A.M. writes:
I am building a 36'x36' Barndominium in the northern USA. At the back of the first floor garage is a 12'x36' entry for the upstairs apartment containing a bathroom and a staircase and a 9'x12' mechanical room where I want to install a masonry chimney and my Liberator Rocket Heater along with some sort of mass heat storage. I would like to copy your "half barrel" design. Would sand store enough heat? How long should it be?
--from Wisconsin
Dear A.M.
Unless you plan on using a pellet feeder I would not put the Liberator or any other Rocket Heater in a room you aren't spending a lot of time in. Rocket heaters need attention every 20 minutes or so. I know of no one who over the long term continue to run up and down the stairs to tend their rocket. Either we drop something in every time we walk by like I am doing today, or we sit on or near the mass feeding it while we work online like my wife does--even then she forgets about it and lets it go out if she is warm regardless of whether the house is warm. An out-of-the-way Rocket is an unused Rocket.
Speaking of warm houses, Rocket Heaters work twice as well with a mass storage, so we will definitely want to incorporate mass. If you have unlimited free wood and a typical 100,000 BTU woodstove I guess you can afford to let your exhaust leave the building at 500 degrees taking all the heat you just made, but the Liberator maxes out at about 30,000 BTU even if you are constantly feeding it dense kiln dried hardwood (20,000 BTU or so using the pellet feeder). A storage mass keeps the heat in the house and slowly releases it through the night instead. The rocket heater's exhaust is initially much hotter than a typical woodstove (burning all the creosote in the heater not in the chimney) but it leaves the house at less than 200 degrees after a mass bench. I can help you design a mass to fit your space either upstairs or downstairs. Either will have its benefits and drawbacks.
Heat is transmitted most efficiently by conduction so you want your mass and heater in your living space. In a barndominium most of the living space is upstairs. Putting your rocket up there means the most warmth with the least fuel (and you are right there to feed it), but the mass bench will need to be carefully designed to not collapse the floor or catch it on fire. There are a number of ways to solve this problem.
My half barrel designs:
https://www.youtube.com/watch?v=KDlHcRznfbg https://www.youtube.com/watch?v=KDlHcRznfbg
https://www.youtube.com/watch?v=_BniUV3kkBs&t=72shttps://www.youtube.com/watch?v=_BniUV3kkBs&t=72s
my cast cob design
https://www.youtube.com/watch?v=ZVxdMe6-Htwhttps://www.youtube.com/watch?v=ZVxdMe6-Htw" target="_blank" rel="nofollow">my cast cob design
https://www.youtube.com/watch?v=ZVxdMe6-Htw
And pebble mass
https://www.youtube.com/watch?v=wcXdMTpHPb0 https://www.youtube.com/watch?v=wcXdMTpHPb0" target="_blank" rel="nofollow">And pebble mass
https://www.youtube.com/watch?v=wcXdMTpHPb0
If you put the heater in the living space on the concrete ground floor you can use a larger mass for more heat storage it will warm up the upstairs floor nicely(if you keep coming downstair to feed it), but it will use a lot more fuel and never quite heat up the upstairs a well.
Conduction i the most efficient way to transfer heat. Sit on a warm bench or a heated car seat and you can be more comfortable with 1/10th the heat. Radiant heat is less efficient but if your heater is in the same room with you in direct line of site and nothing is in the way you can still get a nice bit of heat walking around getting stuff done. The least effective way to transmit heat is by putting your heater in its own room and circulating air around it to push to the other side of the house.
Any heater in its own room on an outside wall will tend to send more heat out through that wall (even an insulated wall) than into other rooms because the temperature differential between inside and outside is a lot bigger than between one room and another. A forced air furnace or woodstove will also leak heat out through every crack around every door and window because one side of the house will be under positive pressure and the other side under slight negative pressure from the circulating air.
If you are going to use the Liberator or any Rocket Heater in the downstairs (living quarters entry) of a barndominium I would not put it in a closed off mechanicals room. I would have that area all open space to warm as much of the upstairs floor as possible and let the open staircase and some downstairs ceiling vents help circulate warm air through the upstairs.
A mass can be any of a number of different shapes. I've built mass benches big and small depending on the size and thermal needs of the space. If your mass is too big the room will never quite warm up. If your mass is too small it will not keep the room warm over night. Mass columns (they are great for heating workshops) and mass walls take up less floor space than mass benches. Mass benches are nice because they keep the warmth low in the room. If your feet are warm the rest of your body needs less heat to feel warm.
In the space you describe a comfortable bench up to 12 feet long and a desk or "inside clean workbench" should compete for space because whatever you build must encourage you to spend time there so you will feed the fire. Sand is an okay mass. Pebbles are a little better if you leave room for air circulation in your pebble box. Cob and dense rock is a better ma, and you can sculpt cob to look how you want it. Cob is harder to get rid of if you change your mind.
One place you do not want mass is in your chimney. The cold mass of a masonry chimney slows down the draw which starves the heater of oxygen which makes it burn inefficiently which leaves creosote which leads to chimney fires. A stainless steel insulated chimney is necessary for a clean burn especially with a rocket heater. An insulated stainless steel chimney will also protect the house from a chimney fire better than a masonry chimney will. It is also much easier to replace after said chimney fire. It's Russian Roulette to keep using any chimney after it has had a creosote fire in it.
Lastly INSULATE INSULATE INSULATE. A well insulated building will stay super comfy with very little fuel. A small Rocket Heater will not be able to keep up with heating a large or poorly insulated building. At Wheaton Labs we ended up replacing the Liberator with a larger Rocket Heater.
https://www.youtube.com/watch?v=jpD-yKw2mNM&t=7s&pp=ygUZdW5jbGUgbXVkIHdvcmtzaG9wIHJvY2tldA%3D%3Dhttps://www.youtube.com/watch?v=jpD-yKw2mNM&t=7s&pp=ygUZdW5jbGUgbXVkIHdvcmtzaG9wIHJvY2tldA%3D%3D
My Liberator does very nicely heating my poorly insulated 1200 square foot doublewide trailer and 600 square foot sunroom in Cleveland down to about 10 degrees (f) then it struggles to keep up unless I close off the sunroom or light the sunroom Rocket. I do love that I can heat my house for less than $100 per year. The propane bill was almost $1000 some month before I switched. Now I use 1-2 banana boxes of hardwood flooring scraps per day. About 2.5 cords worth per year.
You can find the plans for the pebble mass bench stuff like it here
https://freeheat.info/free-heat-plans/?f=51https://freeheat.info/free-heat-plans/?f=51
If enough people bug me about it I will get plans together for the half barrel and cast cob designs
–Mud
January 2025
info@unclemud.com
