Why dont we all want rocket mass heaters?

[paul wheaton]

> you could have the initial firebox opening outside, then have the thermal
> mass inside the house. Possibly having to go outside to tend to it would
> be uncomfortable.

Bingo. Many people have tried this path and found it to be rather annoying.

> I made a tin can rocket stove ...

The rocket mass heater is built on rocket stove philosophies. But we are struggling with people that are worried that everybody in the house will die if you put in a rocket mass heater. It turns out they thought we were talking about running an outdoor cook stove inside of a house. So we are working to stop saying "rocket stove". A "rocket stove" is designed for outdoor use. A "rocket mass heater" is designed for indoor use and the exhaust is routed outside.

I'm not saying that you suggested the two are in any way similar. I just want to point out this important difference in vocabulary.

 

[bholler]

I just found this thread. There are some links to my stuff earlier in this thread. I am not one of the top five brains in rocket mass heaters, but I think I might make the top ten list.

Here is my feeble attempt to answer some questions ...

> Do rocket mass heaters actually work or are they a temper mental nightmare?

They do work. I have one in my three bedroom house in montana. I heated my house last winter with 0.60 cords of wood (I measured VERY carefully last winter). I would usually go to bed with the house at 70 degrees and wake up to the house at 68 degrees. I would usually start a fire when it got to 66 degrees and stop feeding the fire when it got to 72 degrees. This was usually a 90 minute burn every other day. There were some sub-zero days where I would do a three hour burn every day.

> If they do work why are they not common?

I think there are hundreds of thousands of them in existence now. But that is just a guess.

I am surprised that people still buy any sort of wood stove when they could have a rocket mass heater.

> The problem is that it is a huge site built structure that really is not approved at all.

It is mostly a DIY sort of thing.

Approved in portland and ... last I heard ... about a dozen other cities. I've been told that a lot of insurance companies are coming around too.

The thing is that nine years ago they were pretty rough. But they have come a LONG way in the last few years. As of last year there is now an excellent book and ... I put out some DVDs, so my judgement of the DVDs would be very biased.

> they typically have small fireboxes that need lots of loading.

The batch box rocket mass heaters have really big fireboxes. The J-tube style has a super tiny firebox. The one in my house is a j-tube style - and it is smaller than average too. But as I said earlier, most of the winter I burn a 90 minute fire every other day. So I start the fire, go work in my office, go back every 20 minutes to add wood until I forget and the fire goes out.

I lived in a similar sized house with a conventional wood stove, and I remember feeding it a mountain of wood throughout the day on days where it wasn't all that cold outside. The loading of a rocket mass heater is a dream in comparison.

> are you going to want to stand there and feed wood in for a couple hours? Not me.

I don't know of anybody with a rocket mass heater that does that.

> Masonry heaters are much more effective and practical but either way you need a foundation for it and it will take up allot of space.

Rocket mass heaters have a lot in common with masonry heaters. Rocket mass heaters tend to heat a space with less wood, are built in a weekend (instead of three months) and cost a few hundred dollars (instead of $10,000 and up).

Ok how many btu s are you inputting with that .6 cord. You must have a very low btu load to be able to heat with that little wood.

What is the difference between a batch box rocket and a masonry heater?

If they are only approved in a dozen or so cities they have a long way to go before they are a viable solution for most people.

What type of stove were you using? There are stoves out that can burn 48 hours on a single load. And the majority go 8 hours on a load.

You say no one loads wood for a couple hours. But you also said you sometimes burn for 3 hours. And you load every 20 mins. To me between loading and getting the wood to load that sounds like a lot of work that I have no interest in doing.

If you are building a simple masonry heater diy there is no reason for it to take 3 months or $10000. In fact 3 of us built one in a friends shop in a weekend for about $1200. Granted we were all chimney masons though.

I see nothing wrong with rocket mass heaters and I hope they get developed more and get more approval. But honestly the claims of efficiency gains sound pretty far fetched. There is only a certain ammout of btu s available in wood and no matter how you burn it you cant change that.

 

[paul wheaton]

I am burning pine, fir and larch.


> What is the difference between a batch box rocket and a masonry heater?

A batch box rocket mass heater has a very specific shape to the wood feed - and is generally fed through a door, a lot like a conventional wood stove. It is now the most popular design for rocket mass heaters, although I think it still has a few kinks to work out.

A batch box rocket mass heater will have a vertical port in the back that is an exact shape and size, and a low air intake on the door that cannot be plugged. It has an insulated vertical riser that is typically about four feet tall.

It tends to burn cleaner and hotter than a masonry stove, and the exhaust temp tends to be lower.


> If they are only approved in a dozen or so cities they have a long way to go before they are a viable solution for most people.

Nearly everybody can build them outdoors (greenhouses are quite popular). And it seems that the way to get them approved in cities is to have people build them, without permission, in cities where they are not allowed, and then do a huge amount of documenting before doing the work to get them added to the city codes. A path only for the most durable people. Since the exhaust is nearly invisible, I have heard of people building and operating them without permits - just for the sake of heating so cheaply. And, of course, there are places where there are no building codes and there are places where people have a cabin where codes appear to not apply.

There are a lot of people actively working on getting them into the codes of more places. The lawmakers appear to be quite keen on rocket mass heaters, so I have heard it has not been too difficult. But I confess that this is not an area I have put much of my time into.


> What type of stove were you using?

Hmmmm, here is one I was using about nine years ago when I first learned about rocket mass heaters.

I took the picture because I caught two mice with a bucket.

I remember feeding a LOT of wood into the stove and it just never seemed like enough.


> You say no one loads wood for a couple hours.

I don't understand what you are saying.

For a lot of the winter, I will go a couple of days between fires.

> But you also said you sometimes burn for 3 hours. And you load every
> 20 mins. To me between loading and getting the wood to load that sounds
> like a lot of work that I have no interest in doing.

On a regular winter day with temps below freezing at night and above freezing during the day, I will run a fire every other day for about an hour and a half.

On a really cold winter day, with temps holding below zero all day, I will run a three hour fire every day.

Since the wood feed on my j-tube style rocket mass heater is pretty small, then I reload it every 20 minutes or so. My wood feed is about 7 inches by 5 inches and 17 inches deep (to take standard firewood). So it isn't much of a load.

There are people with a similar sized batch box wood feed would guess about how much heat they want for the day and load it once every other day.


> Granted we were all chimney masons though.

There's the catch.

Even still, that has to be some sort of record. Three experts to build it in one weekend. I would have thought three experts working 12 hours a day and 100% of the materials on hand ... pushing to build as fast as they can ..... in a shop instead of a house .... I would have put my money on five days.

In a house with one pro and a non-pro assistant ... I think you will agree with three months and a $10,000 minimum.

> But honestly the claims of efficiency gains sound pretty far fetched.

You are not the first to doubt. Here is me and several experts talking about this very topic:




> There is only a certain ammout of btu s available in wood and no matter how
> you burn it you cant change that.

There is also the question of how well you keep the heat in the house instead of putting it out the chimney.

I think that if I get insulated curtains, but a mudroom/enclosed-porch on the front of my house and make three changes to my rocket mass heater, I can go from 0.60 cords of wood for winter down to 0.4.

 

[bholler]

Ok so how many btu s are in your .6 cord of wood?

How can you compare efficiencies if you don't know what wood stove you had or how much wood you used?

And as far as a masonry heater goes no it should never take 3 months of work at all. And most of the cost is in the finishing materials. Now for our build it was in a shop with a thick enough floor so no foundation needed. And he had precast a couple refractory panels for certain parts 2 in a day 2 weeks earlier. But the rest was all built by 3 of us in 2 10 hour days and honestly not working any harder than a normal day. Yes it was a relatively small simple unit but you claim of 3 months is completely off base.

 

[paul wheaton]

> How can you compare efficiencies if you don't know what wood stove you had or how much wood you used?

You will note that I did not compare the efficiency of the stove in the picture with what I have now. Other than to say that I have a LOT of experience in feeding a LOT of wood (many cords) to conventional wood stoves. You asked what stove was I using - I assumed you wanted to hear about what sort of conventional wood stove I used previously. So I shared one I happened to have a picture of. If you recognize the brand and whatnot - more power to you.

I have not personally racked up several years with a wood stove in one house and then replaced it with a rocket mass heater and then racked up several years with a rocket mass heater. I have, however, racked up many years in many homes with conventional wood stoves, and then when I arrived here I installed a rocket mass heater. Several, actually, in several buildings.

At the same time, i have visited with a lot of people who have racked up years with a conventional wood stove and then switched to a rocket mass heater. One case:

 

[bholler]

And honestly after watching that video it makes me doubt you claims even more. There was an absurd amount of disinformation about wood stoves in it and no real discussion of efficiency of rocket mass heaters

 

[paul wheaton]

And honestly after watching that video it makes me doubt you claims even more. There was an absurd amount of disinformation about wood stoves in it and no real discussion of efficiency of rocket mass heaters

Share your list of misinformation.

Are you saying the part about how peter and donkey talked about using testo meters and see an average of 93% efficiency? And that triggers "no real discussion of efficiency"? How about the part where these numbers have been verified by more than one third party?

 

[bholler]

I am not wasting any more of my time discussing this with you. You are making absurd claims about wood stove efficiencies that have no basis at all in reality so why would I believe any of your claims about rocket stoves.

 

[paul wheaton]

I think you are not going to believe anything I say because you choose to not believe anything I say. And that's fine.

Others are going to believe it because of the massive body of evidence from those that have chosen to try it and have reported excellent results. And others still because they have looked up the scientific measurement done and looked up the repeatability from others on those numbers.

If you are happy with what you have, then by all means, continue to be happy. This path is not for you.

This path caught my attention, so I now have a rocket mass heater in my house, another in my shop, another in my garage/pantry, another outside, one in a tipi, one in an experimental log structure, one in a tiny cabin near my house (for guests), one in an even tinier cabin on skids (also for guests), one in my backyard that is also a smoker, plus a rocket oven, a rocket kiln, two rocket hot water heaters and two indoor rocket cookstoves (one of which just came online today). A total of fifteen contraptions using these general designs. And that's just at my place.

There are the global experts that have traveled from as far away as australia, the netherlands and alaska to be here for our gatherings. And the students from israel, the check republic, borneo, mongolia and more. There are the hundreds of people I have visited with about their successes.

We are all bonkers about rocket mass heaters. So I guess this path is for us. And just because we are having a great time does not imply that this path is a fit for you.

It does seem that some folks here have some questions, so I am trying to help.

 

[georgepds]

I'd like to see a better discussion of efficiency, the bit of 1/10 the wood is hard to believe. Just how big is your house that it takes 6 chords to heat (10*0.6) with a wood stove? I heat mine with 2 chords and a soapstone stove.

.. thanks for the info

 

[electrathon]

How hard is it to start the draft when the stove is cold? The sideways up and down direction seems like it could be tough.

 

[paul wheaton]

I'd like to see a better discussion of efficiency, the bit of 1/10 the wood is hard to believe.

We start with very little wood being used for folks to be very comfortable. And the large body of anecdotal evidence from people that have shifted from a conventional wood stove to a rocket mass heater. Years ago, the claims averaged "one eighth the wood" but more recently (from design improvements) people have been sharing "one tenth the wood".

There have been hundreds of discussions on how and why. At the top of the list is simply exhaust temperatures for rocket mass heaters are much lower - we are keeping more heat inside.

There are a few dozen more items, and let's skip past the stuff about conductive and radiant heat and focus just on convective heat.

A few people in our community insist that a lot of it has to do with the temperature differential. A rocket mass heater tends to heat a room much slower than a conventional wood stove. Some people will run a conventional wood stove to the point that it is over 80 in the house - 10 degrees warmer than "minimal winter comfy". This creates a greater temperature differential between what is inside and outside. The greater the differential, the more heat will be leaking outside.

Next up is the part where people will do things to try to get the fire to go all night. This usually leads to the stove running inefficiently. And as is pointed out in the first video, if the dampers are open, then when the fire goes out, your convective heat is taken outside.

So, a 75% wood stove is actually 59% efficient and is typically operated at 35% efficiency at best, 3% efficiency overnight and in rare cases, negative efficiency as the fire is out and the residual convective heat is being carried outside. A rocket mass heater typically operates at 93% efficiency (on average).

So it isn't just one thing. It's a list of things. And this, is the beginning of the list.

Just how big is your house that it takes 6 chords to heat (10*0.6) with a wood stove? I heat mine with 2 chords and a soapstone stove.

This is a 1300 square foot double wide. The insulation is not great, and I really need to add a an enclosed front porch and winter curtains.

The rocket mass heater I have is designed to be "easy to use" instead of "efficient". So a few design changes would probably get me to less than half a cord.

And this is montana. And last year was a colder than average winter. Not that we hit any record lows, but that we spent a lot more time below freezing than most winters.

 

[paul wheaton]

How hard is it to start the draft when the stove is cold? The sideways up and down direction seems like it could be tough.

It depends on the stove and what the indoor temperature is and what the outdoor temperature is.

If the rocket mass heater design is "easy to use" it will probably be easy to start. If it is designed to be more efficient, then it could be picky and people have tricks to help it through the first fire of the season.

 

[bholler]

We start with very little wood being used for folks to be very comfortable. And the large body of anecdotal evidence from people that have shifted from a conventional wood stove to a rocket mass heater. Years ago, the claims averaged "one eighth the wood" but more recently (from design improvements) people have been sharing "one tenth the wood".

There have been hundreds of discussions on how and why. At the top of the list is simply exhaust temperatures for rocket mass heaters are much lower - we are keeping more heat inside.

There are a few dozen more items, and let's skip past the stuff about conductive and radiant heat and focus just on convective heat.

A few people in our community insist that a lot of it has to do with the temperature differential. A rocket mass heater tends to heat a room much slower than a conventional wood stove. Some people will run a conventional wood stove to the point that it is over 80 in the house - 10 degrees warmer than "minimal winter comfy". This creates a greater temperature differential between what is inside and outside. The greater the differential, the more heat will be leaking outside.

Next up is the part where people will do things to try to get the fire to go all night. This usually leads to the stove running inefficiently. And as is pointed out in the first video, if the dampers are open, then when the fire goes out, your convective heat is taken outside.

So, a 75% wood stove is actually 59% efficient and is typically operated at 35% efficiency at best, 3% efficiency overnight and in rare cases, negative efficiency as the fire is out and the residual convective heat is being carried outside. A rocket mass heater typically operates at 93% efficiency (on average).

So it isn't just one thing. It's a list of things. And this, is the beginning of the list.




This is a 1300 square foot double wide. The insulation is not great, and I really need to add a an enclosed front porch and winter curtains.

The rocket mass heater I have is designed to be "easy to use" instead of "efficient". So a few design changes would probably get me to less than half a cord.

And this is montana. And last year was a colder than average winter. Not that we hit any record lows, but that we spent a lot more time below freezing than most winters.

You really need to learn more about modern wood stoves if you are going to be comparing the two most of us do not struggle at all to burn overnight in fact most of us don't do anything differently at all for overnight. So your claims of reduced efficiency are invalid. And there is no way it should ever take 6 cord to heat a 1300 square ft house in Montana with a modern wood stove. I would expect 3 at most.

 

[paul wheaton]

You really need to learn more about modern wood stoves if you are going to be comparing the two most of us do not struggle at all to burn overnight in fact most of us don't do anything differently at all for overnight. So your claims of reduced efficiency are invalid. And there is no way it should ever take 6 cord to heat a 1300 square ft house in Montana with a modern wood stove. I would expect 3 at most.

earlier you said "I am not wasting any more of my time discussing this with you. " - I take it you changed your mind?

Are you saying that your wood stove is "conventional"? And the the woods stoves favored on these forums are "conventional"?

My guess is that your wood stove is somehow better than conventional. And that most of the stoves discussed here are better than conventional.

At the same time, I would think that if you switched from whatever wood stove you have now to a rocket mass heater designed for efficiency (rather than ease of use) that you would probably be just as comfortable in the same structure with one fifth the wood. Pure speculation on my part, based on visits with a lot of people that had some good skill at wood heat with a fancy wood stove that made such a transition.

 

[electrathon]

@paul wheaton What is the primary difference to a stove designed for efficiency and one designed for ease of use?

 

[paul wheaton]

@paul wheaton What is the primary difference to a stove designed for efficiency and one designed for ease of use?

There can be a few things. Plus there have been quite a few interesting innovations over the last four years or so. But I would say that the #1 thing is how much heat you allow up the vertical exhaust.

A youtube channel person came by my house last fall and took this video. You can see that the vertical exhaust is quite close to the barrel. So rather than trying to keep my exhaust temp under 100 degrees (which is what an efficient system would do) I'm hitting about 140 to 150. So my vertical exhaust ends up being a tertiary thermosiphon.

 

[georgepds]

I want to talk about efficiency a bit, and the perception of heat. I know in my house when it's 72 with the heat pump on , you wear a sweater, when it's 72 with the soap stone stove on , you strip to your skivies. A bit of an exaggeration, but it points to the perception of heat. A radiative surface , like a hot stove, just warms up every solid object in the vicinity.. including you.

The rocket mass heaters have a very large radiative surface that covers the horizontal exhaust pipe ( not to mention the drum itself) .. sometimes it's a couch, and sometimes it's a bed. If you sit on that massive hot object that holds the heat, you are going to feel warm .. warmer than a room with equivalent air temperature but no hot objects in it . Think of that seat heater in a Chevy Volt

Now a wood stove has more radiation than a heat pump ( but you'll never sit on it) , and a large warm mass has (perhaps) more radiation than a wood stove. In any event, making your bed on the hottest thing in the room (even if the room is a TeePee), is just going to make you feel warm. ( I can feel my bum tingling now, just thinking about it)

We need an ASHRE guy to chime in

 

[paul wheaton]

radiant heat is more efficient than convective heat. And conductive heat is more efficient than radiant heat.

If we can mysteriously convince people to stay in contact with the warm surface of a rocket mass heater to continually harvest the conductive heat, I think it is possible to express that the people of the house could be heated with one hundredth of the wood. But ... it is quite rare for that to be even remotely the case.

But when people are warmed through radiant or conductive heat, the air temperature can be quite low and the people feel "too warm".

 

[bholler]

earlier you said "I am not wasting any more of my time discussing this with you. " - I take it you changed your mind?

Are you saying that your wood stove is "conventional"? And the the woods stoves favored on these forums are "conventional"?

My guess is that your wood stove is somehow better than conventional. And that most of the stoves discussed here are better than conventional.

At the same time, I would think that if you switched from whatever wood stove you have now to a rocket mass heater designed for efficiency (rather than ease of use) that you would probably be just as comfortable in the same structure with one fifth the wood. Pure speculation on my part, based on visits with a lot of people that had some good skill at wood heat with a fancy wood stove that made such a transition.

Nothing fancy at all just a 10 year old recency 3100. The same stoves you guys are talking about in the 70 to 80% range. It seems you are comparing to old non secondary combustion stoves which havnt been on the market in years and are in the 20 to 30% range. Like I said you need to learn allot more about modern wood stoves before you can compare them.

 

[SuperJ]

I want to talk about efficiency a bit, and the perception of heat. I know in my house when it's 72 with the heat pump on , you wear a sweater, when it's 72 with the soap stone stove on , you strip to your skivies. A bit of an exaggeration, but it points to the perception of heat. A radiative surface , like a hot stove, just warms up every solid object in the vicinity.. including you.

The rocket mass heaters have a very large radiative surface that covers the horizontal exhaust pipe ( not to mention the drum itself) .. sometimes it's a couch, and sometimes it's a bed. If you sit on that massive hot object that holds the heat, you are going to feel warm .. warmer than a room with equivalent air temperature but no hot objects in it . Think of that seat heater in a Chevy Volt

Now a wood stove has more radiation than a heat pump ( but you'll never sit on it) , and a large warm mass has (perhaps) more radiation than a wood stove. In any event, making your bed on the hottest thing in the room (even if the room is a TeePee), is just going to make you feel warm. ( I can feel my bum tingling now, just thinking about it)

We need an ASHRE guy to chime in

This is all true, and some good points. I work in HVAC controls can definitely attest to the fact the people are satisfied with much lower ambient room air temperatures when the room is heated thru radiant floor heating. With a generic RTU people often want 72F or higher to make the complaints stop, but with a infloor heat sometimes mid to high 60's is totally ok.

I agree with Bholler too, that the 10:1 extreme is probably with some old cobbled together non secondary system run with poor wood, under poor control, to a best case scenario rocket mass.

But, I do have a coworker with an Austrian version of a rocket mass heater who has since substantial reduction in his wood consumption. In his case it has been about a 30% reduction (10 cords to 7 cords) over the last couple years, but he still runs a wood stove and makes maple syrup with wood so his experience is anecdotal.

Nevertheless, I think it's a great discussion on a different style of wood burning heater. It would be a shame to see it completely derailed over the specific numbers.

 

[bholler]

This is all true, and some good points. I work in HVAC controls can definitely attest to the fact the people are satisfied with much lower ambient room air temperatures when the room is heated thru radiant floor heating. With a generic RTU people often want 72F or higher to make the complaints stop, but with a infloor heat sometimes mid to high 60's is totally ok.

I agree with Bholler too, that the 10:1 extreme is probably with some old cobbled together non secondary system run with poor wood, under poor control, to a best case scenario rocket mass.

But, I do have a coworker with an Austrian version of a rocket mass heater who has since substantial reduction in his wood consumption. In his case it has been about a 30% reduction (10 cords to 7 cords) over the last couple years, but he still runs a wood stove and makes maple syrup with wood so his experience is anecdotal.

Nevertheless, I think it's a great discussion on a different style of wood burning heater. It would be a shame to see it completely derailed over the specific numbers.

And I can believe numbers like that. I agree it is a very valid concept with allot of promise but the experts spouting completely unrealistic numbers doesn't help them forward the concept at all.

 

[maple1]

0.6 cords of pine is around 15MBTU. Spread that over half a year and it's 50,000btu/day assuming 100% efficiency. That is very little heat load. There is only so much heat in a certain amount of wood - some claims & numbers I'm reading here are rather out to lunch. To suggest it would take 6 cords of wood to heat a place with a wood stove for the winter, that only needs 50,000 btu/day, is kind of out there.

 

[georgepds]

0.6 cords of pine is around 15MBTU. Spread that over half a year and it's 50,000btu/day assuming 100% efficiency. That is very little heat load. There is only so much heat in a certain amount of wood - some claims & numbers I'm reading here are rather out to lunch. To suggest it would take 6 cords of wood to heat a place with a wood stove for the winter, that only needs 50,000 btu/day, is kind of out there.

just for fun.. that works out to be ~14.6 kwh/day

 

[maple1]

just for fun.. that works out to be ~14.6 kwh/day

Which is only a small 1000 watt resistance heater running for 14 hours a day.

I have no doubt they are efficient units, but they aren't magic.