Cords per tap, gallons per cord
How much wood a season takes
Wood is the traditional fuel, and there are two ways to plan it. By taps: the 2022 Producers Manual budgets 1 cord of dry hardwood for each 75 to 100 taps on an efficient evaporator, and UNH's backyard rule is the same order, at least ½ cord per 50 taps, more with hemlock or pine. By syrup: the figures below are what a full cord (128 cubic feet, a 4 × 4 × 8-foot stack) makes on a conventional wood evaporator. They span three decades and they do not agree exactly, so here they all are with their years.
| Source | Year | Syrup per cord | Basis |
|---|---|---|---|
| USDA Forest Service cost study (Huyler & Garrett) | 1977 data, pub. 1979 | 25–30 gal | Measured on producers' rigs |
| Producers Manual, 2nd ed. | 2006 | up to 25 gal | Producer reports |
| Producers Manual, 2nd ed., worked example | 2006 | ~19 gal at 30% efficiency; 25 at 40% | 2.5 °Brix sap, 20 million BTU cord |
| Ohio State maple blog (Ober) | 2021 | 22 gal | Extension estimate |
Call it 20 to 30 gallons of syrup per cord on a real evaporator, lower on a hobby open pan (next section). At a quart of syrup per bucket tap, that is why the per-tap rule lands near a cord per 75 to 100 taps.
Less than half reaches the sap
Where the heat goes
Most traditional wood evaporators turn less than half of the wood's heat into evaporation. The sources put it differently: under 50% (2006 manual), 40 to 50% (Forest Service, 1979), often 30 to 40% (a 2003 Wisconsin review). Ohio State's blog says 40 to 60% for open evaporators and passes along maker ratings of 85 to 90% for modern forced-air and gasifying arches; those high figures are manufacturer ratings, not independent tests.
Hobby open-pan boiling is far below any of that. In the 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, with a range from 170,000 (a producer with a preheater and reverse osmosis) to 5,343,000 BTU per gallon. The author flagged possible reporting error (face cord versus full cord) and open-pan boiling as causes. For scale, Ohio State puts the theoretical requirement at about 400,000 BTU per gallon from 2% sap. The gap between 400,000 and 2,100,000 is the heat that went past the pan.
What narrows the gap
The cheapest efficiency gain
Dry wood, species and size
Season hardwood 9 to 12 months split (unsplit, at least a year), under a roof with air moving through the pile. Size matters as much as dryness: UNH says split to 2 to 3 inches for backyard rigs; the 2022 manual says pieces should be between 2 and 8 inches wide and cut shorter than the firebox. Small dry wood is what gives the hot, complete burn the arch was designed for, and it is why a basic arch wants reloading every 10 to 20 minutes.
| Species | Million BTU per cord |
|---|---|
| Sugar maple | 21.3 |
| White ash | 20.0 |
| Red maple | 18.6 |
| Hemlock | 13.5 |
| White pine | 12.0 |
Softwoods burn hot and fast but carry roughly 40% less heat per cord than the dense hardwoods, which is why UNH tells backyard boilers to plan on more wood if they burn hemlock or pine. The 2006 manual also passes along maker claims that a forced-draft fan recovers 10 to 20% more heat and cuts wood use 30 to 50%; treat those as 2006 sales figures until you measure your own.
Dated numbers, clearly labeled
Oil and propane
Every oil and gas figure in the sources predates modern heat recovery, so read the years. For fuel oil, the 2006 manual works out that a 100%-efficient rig would need about 2.2 gallons of No. 2 oil per gallon of syrup from 2.5 °Brix sap; traditional oil evaporators at 65 to 75% efficiency use more than 3 gallons. The 1977 Forest Service study measured 3.5 to 4 gallons, and Ohio State's blog says under 4. With reverse osmosis and heat recovery the manual puts it at about 1 gallon, and one steam-plus-RO producer reported half a gallon.
For propane, the single sourced figure is from the same 1977 study: purpose-built LP-gas evaporators burned about 5 gallons of LP gas per gallon of syrup at 65 to 70% efficiency. That is a commercial rig with a proper arch. A turkey-fryer burner under an open pot wastes most of its heat, and Cornell's beginner notebook warns the propane cost can approach the value of the finished syrup. No extension source opened for this guide gives a propane-per-gallon or tanks-per-gallon figure for a hobby burner, so any “one 20-pound tank makes a gallon” claim you read is unsourced. Propane and natural gas are clean and adjustable, which is why they are common for small hobby rigs and for finishing and canning units.
Boil less water
What reverse osmosis changes
Fuel use is a water problem. At 2 °Brix you evaporate about 42 gallons of water per gallon of syrup; concentrate the sap to 8 °Brix first and it is fewer than 11. That is why the oil figure drops from more than 3 gallons to about 1 with RO, why the Wisconsin survey's best producer (preheater plus RO) used 170,000 BTU per gallon, and why a hobby RO unit in Cornell's notebook halved a 25-tap boil from about 8 hours to 4. The trade-offs, flavor findings and membrane care are in the reverse osmosis guide.
Your numbers, not ours
Putting a price on it
With the ratios above, the cost is arithmetic: taps × sap per tap gives gallons of sap, the sap-to-syrup calculator turns that into gallons of syrup and water to remove, and 20 to 30 gallons per cord (or 3 to 4 gallons of oil, or about 5 of propane on a purpose-built rig) turns the syrup into fuel. The cost calculator carries it through to a price per quart, with the caveat that hobby open pans will burn more than any of these figures suggest.
