Long RMH in earth sheltered greenhouse, requesting design input

Hi All,

I am working on a large earth sheltered greenhouse build, and am in the design phase for a central Rocket Mass Heater bench.  Hoping to glean some practical experience from folks.

The greenhouse is 60 feet long and 24 feet deep with a northside 6ft wall buried in the earth, and the southern 2ft concrete footings buried 2ft below grade. So the whole building is sunk into the surrounding earth.  So it has a lot of mass to the building.  

You can see more about it on this article:

http://windward.org/2.0/notes/2015/2015andrew06.htm

The greenhouse interior is about 16ft tall, so the exit stovepipe will have 16ft+ of straight up rise.

I am working from Earnie and Erica's book, particularly the greenhouse 8" example in the book.  In that example the bench has 40ft of pipe that runs in a straight line and does not take a 180 degree turn.

My desire is to have a simlarly long bench  (roughly 34ft instead of 40ft)  but have the pipe double back on itself (a 180degree turn) and head back to the heat riser before heading up and out of the building..

So functionally, I would have almost twice the length of pipe as the design on paper.  I understand that I need to help such a system establish and maintain a good draft.  The design elements that I intend to employ to do that include:

1.) the exit flue is next to the barrel, so it is heated by the initial fire.
2.) minimal bends, only one 180 degree bend.
3.) have pipe running through mass always on an inclining plane to the vertical exit flue.
4.) The 16ft+ rise on the exit pipe.

Other design considerations which I am aware might help, but don't have a clear image of how to do include:
1.) have a means of lighting a small fire (a candle or piece of paper) in the exit flue to establish initial draw.
2.) make the heat riser taller and very well insulated.
3.) create a way (in the manifold) to vent initial fire directly into exit flue to establish draw.

To be clear about my questions for y'all:
1,.) is a 60ft long system even feasible? If not, is there generally regarded an upper range of length?
2.) are there other particular design considerations for a long system that I should bear in mind?
3.) any good examples of manifold that allow initial venting of stove directly to the exit flue to warm it up?

Do you have an optimal temperature range in mind for the plants in the greenhouse? Which plants? Is there any insulation between the floor/walls and the ground? The larger the differential between the ground temperature and the existing earth and mass, the larger the heater would need to be to provide the amount of heat you need.

What shape is the roof? Going by the numbers given, it sounds like you might have somewhere upwards of 20,000 cubic feet of space to heat. Are the above grade parts of the greenhouse that do not provide solar gain insulated? There are a lot of considerations to figure out what would work best, but I am willing to bet that you would need a larger diameter system, along with the accompanying burn chamber. I don't think an 8" system would be adequate to heat that much space unless you fed it constantly. A larger batch style burner may be a good fit for this. If you make a large system with a barrel at one end, the initial radiant heat will make that side of the greenhouse warmer, and the opposite end not getting as warm. This could affect how you want to design things and lay out the placement of your plants. Fans can move warm air, but not radiant energy.

I think most of the ideas you have are on the right track. Having a taller chimney, using the barrel to add heat to create better draft, and other things mentioned all sound good. I think increasing the diameter of the system with a larger fire would make it much more feasible, and provide the heat you need. I'm sure someone with more experience can give you a better idea if you can provide more details of the structure and what you expect from it.

As Daniel says, we need more information to give really good advice.

For starters, a 60' long (plus elbows) duct run for an 8" system is likely to be problematic. In your case, I would strongly advise a long, low bell instead of ducting. A 40' long x 1'-6" high x 1' wide cavity in a masonry enclosure would give the heat exchange you need without much friction. At 160 sq. ft. of internal surface area, it would want a large combustion chamber to support it and have enough residual heat to create draft. batchrocket.eu/en/building has the calculations you need for this, under the "bell sizing" section. I think you would want a batch box instead of a J-tube for this; a 10" bb system would match the bell size.

Ducting inside a bench in a greenhouse is likely to rust out soon, so a masonry bell will be more durable.

I'm going from memory here--always a tricky thing to do, but seems the number I remember for an 8 inch riser is a possible forty to fifty feet or so, Erica's design was near the limit.

your design has to be counted as 68 feet of pipe, two bends to make the 180 degree turn, then a third bend to go topside at 5 feet per bend, plus 16 more feet going up. or at least that's the way I always counted it

At the very best you will have to put a chamber in the vertical exhaust to stimulate the draft with a burning newspaper, at worst you'll have a smoking monster that can't decide if it's coming or going with or without the newspaper.

Remember when you start the stove cold, there is no heat in the barrel to help warm the vertical pipe and the fire will need to burn a few minutes before there is--it will be fighting a column of heavy cold air

You can always add a fan to force the exhaust out, and if it was me I would be looking for a way to exhaust horizontally to the outside so I wouldn't be pushing against a cold column of air.

Daniel asked a lot of pertinent questions, here are some more-- what is your coldest outside temp, how frequently do you go without sun, for how long? do you have lots of additional thermal mass to store the sunlight, did you insulate your footings on the inside and down the full two feet? is that the frost line?

Have you seen the greenhouse in Canada that stores solar energy in the ground in summer, and retrieves it in winter with just a fan ?



And if you're not familiar with batch burner rockets, there's no time like before you invest time and energy in a system only to discover that there's some downsides that could have been avoided.

The rocket stoves downside is the small amount of wood in the firebox and constant reloading (and very small stick size) Batch burners address those issues very well with little or no loss of efficiency.

have fun

Hi Glen,

I thought about a bell instead of ducting, but I'm not sure there's any calculations on that maximum length. I was just reading some posts from Peter today and he mentioned that the maximum Bell ISA had never been tested because it never seemed to be an issue, only the minimum size.

I think this project might put that to the test however with  a 34 foot long bench.

I did however think it might be better to do several smaller batch and bell systems to more evenly distribute the heat, They could even become warming beds for starting plants with large masses of soapstone or concrete slabs on top to keep a really steady temperature under the seedlings

I have to wonder if a rusting exhaust pipe is really an issue, or if the cob would simply continue to hold the shape (and seal) long after the metal was gone.   I suppose that's something to think about also

I just had a thought, how about a continuous hollow bench the length of the greenhouse with two or  three batch burners all feeding into the same bell space, and using the same exhaust( which would need to be big enough to handle all the burners)  Or they could be lighted successively, so the exhaust could be sized smaller, or...  

In the batchbox.eu page I linked to, the calculations for ideal sizes of systems are given. A bell like I suggested would want a 10" batch box, which would hold a lot of wood per burn.

I didn't see the link, but would like to. The quote from Peter is at the bottom of this  page

I have a greenhouse with a wood burner and have come up with a design that I think will work for me using 3 IBC totes sitting on a RMH bench. The water will provide plenty of mass and I can use the heated water as well for my aquaponics setup. I like Bob's idea of just running the exhaust straight out to achieve the 60 ft. run. since it's in the center of your GH you could bury the duct underground and utilize the soil as more mass and not lose much heat to outside influences. The RMH could still be above ground with a deeper "bell" coming out below ground level.

Hi All, Thank you for your questions and considerations.  

Here is some more info.

Here is my rough pass at the design from a few years ago:




One big difference in the design is how long I imagine the bench needing to be. But hopefully, this gives y'all a rough sense of what I am talking about.

This greenhouse is very much like an earth ship (except concrete and not tires for retaining walls)   The south wall is pretty much all solar gain, and the side walls and northern roof are insulated. So it does not loose as much heat as a hoop house or full greenhouse.   The footings are not insulated,  but are all buried under several feet of earth on the exterior of the building.

The ultimate design is similar to our communities dining hall, both in structural design, shape and square footage.  The main difference is how much solar exposure the polycarbonate south side allows.  

Attached are a few images of the interior I took yesterday

To answer some of your specific questions:

The optimal temp range around the plants in the greenhouse (the souther side) I would like in the greenhouse is in the 40 degrees F range.  Enough that cold winter crops will not die.  

There is also going to be a shower in the greenhouse which should provide some additional warmth/warm humid air.

The coldest temps we get here, in a prolonged manner, are in the negative single digits.  -5 degree F every other winter.  Otherwise the lows are in the single digits.

We can go for more than a week without direct solar gain.  Basically, we get stuck in a frozen cloud every so often.  Doesn't happen every winter, but when it does the temperatures drop.  Otherwise, it is usually pretty clear in the winter.

the Internal volume of the building is a little over 20,000 cubic feet.

The frost line is 24 inches. The footings are not insulated, but the could be. However, the whole building is half buried underground, so there is a large thermal mass that it is tied into.

My thought was to insulate the bottom of the mass heater to get the heat to rise up and into the building instead of into the ground.

Yes, I have seen Geoff Lawton's video of the Annualized Geosolar system in Canada.  I did calculations on doing such a system.  I found out that I would need to bury the pipes roughly 10 ft underground and put about 4 inches of rigid foam insulation over it.   The reality for us is that we do not have 10 ft of soil to dig into. The cost of the piping alone was in the 5k range. While the system is easy and passive to use, the installation seems beyond my grasp.

I have been looking at this design along side earnie and erica's

https://midwestpermaculture.com/2016/04/epic-greenhouse-rocket-mass-heater/





It has the large batch capacity, but also uses the j-tube design.

The rough batch design on your picture rocket is interesting, but likely you could be much more efficient following the guides for batch burner designs and dimensions--they are well engineered at this point to provide optimal burns.


I totally get that after the GH is built is not the time to try and dig deep trenches--but do good insulation inside the footings and maybe a bit deeper to confine the heat in the ground inside the GH--this essentially turns the whole ground mass into a thermal battery--although granted the exchange of heat in summer will be limited, so while you may not be noticeably gaining heat, at least you won't be losing it sideways during the cold months--certainly the south and side walls  could make a really noticeable difference.

I have been thinking about using a collector and metal rods  driven in the ground to transfer summer heat into the subsoil, but have not tried it yet maybe even some water pipe and a solar water heater circulating just a foot or so down through more sensitive areas

I would still think about a longer bell/bench, batch burner, and possibly two burn units on opposite ends of the bench, and take the exhaust out anywhere you want along the bench/bell

wow, that Batchrocket.eu site is very helpful!  

Thank you for that resource.

I ran calculations on their spreadsheet.  Here is what got.

Heat loss of the building to be heated:  15kW

A 250mm (10in) riser system running 2 double burns a day roughly 50lbs of wood per fire can meet that need.

with a 10in riser diameter, the internal surface area of the bell would need to be roughly 160 square ft