In addition to variable fan speed to reduce gasification, chimney draft also needs to be controlled, and these two are somewhat independent. I have a speed control on the draft fan on my Tarm, and it does work to slow down gasification. Depending on temperature, wind, and other factors affecting draft, however, slowing down the draft fan does not always result in a lower draft being pulled through the firebox by the chimney, as measured by stack temp. Another consideration is the MC of the wood load, which often varies somewhat from load to load, even with wood from the same stacks that have been seasoned the same length of time. Drier wood generally burns hotter and gives off more gas more quickly. Another consideration is even if gasification is slowed down, is the firebox turning out wood gases that due to reduced air flow are not being combusted in the gasification tunnel and simply escaping up the chimney? If this is happening, then the boiler is operating in part just like an ordinary wood stove. It seems to me that although likely possible, the parameters to be measured and controlled to achieve the most efficient final combustion are several and interrelated, and achieving the desired result is fairly complex.
This for me all gets back to the ultimate goal of achieving nearly complete combustion/gasification, on the one hand, and, on the other, capturing as much of the resulting heat as possible without condensation of the final flue gases. I have to assume that the Tarm is pretty well engineered to achieve nearly complete combustion.
As to capturing the heat, I am limited by the hx tube engineering in the Tarm, which I have supplemented by adding the chain turbulators combined with regular cleaning of the hx tubes. I also have plenty of storage which greatly has reduced idling, although idling begins and then increases as tank temp rises. This means that the Tarm is transferring more heat to its boiler water than my system demand (actual demand plus storage demand) can use.
The "simple" solution appears to be a modulated demand heating system that can absorb the extra heat when needed to prevent boiler idling.
What about a 2-stage storage system, combined with a heating system that can efficiently operate at, say, 110-130F water (assumed "no-idling" range for boiler)? The primary stage storage would be relatively large and the secondary stage storage relatively small. System demand would first deplete the secondary storage to as low a temp as would be usable, then switch to the primary storage until it approached low temp. The boiler then would be fired, first to raise the primary storage to the "no idling" temp, and then until the wood load was burned out switch to raise the secondary as needed. Since the secondary would start at a very low temp, there should be no idling as the wood load burned to completion.
So far, with less than 1 week of experience of using the boiler, the Tarm Solo is exceeding my expectations. I must admit a tinge of envy wondering what the newer model is like, but the fact that there is a newer model does not make the previous model any less capable than it was before. Ever been married? Ha ha... I better not go there.
