It's confusing, no doubt about that. And cost is always an issue with me, too.
All I have to go on is experience, so I may be wrong. But this is the way I understand the situation:
A gasifier is a fundamentally different design than what I'm calling a conventional wood-fired boiler. In the latter, you have a firebox surrounded by a water jacket, usually sitting on top of a grate and an ash chamber. Air comes up through the grate and burns the wood, with the smoke exiting out the back and up the chimney. You can put in various baffles and water channels to increase the heat transfer efficiency, but you can't get the temperature in the firebox high enough to achieve secondary combustion. So the result is smoke and creosote under most conditions. You can get conventional boilers with blowers or with thermo-mechanical controls, preheated combustion air strategies, etc. but they all operate basically the same way. And OWBs are simply bigger versions of indoor boilers. They're just parked out back, which has some distinct advantages and disadvantages that don't affect their efficiency.
Gasifiers are based on a different design. A gasifier has a very similar firebox, but it's sitting on a refractory-lined combustion chamber which the smoke produced in the firebox is drawn down into and ignited at about 2,000 degrees. So instead of going up into the chimney, all smoke and wood gas is sucked down into the lower combustion chamber and burned up. What's left is hot gas, which then travels through a firetube heat exchanger immersed in water on its way to the chimney outlet. Compared to a conventional boiler, gasifier stack temps are very low (300-400 vs 500-1000), indicating efficiency, since the heat is going into the water and not up the stack. And what does go into the stack is clean--no smoke and no creosote, so no need to maintain a high stack temp.
The bottom line, as I mentioned earlier, is that you can't achieve secondary combustion in a water-jacketed combustion chamber. Conventional boilers write off the loss and send it up the chimney. Gasifiers get the job done somewhere else and reap the benefits. As a result, conventional wood-fired boilers are typically 30-50 percent efficient, whereas a gasifier is going to be 80 to 90 percent efficient. That's overall efficiency--combustion efficiency plus heat transfer efficiency. Conventional boiler claims of high efficiencies are only giving you one or the other, depending on their particular strengths, not the necessary combination of both.
So yes, you'll pay a lot more for a gasifier. But if it gets the job done with half the wood and with no smoke, then it's going to make up the difference eventually. You might save money over the short term, in other words, but it won't come without a cost. Having to buy or cut twice as much wood is just one example. Fouling the neighborhood air is another.
As to recommending one conventional boiler brand over another, I think it gets down to the bells and whistles and manufacturing quality. Thicker boiler plate is always better. I think round is better than square. I've owned a Marathon and a Royall and enjoyed using both of them. If you want a conventional boiler that's going to last forever, consider either a Royall or one of those imported cast-iron rigs that New Horizon sells for about $4,000.
