DIY maple evaporator: cinder-block arch, barrel stove and pans

The 2022 Producers Manual says most evaporators comprise at least four distinct parts: the arch (the firebox), the heat source, one or more pans, and the stack. A backyard rig has the same four. What changes at hobby scale is that the pans are flat and the arch is whatever you can build. The extension bulletins describe these rigs in a line or two, not as plans; no extension plan with block-by-block dimensions was found for this guide.

Cornell's beginner notebook describes the classic: a firebox of concrete (cinder) blocks sized to the pans, at least two blocks high, fitted with a smokestack to lift smoke overhead and improve the draft. UMaine's version is the same idea with several catering pans across the top for more boiling surface. Two facts from the manuals explain why the shape matters:

• About 80% of the heat a wood fire delivers to the pans is radiant, so wood arches are built shallow with vertical walls to keep flame against the pan bottoms. A block firebox that is sized to the pans does that; a wide fire pit under a small pan does not.
• A basic wood arch needs reloading about every 10 to 20 minutes (2022 manual). The 2006 edition described alternating-door firing every 3 to 7 minutes. Either way, someone stays with the fire.

The pans sit on top of the blocks; fire and smoke pass under them and out the stack. Cornell's other warning applies here: smoke that swirls across an open pan flavors the syrup, which is the real reason for a stack on a rig this small.

Ohio State's hobby fact sheet says barrel stoves can be modified to support a pan for boiling, and lists an outdoor fireplace or fire pit, a woodstove, a gas grill, a camp stove and an outdoor gas range alongside it. UNH's list is shorter: arch, fire pit, woodstove or gas grill. That is the extent of what the extension sources say about barrel evaporators. Nothing opened for this guide gives a barrel-stove design, a pan size for one or an evaporation rate, so treat plans you find elsewhere as untested by the research community.

The propane turkey fryer gets a little more: Ohio State says it works for a few taps and can make fine syrup if no frying oil is left in the pot; Cornell says keep the sap 2 to 4 inches deep and warns the propane cost can approach the value of the syrup. Fuel figures are in the wood and fuel guide.

Cornell: flat pans at least 2 inches deep and 12 inches square, and bigger is faster. UNH: high sides, about 6 to 8 inches minimum, so a rolling boil stays in the pan. Several pans in series, fresh sap in the first and ladled forward, beat one pan, because the sap moves toward syrup in stages instead of sitting at heat all day. A flue pan, with channels folded into its bottom, boils much faster than a flat pan of the same footprint, but that is a bought part, not a shop project.

While boiling, keep at least 1½ inches of liquid in a flat pan (Cornell, UMaine and Ohio State agree) or it can scorch.

Food-grade applies to everything the sap touches, not just the pan: Ohio State says no oil or chemical containers, and UNH says no plastic buckets not intended to hold food, such as drywall-compound pails.

A stack does two jobs: it lifts smoke off the pan and it pulls air through the fire. On height, the sources disagree, so here are both:

The 2022 manual gives a way to check the result on a real rig: wood-fired stack temperatures should generally run 600 to 800°F (315 to 425°C). Hotter means heat is going up the chimney; cooler means the front of the rig may not finish syrup. Nobody measures that on a block arch, but the manual's point carries over: draft and dry wood decide how fast the pan boils.

On air, the manuals list a fan forcing air into the arch among the things that speed evaporation, and the 2006 edition reports maker claims for forced draft of 10 to 20% more heat recovered and 30 to 50% less wood. Those are manufacturer claims from 2006, not measurements. The sources opened for this guide say nothing specific about grates in a block arch, so that detail is yours to work out. A spark arrester on the stack is strongly recommended for every wood-fired rig, most of all late in the season once the snow is gone, in wind, with softwood, and with forced draft.

1. 1

   Clearances: no official number

   Both manuals say only to leave plenty of open-air space between the stack and anything combustible, follow the maker's instructions, use a roof jack and guy the stack; working stack temperatures commonly exceed 500°F (260°C). One maker's manual gives 24 inches between the stack and any wall, closer only with a heat barrier such as ceramic blanket, and says to contact your insurance company.
2. 2

   Never leave the fire

   Cornell: do not leave a fire going while you are asleep or away. Outdoors, mind wildfire risk, check with the local fire warden about permits, and put the fire fully out. If you use any accelerant to start it, never gasoline or lantern fuel.
3. 3

   Have an extinguisher

   The 2022 manual's minimum is one 10-pound ABC dry-chemical extinguisher, with everyone trained to use it. Water extinguishers freeze and are for wood and paper fires only.
4. 4

   Carbon monoxide

   Any fire, wood, oil or gas, makes CO, which is invisible and odorless. The manual's advice is written for sugarhouses: give the fire make-up air and install CO and smoke detectors, above all anywhere people sleep. No extension source opened for this guide gives propane-cylinder or turkey-fryer rules specific to sugaring.
5. 5

   Burns are the common injury

   Hands from reaching over the pan, faces from steam. Heat-proof gloves, a non-absorbent apron, safety glasses, long sleeves, closed shoes, and don't lean over the pans. Cool a burn in cold water for 10 to 15 minutes; no ointment, salve or butter.

It can make excellent syrup. Ohio State says so of a turkey fryer, and the pan-in-series block arch is how most beginners start. What it cannot do is boil fast or boil efficiently. A flat pan removes ¾ to 1½ gallons of water per square foot per hour (Ohio State); a 2 × 4-foot flue-pan evaporator does about 15 gallons an hour on the 2006 manual's ratings. In a 2003 Wisconsin energy survey of 62 producers, wood users averaged 2,100,000 BTU per gallon of syrup against 331,000 for oil and gas users, and the author named open-pan boiling as one likely cause. Batch boiling in one pan also tends toward dark, strong syrup, as the 2006 manual notes, unless you keep the pan below about 50% sugar until the sap is all in.

Cornell's line is about 50 taps: past that, small factory-made continuous-feed evaporators become the practical choice. Below it, a block arch and a couple of good stainless pans are the right tool, and the sizing guide will tell you how many hours they will cost you.