The solution for the heating shortage is embarassingly simple. I'm paul wheaton, an expert on rocket mass heaters. Ask me anything.

The numbers you spout sound unbelievable. Is there a thermodynamic proof somewhere supporting what you're saying?

The way "efficiency" is measured for a conventional wood stove is ... interesting. Here is a three minute video i made to try to explain how they are so efficient

https://www.youtube.com/watch?v=A-xK7vZd7nk

If the stove makes "the same amount of smoke as a candle" why do the stoves exhaust outside through a pipe?

Shouldn't you be able to burn them indoors without worrying about getting the exhaust outdoors if it's the same as a candle?

Because of this, some people have actually tried to do this. And I wish to be emphatic - send the exhaust outside. For the same reason that gas burners on a home stove are a bad idea: the flame is consuming oxygen. the same oxygen you need for breathing!

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The problem with burning wood is smoke and the need for so much wood. And people die from chimney fires. A conventional wood stove is a metal box which gives off heat immediately. A rocket mass heater burns the wood in an insulated chamber so it the fire is much hotter, burning all the smoke and creosote. So, in a way, we have domesticated the chimney fire. A more efficient burn, plus not nearly as much heat out the roof.

About 10 times safer than a conventional wood stove, because it has a fire in it about one tenth the time.

If you’re insulating the fire, how does the heat get into the room around it, instead of being exhausted out of the building?

This is my really crappy video trying to show how it works: https://www.youtube.com/watch?v=fwCz8Ris79g

How is this any different than a modern high efficiency firebox? Clean air standards mean they have very low smoke and particle emissions. Or would you consider those to be "rocket mass heaters"

The modern wood stoves are a bit more efficient, but they don't have a mass and they still send a lot of heat out the roof.

Being that so-called rocket stoves can be built with milennium-old technology, why are they still virtually unknown and used almost nowhere? What's the catch?

As for why they are not used more - i don't know. The only thing I can think of is that there are some people talking about "rocket mass heaters" and what they have mad is not a rocket mass heater. That can be confusing.

As for a catch, there is a catch! A rocket mass heater puts much less heat out the roof. And because of that, can be hard to start in certain conditions.

Another catch: if you want it small, you can go with a "rocket heater" which will stop being hot when the fire goes out. The mass of the "rocket mass heater" will give you heat for days after the fire goes out. And the mass can be massive. Most people have a lovely bench in their home for the mass.

Nobody cares about proof that this is you, we want proof for all these wild claims you’re making

Here is a rocket mass heater we built in an hour and ten minutes. Of course, we sorta built a lot of it off-site the day before. But we moved the pieces to the demonstration with bicycle power and fired it up

https://www.youtube.com/watch?v=GkHOwmKyL7A

Here's proof of it burning so clean. And it also proves that it keeps most of the heat indoors.

https://www.youtube.com/watch?v=kGaGtO8MkQk

What is the next bit of proof you would like?

the average american adult carbon footprint is 30 tons per year. The average montana home heated with electricity is 29 tons. 20 tons for natural gas and 0.4 tons for a rocket mass heater

You can heat a house in Montana for a year with 800lb of wood? That's incredible (literally, not credible).

Please show some proof.

Here is a video i made 4 years ago. I heated my montana home (3 bedroom 1300 square feet) wtih 0.60 cords of wood. Conifer wood. I did not weigh it.

https://www.youtube.com/watch?v=7hcZ1RvW440

I maintained an average temperature of about 69 degrees. And it was a particularly cold winter. This building had zero curtains, and so-so insulation.

How difficult is it to clean one of these?

About once a year I open it up and vacuum out the ash - less than five minutes. There is zero creosote.

Given that the heating mechanism is radiant, how is that best distributed throughout an entire house?

As americans, we prefer convective heat - although radiant heat and conductive heat are more efficient.

So, for convective heat: an example is my office in my house. The rocket mass heater is located in my living room. The living room will get up to 75 and if I leave my office door open, my office will be 72.

That doesn't seem ideal.

It seems ideal for me and my values.

More importantly, there is talk of people in europe going without any heat at all. I suspect that could be ideal to many of them.

How much space do these heaters take? In videos I've seen they look bloody huge!

If you say "bloody" then I am guessing you are in europe. There is a commercial rocket heater there called "the gamera" - built in bulgaria. Although my guess is that it is now sold out. The point being, you can look it up and see that it has a pretty humble size. But it would be a "rocket heater". You would add a mass to get a "rocket mass heater". The greater the mass, the longer you get the heat to last.

I can see this being great for off grid cabins.

However, I live in the Atlanta suburbs and have a moderately well insulated house with a gas tankless hot water heater and a gas 98% efficient furnace with a timed thermostat and I use about $50/month in gas a month during the winter not counting taxes and fees. I have a high efficiency air filter that has to be swapped out twice a year and I get it inspected once a year. That's it. Otherwise I don't have to lift a finger.

I probably have enough random tree limbs come down that I could use this as supplemental heat, but that almost seems more work and effort than it's worth. I suppose they are handy for power failures, but I can run my gas furnace with a small generator (I use a ~$100 transfer switch).

I also presume that it needs very dry hardwood that has been seasoned for a few years?

If you want to convince people to switch, you have to convince them that it's better. How is it better than what I have now?

I burn conifer wood that I stacked last spring.

If a person has plenty of money and a thermostat, they probably won't be interested in a rocket mass heater.

I don't follow politics, but my understanding is that a lot of people in europe are about to be very, very cold. And if they knew of rocket mass heaters, they might build one now so they can be comfortable through the winter.

How does it tie into a home heating system? Is there and carbon monoxide put off by it?

The one I have in this house has been here nine winters (3 bedrooms in montana). I set the thermostat on the old heater to 50. Last winter, the old heater never came on.

About 8 years ago, the rocket mass heater expert Petr van den Berg came to my place and built an 8 inch batch box rocket mass heater. He used a testo meter on the exhaust for the whole burn. For most of the burn, the CO went to zero - lower than a natural gas burn. You can see his posted results here.

How does it compare to a mini split in terms of efficiency ?

The fuels and functions are quite different.

The nice thing about a mini split is that it keeps your house at the perfect temperature without any further effort from you. But it does have operation cost.

The rocket mass heater will need to be fed every couple of days, but the operation cost is very low - possibly zero.

So when the people in europe are looking at having no heat, this could be a great solution.

Can the smoke be made clean enough and cold enough that mechanical extraction is possible? In most of the designs I've seen, where the fire goes sideways, I am concerned that the smoke could go inside the space if the fire weakens unattended.

There have been a few people that have done mechanical extraction. I think that if a person ever gets to the point that they are contemplating that, they've done something very wrong in their design. It should be fully passive.

For my house, when I am about to start the fire, i pull the bricks back and there is already a strong draw in the correct direction. As the fire begins, the riser is warmed and the vertical exhaust is warmed, creating an even stronger draw. So the smoke goes where it is supposed to go - into the riser - where it is burned as fuel.

How is this different than carbon capture technology for coal power plants? (Legit curious, not being snarky)

My work is hyper focused on what people can do at home - so the desire for coal power shrivels up and blows away.

Could a floor heating system be designed with this? I am trying to design a home with a permanent heating system and floor heat tends to be a very efficient way to heat a home. Fireplaces are permanent but don't distribute heat as evenly.

Yes. It has been done many times.

Is manual loading/re-loading the sticks or paper/carboard waste the only way to load it or is there an automatic loading design for it too or in the works?

There is a commerical rocket heater called "the liberator" that actually works best with pellets - although it can burn sticks too.

Does "the liberator" have to be loaded manually or can it be loaded automatically? Are there any auto loading rocket mass heater models in the works or is everything manual at the time being?

The liberator comes with a big pellet hopper. So you might load it once a week.

Can it be incorporated into a standard HVAC system to send the heat all throughout a home or building space?

I suppose as much as any wood stove can. But I think that this is more of a solution for people in europe that might find themselves without heat this winter. So not as much of an HVAC type of solution and more like a wood stove solution.

What does it look like?

Here is a bunch of pictures of rocket mass heaters

What are the pros and cons of this compared to installing some solar panels powering a heat pump?

The other answers are all spot on. Cost.

Plus, it would depend a bit on the life span of the solar panels and heat pump.

Another thing about solar panels: you end up having to be a bit of an expert to keep them running.

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People have done it - although it can be a little weird.

What are typical building/installation costs? Would you be able to build one into an old house or would it need to be new construction?

Nearly all the builds I know of are in existing structures.

Cost is typically about $200. But I have seen them go as high as $2000 and as cheap as $20.

Could a rocket mass stove be used safely in a garage or woodshop? How do I get started on building one?

If you are going to build one in a garage or woodshop, we typically do it without a mass. For a home, there is a radiator and the mass. The mass holds heat for days, keeping you comfortable through the night and in the morning. But your tools don't care about being warm. And you might go into your garage or shop sporadically. So you might have two radiators and zero mass - that way you can heat the space quickly.

Hey Paul, just heard you on Jack's podcast and it got me thinking about building a RMH in my next house.

I have a few questions for you, first- one of my favorite parts of having a wood stove as a kid was the ambiance, have you ever built a RMH with a stove window for the light to go into the room?

I'm also wondering if you've ever played with putting the mass below floor level to let the heat rise naturally to the floor? I'm sure there's a limit to the thermal siphon, but not sure where that lies.

Also, I saw in one of your videos that you were playing with one of these and had an external air intake on it that you didn't like and took off, I'm curious why it didn't work out?

Thanks!

That was a fun podcast!

We have two different rocket mass heaters here with glass. Check out matt walker's stuff and peter van den berg's stuff. Glass is fun, but you do lose heat at a time where you are trying to add heat.

Underfloor has been done many times with great success.

External air intake is required for code in some places. With a rocket mass heater, you take up one tenth the air from a home (if you are using internal air intake) so you are 90% of the way there without the extra infrastructure. But I tried it for a winter with an and without - and I just like it better without the external air intake. The indoor air is fresher.

Cool, thank you. Is there a rule of thumb for the ratios of height of chimney to the distance I'm drawing the exhaust down to go underfloor?

That part I don't know.

Rockets are crazy dangerous and complex. We literally say ‘rocket science’ when we mean something is hard and complex. Why would you name it that if you want people to embrace it?

There has been a lot of debate over the name. It got the name from the sound it makes during the burn.

so why in your opinion are we not doing this? why am i only hearing about this now!?

I am baffled about this too. I learned about rocket mass heaters in 2009 and asked people then the same question. Since then, I have tried hard to tell people about it.

Are there any comprehensive guides that teach you how to step by step build a rocket mass heater out there? Preferably ones that are accompanied by instructive videos..

The premier book at this time is "The rocket mass heater builder's guide" by erica wisner. The main build in the book is the same build in the movie "building a cob style rocket mass heater" (i made the movie).

This sounds like it's a very good heating solution, also simple enough so people could be using this for centuries now. Why it is not so widespread and a relative novelty today?

I'm not sure if this could have been done centuries ago. I would have to think about that. In the core, we try to get to temperatures of 1800 to 2000 degrees F. We do that with combinations of firebrick on high temp wool. I'm not sure what we could have used for the high temp wool centuries ago.

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Yes. While I frequently see them being built in a day by experts, I have seen a lot of first timers build them in a pretty casual week. I know one guy that tried to build one super fancy and he took three weeks for his first build.