The boiler has a minimum temperature of 20 and a maximum temperature of 500. Every 45 ticks the temperature of the boiler decreases by 1. Every 12 ticks 1 energy is consumed from the current fuel and raises the temperature by 1. This means that when heating up, a lump of charcoal will only last 96 seconds, but when the boiler is at max temperature, a lump of charcoal will last 360 seconds, equating to 43200 Liters of steam.
The boiler can hold 16,000 Liters of steam and 16,000 Liters of water. As long as its temperature is above 100, every 25 ticks it will consume 1 Liter of water and produce 150 Liters of steam, equivalent to 6 mB/t of steam. How close the temperature is to max temperature does not affect the rate of steam production. If the steam tank fills, excess steam will be discarded as it is produced, releasing smoke particles and a hiss sound each time steam is discarded.
Water can be added to the boiler by right clicking on the machine with water buckets, or by placing any sort of cell with water in the water input slot in the GUI. Like all GregTech machines, it will give the empty cell back. If the temperature is above 100 and the boiler runs out of water, adding water will cause an explosion, so wait for it to cool off first.