Fuel properties
The energy content of firewood by species and moisture, the effect of seasoning, and how fuel choice changes UNI 60 output and maintenance intervals.
Combustion heat of absolutely dry wood is about 8,112 BTU/lb regardless of species — so what separates oak from spruce is density, and what separates good wood from bad is moisture.
Accepted fuels
UNI boilers are universal and work with all solid fuel: wood, coal, peat, mulch and woodchips.
Never burn waste
The combustion of waste, plastics or flammable liquids can release poisonous gases. Use only recommended fuels. If there is danger of explosion, fire, or the emanation of combustion gas indoors or steam — stop the boiler.
Moisture categories
| Category | Moisture content |
|---|---|
| Room-dry wood | 7 % – 20 % |
| Air-dry wood | 20 % – 50 % |
| Driftwood | 50 % – 70 % |
Higher moisture in wood, the lower its calorific value. The lowest moisture content to cut a tree is in early winter. Firewood laid for a year under a canopy contains 20–25 % moisture; after two years, 13–17 %.
Density by species
Weight of one cubic metre:
| 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 |
Volume calorific value
The volume calorific value of wood depending on type and moisture. 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 |
Average coal calorific heat output is 6,500 kcal, or 25.79 kBTU.
The two decisions that matter most
- Species — oak carries roughly 2× the energy per volume of poplar at the same moisture.
- Moisture — the same oak loses roughly 66 % of its value going from 12 % to 50 % moisture.
Get both right and the same firebox delivers several times the heat.
Fuel and maintenance intervals
Deposits and gum formation by condensation depend on the fuel type — wood more than coal — on the chimney draft, and on the operation mode.
| Fuel | Chimney fouling | Notes |
|---|---|---|
| Dry hardwood | Moderate | Best balance of output and cleanliness |
| Wet wood | Severe | Tar and condensation; expect frequent sweeping |
| Coal | Lower than wood | Higher calorific density, more ash volume |
| Woodchips / mulch | Variable | Depends heavily on moisture |
MBtek recommends boiler cleaning in mid-winter at least once a week — see Cleaning.
Sizing your fuel
| Property | UNI 60 |
|---|---|
| Maximum log length | 26 in |
| Firebox volume | 58 gal |
| Burn time on a full load | Up to 24 hours |
| Fuel storage clearance | 4 ft minimum from the boiler |
Ending the heating season without leaving problems for next year.
Frequently asked questions
Daily operation
Running the UNI 60 day to day — loading the firebox, removing ash, watching pressure and temperature, and the habits that keep efficiency up.
Seasonal shutdown and start-up
Close the UNI 60 down at the end of the heating season and bring it back safely — cleaning, freeze protection, water in the system and the pre-season electrical check.