What size evaporator do you need?

An evaporator is sized for the big day, not the average one. The sources give a range for what a tap delivers in a good run, and the spread is real:

• 2006 Producers Manual: a good run “may produce 4 to 5 quarts” per taphole; a gallon per tap is a large run.
• UNH (backyard): about 1 gallon of sap per tap in a good run.
• Ohio State (hobby): expect around 2 to 3 gallons per tap during a good run.
• 2022 Producers Manual: its design example assumes an average of 1 gallon per tap per day.
• Cornell's tubing worksheet: 0.2 gallons per tap per hour during excellent flow, which works out to nearly 5 gallons over a day.
• Top of the range: UVM's Wilmot measured a 3/16-inch gravity line peaking at over 6 gallons per day per tap (reported in The Maple News, 2013).

Over a season, the manual's rule of thumb is about 10 gallons of sap, roughly a quart of syrup, per bucket tap; well-run vacuum systems collect 15 to 25 gallons or more. For the season total, use the yield estimator. For sizing, use a per-day number: 1 gallon per tap is the common planning figure, and Ohio State's 2 to 3 is the margin you want for the exceptional day.

Sap has to wait somewhere between the tree and the pan. The 2022 manual sizes storage per tap, per day of flow between boils: at least 1 gallon for buckets and bags (more with big trees), 1.5 gallons for gravity tubing and 2 gallons for vacuum tubing. Its processing chapter puts it as at least 1½ gallons per tap each day, up to 2 on high vacuum. Cornell says the safest estimate is 2 gallons per tap, and the 2006 edition planned on 2 to 4.

The manuals, UNH and Cornell all size an evaporator the same way:

1. 1

   Gallons on a big day

   Taps × sap per tap. UNH's example: 25 taps × 1 gallon = 25 gallons. Cornell's: 100 taps × 1.5 gallons = 150 gallons.
2. 2

   Divide by the hours you can really boil

   UNH: 25 gallons ÷ 6 hours = 4.2 gallons per hour. Cornell: 150 gallons ÷ 10 hours = 15 gallons per hour. The 2006 manual says a 10-hour workday is the most you should plan on, with margin left for the exceptional run and for setup and cleanup.
3. 3

   Match a pan to that rate

   UNH's 4.2 gallons per hour lands on a 2 ft × 33 in pan rated about 5. Cornell's 15 gallons per hour is a 2 × 4 ft evaporator with a flue pan. The ratings are in the next two sections.
4. 4

   Check the short end too

   The 2022 manual's rule of thumb: a boil shorter than about 3 hours is inefficient once you count start-up and cleanup. If a typical run finishes in an hour, the rig is too big for the taps, or you need more taps.

The rate is gallons of water removed per hour. At 2 °Brix, 42 of every 43 gallons of sap is water, so the two are nearly the same number; with pre-concentrated sap they diverge. The sap-to-syrup calculator gives the water figure for any sugar content.

A flat-bottomed pan evaporates ¾ to more than 1½ gallons of water per square foot of liquid surface per hour, depending on the heat source and the pan (Ohio State). A 12-inch-square pan is one square foot. UNH's backyard table gives whole-pan ratings for the small sizes: a 12 × 20-inch pan about 1 gallon per hour, a 19 × 36-inch pan about 3, and a 2 ft × 33-inch pan about 5. Those work out to roughly 0.6 to 0.9 gallons per square foot per hour, the low end of Ohio State's range, though UNH does not say what kind of pan it assumed.

The same arithmetic explains why “a 2 × 4 does 15 gallons an hour” is only true of a 2 × 4 with a flue pan. A flat 2 × 4 pan, 8 square feet, lands at about 6 to 12 gallons per hour on Ohio State's figures.

A flue pan has deep channels folded into its bottom that multiply the heated surface, which is why it boils much faster than a flat pan of the same footprint. The 2006 Producers Manual averaged several manufacturers' ratings for conventional flue-pan evaporators and called the result an approximate guide only. The 2022 edition dropped the table and says each design has its own rate. One maker's current chart runs higher; both are shown.

The maker's chart carries its own caveat: real output depends on wood, firing and draft. Treat the 2006 column as what a rig does on an ordinary day and the maker's column as what it can do when everything is right. Two add-ons move these numbers: a steam pan (piggyback unit) that pre-concentrates sap with the evaporator's own steam raised evaporation rate by an average of 65% in the 2006 edition, and a preheater in the hood gains 5 to 15% (2022) or about 15% (2006).

Here is the UNH and Cornell method applied to common hobby tap counts, at 1 gallon of sap per tap on the big day and a 6-hour boiling window. The arithmetic is ours; every rate and rating comes from the sources above. If you expect Ohio State's 2 to 3 gallons per tap, double or triple the sap column and move down a row or two.

Cornell draws the line at roughly the 50-tap row: past about 50 taps, small factory-made continuous-feed evaporators become the practical choice. Below it, the rigs in the DIY evaporator guide are what most people run.

The one measured hobby figure in the sources comes from a producer account in Cornell's beginner notebook: 25 taps on a 2 × 4-foot flat pan took about 8 hours per run, and 4 hours after a small reverse-osmosis unit sweetened the sap from about 2% to 4 to 5%. Ohio State's per-gallon figures point the same way: 9 to 18 hours per gallon of syrup on a 24 × 18-inch pan.

For flue-pan rigs the sources give ratings, not hours, so the hours are division: 100 gallons from 100 taps on a 2 × 4 rated 15 gallons per hour is a little under 7 hours; the same sap on a 2 × 6 rated 25 is about 4. Both sit inside the manual's 3-to-10-hour window. If your boils run long every time, the two fixes are a bigger pan or less water to remove, which is what reverse osmosis is for.