Recommendations for rocket maple sap heater

Hello all!

I have a time where I have a use for a rocket heater. My wife and I process sap for maple syrup. We have an outdoor cinderblock set up that was smoky and inefficient. We want to set up a sugar shack. I want to take advantage of less wood to chop and hotter burn of a rocket heater. The wife is skeptical. We live in the thumb of Michigan when it can get really cold.

I would like some help so I'd thought I'd post on the permie forum. I have a head on my shoulders, two friends who can do metal work and masonry, and an outside workshop so I don't burn down my house. The work I want this heater to do is boil sap at a really good rate, prewarm a different batch before adding to the really hot plate, move the finished syrup easily to a pot, create a shortcut for the vent for first burns to bypass heating the mass, and, if I can, heat a bench so us humans can warm up directly while working on the sap. At the moment, I'm looking at a batch box rocket mass heater since I can add twice as much wood to get it going hot. A regular j tube heater is easier to see in my head. I have a harder time with a batch box rocket. The sugar shack is uninsulated. I know recommendations for these outside systems are 8 inches diameter insulated risers and vents which is a taller riser which makes it higher and that could be harder to boil the sap.

Any recommendations for designs or pointers? Thanks!

I have built two rocket fired maple syrup evaporators, both L-tube style. A 30" or so long feed/firebox and a 30" or so riser shooting directly at the bottom of the pan work well. A "mass" style setup where the fire heats the mass which heats the sap is too many steps and is not likely to get a good boil on a useful scale. The evaporators I built have 8 and 6 square foot pans, and the core of each is an 8" chimney stack and an 8" wide x 9" high firebox (the largest simple standard firebrick sizing) with corresponding riser. With good conditions, I can get a good though not all-over boil on those sizes of pans. Any larger pan would need a larger core.

What size sap pan(s) do you have? Those are expensive components that are not practical to make without full stainless steel sheetmetal working capacities. The rest of an evaporator can be homemade with a bit of creativity. I used old home appliances for the structural shells of my evaporators, one was an upright freezer and a kitchen stove while the other was a modest chest freezer, my wife's first appliance decades ago, given a new life. Some fairly crude sheetmetal work with hand tools and hot linings of firebrick and some cob made serviceable units. In both cases I was working with existing sap pans. I had plans to make a larger evaporator using a commercial 6' x 4' syrup pan on a base made from an old 500 gallon oil tank with a rocket core inside it, but decided it was domestically unwise to add another annual time commitment.

My current 6 square foot evaporator.

I think for your desires, I would make a dedicated evaporator similar to mine for high feed rate and efficiency, and add a separate unit which could be a J-tube or batch box operating a preheater and a mass bench. Arrange the evaporator to have its drain flow into a pot which you can carry to the secondary unit's barrel top for precise finishing.

Hi Doug;
I was planning on responding with a suggestion for an L tube.
But I was really hoping that Glenn would reply first, as knew he had built sap evaporators in the past.
I'm glad I waited.
I second everything he has suggested, including a separate J-tube with a bench to sit on.

Good to see your post!

I actually joined this forum because I had this EXACT same desire/plan: to build a small sugar shack with rocket stove that I could use to boil sap.

I was thinking I’d buy this build kit:

https://www.dragonheaters.com/barrel-build-rocket-heater/

I’m not commercial and am not selling syrup. I only have enough trees for personal and close family and sometimes friends. But it’s a sort of tradition me and my kids enjoy and look forward to. Anyhow,  I don’t have any nice evaporator pans (I’m just using what are essentially restaurant buffet pans).

Anyhow, I was just going to set my pans on top of the barrel seen in this design/kit

Welcome to Permies, Mark! For your restaurant-pan sized evaporator, your idea would probably work fine. You would really want to build an enclosure that the pan(s) could sit in with heat flow space underneath them, so you don't just have one super hot spot and ambient air around the edges. You might be able to get enough heat flow with a J-tube, but I think you would find that an L-tube works better and is easier to keep fed. Building an L-tube with an 8" wide x 9" high x 31" long firebox and a 27" or 31" total height riser (from the firebox floor) is very easy, requiring no firebrick cutting or real masonry skills, just large Lego stacking. See my photos above. The enclosure which you would need to build anyway to stabilize the core and pans can be just about anything, even stacked concrete blocks. You would need about 40 standard firebricks, and if you use blocks for the support structure, about 30 standard 8" concrete blocks. I like to use Durock or similar cement board for the firebox floor - it won't last forever but is cheap and easily replaceable. If you use firebrick for the floor, add another 16 or so to the brick total. You would want to fill the space between enclosure and firebrick core with perlite, best if mixed with a small amount of miscellaneous powdered clay and moistened before filling. This will support the firebricks securely.

All this material will cost hundreds of dollars less than a Dragon Heater core alone.

By the way, pans set on top of the barrel will be much less effective than if they are directly exposed to the flame from the riser.

Glenn Herbert wrote:My current 6 square foot evaporator.

question about the chimney - are you using 6" or 8" diameter ducting?  Hoping 6" ducting is sufficient for an 8" L-tube rocket.  

Are using using regular galvanized zinc HVAC ducting instead of the black single-wall "stove pipe"?  The galvanized is cheaper but I was concerned about high temperatures causing the zinc coating to burn off with toxic fumes.

Glenn Herbert wrote:Welcome to Permies, Mark! For your restaurant-pan sized evaporator, your idea would probably work fine. You would really want to build an enclosure that the pan(s) could sit in with heat flow space underneath them, so you don't just have one super hot spot and ambient air around the edges. You might be able to get enough heat flow with a J-tube, but I think you would find that an L-tube works better and is easier to keep fed. Building an L-tube with an 8" wide x 9" high x 31" long firebox and a 27" or 31" total height riser (from the firebox floor) is very easy, requiring no firebrick cutting or real masonry skills, just large Lego stacking. See my photos above. The enclosure which you would need to build anyway to stabilize the core and pans can be just about anything, even stacked concrete blocks. You would need about 40 standard firebricks, and if you use blocks for the support structure, about 30 standard 8" concrete blocks. I like to use Durock or similar cement board for the firebox floor - it won't last forever but is cheap and easily replaceable. If you use firebrick for the floor, add another 16 or so to the brick total. You would want to fill the space between enclosure and firebrick core with perlite, best if mixed with a small amount of miscellaneous powdered clay and moistened before filling. This will support the firebricks securely.

All this material will cost hundreds of dollars less than a Dragon Heater core alone.

If I’m following correctly, the box can be made of cinder block with clay/perlite mixture to stabilize and insulate the fire brick core. The flame would be exposed to the pan bottom. The open flame would also be in contact with the top row of cinder block the containment wall is built from? Or is perlite/clay extending up the top row of cinder blocks also?

I used standard 8" galvanized ducting for chimneys. A J-tube core generally wants to have its feed damped somewhat (for my heater about 80% cover) to reduce excess air which just cools the fire. An L-tube evaporator is going to want more combustion happening at one time, so more airflow. I would not recommend reducing the chimney size for this. The bottom foot-ish of the chimney gets enough heat that it corrodes after a few years - I patched mine in one spot - but the rest is fine. I made a liner for the exhaust collector/transition from thin scrap stainless sheet I had to improve the longevity of this high-heat area.

If you build the shell/base from block, I would set the floor of the flue channel beneath the pan on that, then add a layer of bricks for a perimeter spacer to support the pan. Depending on the system size, 2 1/2" or 3 1/2" should give good flow while keeping the heat near the pan.