh-howard/house-dreams/ where he built a house, mostly himself and by hand. He included a Rumford fireplace and grubka masonry mass heater in the basement to heat the house. The explanation of how it worked fascinated me, particularly since it was meant to be fired once, burn for a couple of hours, and then left with the damper closed, circulating and radiating heat for the rest of the day.

I am wondering if this is the RMH but on a massive and more complex scale. It was a multi-ton masonry mass with flues built into the basement of the house, with a central chimney that carried heat into the rest of the building. He details the build in the chapter Of Hearth and Home. It may be something that has to be in the original construction plans of a house and can't be retrofitted or added to a basement later, but it's an interesting read on the concept.

It sounds like a type of "masonry heater" which have been popular in northern Europe for centuries. "Grubka" might be something like a traditional Polish version of it. Rocket mass heaters are a subset of masonry heaters with an extremely efficient firebox and commonly low-tech/low-cost materials for much of the build.


Grubka appears to be a Polish name, but has also been described as a Russian name for a masonry heater.
https://www.adobebuilder.com/grubka-european-masonry-stove.html

Yes, in the book it is described as a Russian masonry heater. The "technology" sounds very similar to the RMH but way bulked up.

Traditional masonry heaters (there are many different flavors) tend to employ circuitous channels (often going up and down) built into solid masonry, and have to be meticulously planned by an experienced mason. Ducted RMHs take the complication out of the planning, simply requiring attention to accounting for bends in figuring the length of the ducting. Bell style masses further simplify the planning, and are used in some masonry heaters as well as RMHs.