How the UNI 60 works

Inside the UNI 60 — the firebox, the three-pass flue path, the forced-draft fan and the TIS Tronic 260U controller, with the labelled factory section drawings.

The UNI 60 burns a full firebox of wood slowly under fan-controlled air, then pulls the hot flue gas back through three heat-exchange passes before it reaches the chimney. The controller modulates the fan to hold a target water — or exhaust gas — temperature, so one load can run for hours instead of burning out in one rush.

The combustion path

Sectional drawing of the UNI boiler showing the body, furnace, third flue-gas pass, chimney, grate, ash pan, doors, pipe connections, fan and controller, with airflow arrows — **Picture 1 — boiler structure.** Arrows marked 18 show the direction of air during combustion. Source: UNI Boiler Manual §6.0.

The numbered callouts in the drawing map to these parts:

#	Component	#	Component
1	Body	10	Service flue gas door
2	Furnace	11	Supply pipe connection
3	Flue gas 3rd pass	12	Return pipe connection
4	Chimney connection	13	Drain pipe
5	Grate set	14	Temperature sensor well
6	Support	15	Fan
7	Ash pan	16	Controller
8	Fuel loading door	17	Ash door
9	Ash removal door	18	Direction of air during combustion

The ash pan sits outside the boiler

Unlike the drawing's internal position, the ash pan on delivered units goes outside the unit, at the front of the ash door.

What happens on one load

Loading. Split wood goes in through the fuel loading door (8) onto the grate set (5). The UNI 60 firebox holds 58 gallons and takes logs up to 26 in long.

Ignition and firing up. The controller runs the fan hard for the firing-up period to establish the fire quickly. In Brager Expert exhaust gases mode it holds the exhaust target at 300 °C during this window.

Controlled combustion. The fan (15) forces air into the furnace along the path shown by arrows 18. Because air is metered rather than free, the load burns from the top or the bottom over many hours instead of all at once.

Three-pass heat extraction. Flue gas leaves the furnace (2), turns through the flue gas passes and the third pass (3) above the firebox, giving up heat to the water jacket on the way, and exits through the chimney connection (4).

Supervision. Once the water reaches setpoint the controller drops into supervision: the fan stops and only pulses briefly at a low speed to keep the coals alive without overheating the boiler.

Quenching. When the fuel runs out — detected by falling water temperature or falling exhaust temperature — the controller stops the fan, lights the STOP indicator, and optionally stops the circulation pump.

Inside the water jacket

Three-dimensional cutaway of the UNI boiler showing water jacket, mineral wool insulation and steel plate thicknesses around the firebox and heat exchanger — **Boiler internal section view.** Water surrounds the firebox on all sides; mineral wool insulation sits between the water jacket and the outer casing. Source: UNI Boiler Manual, fire safety rating addendum.

Water wraps the firebox and the flue passes, and the whole assembly is wrapped again in mineral wool. The plate thicknesses called out in the drawing range from ¼ in on the outer surfaces to 1½ in at the heaviest sections, which is what gives the boiler its thermal mass and its 1,224 lb weight.

The insulation and steel both carry fire safety classifications:

Material	Thickness	Class rating
Mineral wool insulation	1½ in	Class A1
EN 10051/10 mild steel	¼ in	Class A*

* Subject to individual material tests. Class A materials have a flame spread index of 0–25 under ASTM E84 / E119.

What the controller adds

The TIS Tronic 260U turns a wood fire into a thermostatically controlled heat source. It:

drives the fan with variable speed, which is the only real throttle a wood boiler has;
reads the boiler water temperature (T1) and, optionally, the exhaust gas temperature (T3);
starts and stops the central heating pump (Q3) and the domestic hot water pump (Q4);
responds to a room thermostat on I2;
and is backed by an independent STB bimetallic sensor on I1 that cuts the fan if the water passes roughly 90 °C, regardless of what the software is doing.

That last point matters: the safety chain is hardware, not firmware. See Electrical and sensors.

Fuel flexibility

The UNI is a universal solid-fuel boiler — it runs on wood, coal, peat, woodchips and mulch. Fuel choice changes both output and maintenance intervals: wood deposits more tar and soot in the chimney than coal, so wood-fired units need more frequent sweeping. Details are in Fuel properties.

Never burn waste

Burning waste, plastics or flammable liquids releases poisonous gases and will damage the boiler. Use recommended fuels only.

NextSpecifications

Every published figure for the UNI 60, plus the full UNI range for comparison.