Fuel and storage
Which fuels the UNI 60 accepts, how dry wood must be, the calorific value of common species, and the 4 ft fuel storage clearance rule.
The UNI 60 burns any solid fuel, but the moisture content of your wood determines what you actually get out of it. Dry oak carries nearly three times the usable energy per volume of the same oak at 50 % moisture.
Accepted fuels
Boilers are universal and work with all solid fuel: wood, coal, peat, mulch and woodchips.
Never burn
Waste, plastics or flammable liquids. The combustion of these releases poisonous gases. If there is any danger of explosion, fire, or the emanation of combustion gas or steam indoors — stop the boiler.
Moisture is the whole game
Combustion heat of absolutely dry wood is practically independent of species — about 8,112 BTU/lb. What differs between species is density, so the useful comparison is energy per volume, not per pound.
Wood humidity falls into three categories:
| Category | Moisture content |
|---|---|
| Room-dry wood | 7 % – 20 % |
| Air-dry wood | 20 % – 50 % |
| Driftwood | 50 % – 70 % |
Seasoning timeline: the lowest moisture content in a standing tree is in early winter. Firewood laid for one year under a canopy contains 20–25 % moisture; after two years, 13–17 %.
Density by species
Weight of one cubic metre of wood:
| Species | Weight |
|---|---|
| Oak | 1,540 lb |
| Birch | 1,410 lb |
| Maple | 1,160–1,440 lb |
| Alder | 925–1,410 lb |
| Pine | 1,010–1,360 lb |
| Spruce | 990 lb |
Calorific value by species and moisture
Volume calorific value of wood depending on type and moisture. Figures are calorific output for 2.2 lb.
| Species | kcal/dm @ 12 % | kcal/dm @ 25 % | kcal/dm @ 50 % | kWh/m @ 12 % | kWh/m @ 25 % | kWh/m @ 50 % |
|---|---|---|---|---|---|---|
| Oak | 3,240 | 2,527 | 1,110 | 3,758 | 2,932 | 1,287 |
| Larch | 2,640 | 2,059 | 904 | 3,062 | 2,389 | 1,049 |
| Birch | 2,600 | 2,028 | 891 | 3,016 | 2,352 | 1,033 |
| Cedar | 2,280 | 1,778 | 781 | 2,645 | 2,063 | 906 |
| Pine | 2,080 | 1,622 | 712 | 2,413 | 1,882 | 826 |
| Aspen | 1,880 | 1,466 | 644 | 2,181 | 1,701 | 747 |
| Spruce | 1,800 | 1,404 | 617 | 2,088 | 1,629 | 715 |
| Fir | 1,640 | 1,279 | 562 | 1,902 | 1,484 | 652 |
| Poplar | 1,600 | 1,248 | 548 | 1,856 | 1,448 | 636 |
For comparison, average coal calorific heat output is 6,500 kcal, or 25.79 kBTU.
Read the table as a purchasing decision
A cord of oak at 12 % moisture delivers roughly 2.9× the heat of the same cord at 50 %. Buying green wood and burning it the same winter is the most expensive fuel decision you can make.
Storage
Keep it 4 ft from the boiler
Do not stack the fuel near the boiler at a distance of less than 4 ft.
Split and stack under cover
Stack under a canopy with airflow through the pile. Covered but ventilated beats sealed under a tarp, which traps moisture.
Work two years ahead
Cut and split this year for the winter after next. That is the difference between 20–25 % and 13–17 % moisture.
Size for the firebox
The UNI 60 takes logs up to 26 in long and holds 58 gal of volume. Cut to length before stacking; re-cutting in the boiler room is a job nobody enjoys.
How much will you burn?
Estimate from the boiler's rated output and your building's heat demand, not from the firebox. The UNI 60 runs 85–205 KBTU, and a full load can sustain up to 24 hours of operation. In practice, consumption depends on:
- the building's heat loss and how cold the season is;
- fuel species and moisture (see the tables above);
- whether the system has a buffer tank, which lets the boiler run at full, efficient output;
- and whether the boiler also makes domestic hot water year-round.
Planning is done. Time to move 1,224 lb of steel into the room.
Frequently asked questions
Buffer tank and hot water
Why a UNI 60 needs thermal storage, how the buffer tank fits the scheme, and how the TIS Tronic 260U prepares domestic hot water in Summer and Winter modes.
Positioning the boiler
Unpack, move and set the UNI 60 in place — floor, levelling, clearances and the flue connection, before any pipework or wiring begins.