Closed system vs open system
No oxygen can permeate in water, hence keeping the corrosion away. You may not need to keep the outdoor boiler away from the indoor system as it helps in saving on costs and inefficiency of plate exchanged. This system also doesn’t need a big circulator as there is pressure in the system, is durable to not treat the boiler water and no need to keep the system resupplied with water.
If boiling takes place in the boiler, then the water steams off and not ruin the plastic underground piping. But all closed system needs a pressure relief valve so that it doesn’t explode if the pressure is high. The temperature may not get as high in the closed system. But greenhouses, livestock buildings and lumber kilns may need high water temperature to work well.
Combustion rating: 100 percent
This is determined by ask content, creosote accumulation, and smoke. If no smoke and little ash and little creosote, the fire burns at peak efficiency. In a solid fuel boiler, this can be achieved by combustion air injected in the firebox at right spots.
Heating efficiency > 85 percent
The heat exchange area must be sufficient to transfer all the heat from fire into the water. This is accomplished by multi pass heat exchanger. Here, the heat from the fire is forced to pass via a set of tubes, find, or water pipes bank. The bigger the firebox, more tubes, fins or pipes are needed. If there is water around the boiler around the firebox then this is first heat exchange pass. If it is not, then heat must pass via tubes, fins or pipe banks before it exits the chimney. A clean heat exchanger is a must for efficient transfer of heat to find what fuel was used to further determine the efficiency rate of the boiler.
What kind of firebox?
Big enough to last for 10 to 12 hours but not for a couple days. Oversized ones can cause smoldering, inefficient and smoky fires. More wood is used because heating value from the burning gases cannot be determined. When lit hot, the stack temperature of these boilers can attain dangerous levels.
The best are the ones with water around to give the best heat exchange. Some are installed with refractory that makes fire hot with hot firebrick. This is ideal for burning wood, but not coal.