Max Size Heater?

List,

I am wondering what the largest burn area/tunnel has ever been built and functioned properly? Or how large do I need?

Lumber is plentiful. Heat is for 15 x 30 x 8 Johny's style x mill thick plastic roof green house.

I envision using metal ducting T'ing off the entrance point after sufficient cool down has occurred. (Should be safe to do, correct? )

Michael Richards wrote:List,

I am wondering what the largest burn area/tunnel has ever been built and functioned properly? Or how large do I need?

The largest I have heard of was 250mm / 10 inch but, that was for a large warehouse.

Michael Richards wrote:
Lumber is plentiful. Heat is for 15 x 30 x 8 Johny's style x mill thick plastic roof green house.

Not very big but very low insulation.  Where are you, what planting zone?  The greenhouse needs to stay above 33°F.  That is much easier in zone 9 then zone 3.

Michael Richards wrote:
I envision using metal ducting T'ing off the entrance point after sufficient cool down has occurred. (Should be safe to do, correct? )

No idea what you mean by this, can you add a drawing?  

A greenhouse needs low heat for hours not much instant heat.  Use a long thermal bench and cover much of the barrel with cob.  Put the heat in the bench not in the air.  

Note Uncle Mud has a webinar due soon on exactly this subject.  

https://permies.com/wiki/202866/Webinar-Heating-Greenhouses-Rocket-Mass

Tom

Southern MO so I think it's called zone 6.

Since the burn tunnel can get hot enough to damage metal, I am considering using fire brick to the point the air stream has cooled enough to not damage metal ducting. From that point I plan to T off in 2 directions.  I am of the opinion heat will transfer more quickly from that then a cob coated duct of sort. Some small low side of ducting (..to keep smoke from escaping) "diffusers" of sort will heat the air quicker IMO.

I don't have a drawing, or know how to make one, so it's in my mind as to how I plan to proceed.

  You do not need to split the heat flow in the bench.  Your greenhouse is tiny a standard 6" rmh (load area 3',  barrel 2', 18ft out n back in bench, total 23ft) just fits.  It sounds like you need more basic understanding of the way a rocket mass heater works.   See Paul's page. https://richsoil.com/rocket-stove-mass-heater.jsp

Here is a 1hr class by the experts.  Note the 6" build about 40min mark, Out and back.  https://m.youtube.com/watch?v=3mZQdu2wNi4

After the barrel the gasses are cool enough for metal duct to survive.  Everything before and inside the barrel must be refractory.  Even old school clay bricks have a limited life (1year?) as parts of the burn tunnel or the riser.  

A suggestion I just saw looking up that rmh link, don't use a barrel use a brick bell.  You don't need instant heat and in zone 6 you don't need huge amounts of heat.  A brick bell / box over the riser will store heat, instead of a barrel.  Masonry moves heat about 1” per hour so 4" brick would hit peak heat in the greenhouse about 4hours after the burn. If you doubled most of the walls that would be 8" or 8hrs a 10PM fire would peak the heat output @ about 6AM near perfect match for a greenhouse heat load.  

Also insulated walls are usually cheaper than glazing.  The north roof and wall of the greenhouse are better insulated than glass.  Less heat loss, less cost. Almost no change in solar gain.  Tom

Total Sarface Area = 2(lw + lh + wh)
SA = 2(30*15 + 30*8 + 15*8)
SA = 1620 sqft
Heat Loss = U-value x SA x delta T= 1 x 1620sqft x (70F-20F) = 81,000BTU/hr  (2MBTU/day or 310lbs/day or 1/10th of a cord of wood per day)

1cord of wood is around 20MBTU, So you will go through 3cord of wood per month.
Do you plan on insulating your floor and north wall?

How do you plan on feeding the 310lbs/day of wood to your rocket mass heater? Will you do 24 burn session per day (13lbs of wood), on the hour every hour. Or will you only do 6 burn session per day @ 52lbs of wood.

So technically even a 6inch tube will work. But you can do a 10inch or 12inch if you want to.  

Tom Subject: Max Size Heater?
 You do not need to split the heat flow in the bench.  Your greenhouse is tiny a standard 6" rmh (load area 3',  barrel 2', 18ft out n back in bench, total 23ft) just fits.  It sounds like you need more basic understanding of the way a rocket mass heater works.   See Paul's page. https://richsoil.com/rocket-stove-mass-heater.jsp

I WAS planning on 2 ducts to more evenly heat the air. What is "rmh"?  I watched most of the video you suggested. I don't need the mass surrounding ducting because it will provide instant heat when needed more readily, despite your saying I don't need that. I may come up with something later to store heat it's just currently not in my planning.  

I was toying with the idea of using thick cedar boards and whatever to insulate all the walls all around. My chainsaw mill should be churning out planks soon.

S Benji
Total Sarface Area = 2(lw + lh + wh)
SA = 2(30*15 + 30*8 + 15*
SA = 1620 sqft
Heat Loss = U-value x SA x delta T= 1 x 1620sqft x (70F-20F) = 81,000BTU/hr  (2MBTU/day or 310lbs/day or 1/10th of a cord of wood per day)

1cord of wood is around 20MBTU, So you will go through 3cord of wood per month.
Do you plan on insulating your floor and north wall?

How do you plan on feeding the 310lbs/day of wood to your rocket mass heater? Will you do 24 burn session per day (13lbs of wood), on the hour every hour. Or will you only do 6 burn session per day @ 52lbs of wood.

So technically even a 6inch tube will work. But you can do a 10inch or 12inch if you want to.  

Thanks for all your math! It seems like your figures are not compensating for the reduced heat load once my planned 9 wicking beds have warmed up. Still, again thanks, I was thinking of doing some insulating, now I know for sure it's going to be required.

I am not sure what you mean by ducts or where they will go or at what hight but if you just want radiant heat with no overnight storage, then a thin wall bell will work best ie two barrel stacked on top of each other.

Uncovered horizontal ducting does not work well at all.
It sheds its heat too fast, you will get condensation and you will lose the draft.
Lose the draft and your feed tube becomes the chimney...   not fun.

Fox suggested double barrels for quick radiant heat, I second the idea.

Michael Richards wrote:
Thanks for all your math! It seems like your figures are not compensating for the reduced heat load once my planned 9 wicking beds have warmed up. Still, again thanks, I was thinking of doing some insulating, now I know for sure it's going to be required.

The heat lost of 81,000BTU/hr or 2MBTU/day through the "glass walls and ceiling" will not really change if you chage your heat source from fossil Fuel radiant floor heating to RMH radiant floor heating. But one could cut heat loss by 30% if the air temp was lowered from 70F down to 55F (with the idea that plants care more about the "radiant floor heating" soil temp of 75F).

How are you going to keep it warm throughout the night?

The thermal mass of the "wicking beds" will moderate the temperature swings, i.e. the temps wont drop immediately after sunset... however, the heat loss of the greenhouse remains the same. Only way to change that is changing construction/materials.

The fuel requirements, I don't think is as dire as S.Bengi says, since sunlight is going to contribute some heat (weather and day-length dependent) to both the interior of the greenhouse and the outdoor temperatures (daily highs/lows). The heat load at noon on a 20*F day might only be 10*F, rather than 50*F in the example (70*F inside - 20*F outside). Also, the 50*F delta T might not be necessary if growing cool-weather crops, so obviously use your own figures.

A system that is capable of your highest expected heating load is necessary to get you through overnight lows, and bad weather.

S Bengi wrote:Total Sarface Area = 2(lw + lh + wh)
SA = 2(30*15 + 30*8 + 15*8)
SA = 1620 sqft
Heat Loss = U-value x SA x delta T= 1 x 1620sqft x (70F-20F) = 81,000BTU/hr  (2MBTU/day or 310lbs/day or 1/10th of a cord of wood per day)

1cord of wood is around 20MBTU, So you will go through 3cord of wood per month.
Do you plan on insulating your floor and north wall?

How do you plan on feeding the 310lbs/day of wood to your rocket mass heater? Will you do 24 burn session per day (13lbs of wood), on the hour every hour. Or will you only do 6 burn session per day @ 52lbs of wood.  

@Kenneth, you are correct in that solar gain will help out alot.
Avg Daily Heat Loss = 2.0MBTU
Avg Daily Solar Gain = tranmission x Collector Area x Effective Solar Hours = 0.5 x 30ftx15ft x 2 = 450kWHr = 1.5MBTU or 75% of the heat loss/heat needed
Avg Daily RMH Makeup Heat = 2.0MBTU - 1.5MBTU = 500,000BTU/day = 77lbs/day of wood

S Bengi wrote:Total Sarface Area = 2(lw + lh + wh)
SA = 2(30*15 + 30*8 + 15*8)
SA = 1620 sqft
Heat Loss = U-value x SA x delta T= 1 x 1620sqft x (70F-20F) = 81,000BTU/hr  (2MBTU/day or 310lbs/day or 1/10th of a cord of wood per day)

1cord of wood is around 20MBTU, So you will go through 3cord of wood per month.
Do you plan on insulating your floor and north wall?

How do you plan on feeding the 310lbs/day of wood to your rocket mass heater? Will you do 24 burn session per day (13lbs of wood), on the hour every hour. Or will you only do 6 burn session per day @ 52lbs of wood.

So technically even a 6inch tube will work. But you can do a 10inch or 12inch if you want to.  

Not going into the math in detail, but, could you really get 52 lbs of wood "each time" into a 6" system, 6 times per day.???  2.5 hour burn x 6 times, equals a lot of time that a person has to be around. and at 7 days a week, starts to add up. No matter if the stove set up works, this time factor would kill it for most folks, in my honest opinion.

Michael Richards wrote:

I WAS planning on 2 ducts to more evenly heat the air. What is "rmh"?  I watched most of the video you suggested. I don't need the mass surrounding ducting because it will provide instant heat when needed more readily, despite your saying I don't need that.  

rmh = Rocket Mass Heater.  

Building a low mass heater in a greenhouse would give an extreme miss match to the heat profile vs the heating demand of the greenhouse.   Your greenhouse will need heat in the early morning every winter morning.  The only way to supply that with low mass is a thermastaic control on-demand heater, or staying up all night feeding the fire.   The high mass rmh gives the heat profile you need without controls.  Build the bench and let the mass do the timing for you.  As I stated above 8 inches of masonry will time shift the burns heat from 10PM to 6AM.

Michael Richards wrote:
Thanks for all your math! It seems like your figures are not compensating for the reduced heat load once my planned 9 wicking beds have warmed up. Still, again thanks, I was thinking of doing some insulating, now I know for sure it's going to be required.

His math also maintains 70°F 24 hrs of 20°F all day, worst case planning.  I'm assuming it just can not freeze.  Your greenhouse will seldom need heat in the day time and it takes a great deal of energy to get a greenhouse to step down from 32°F to 31°F.  Tom