Some recent excellent discussions about Veissmann's (as yet unavailable in the USA) modulating wood-fired boiler (I think it's the Vitolig 200) have got me pondering the whole concept of modulation. I guess, to start with the fundamentals, "modulation" can be defined as the boiler controller automatically adjusting the boiler's heat output to conditions. So, for example, a 100 KBTU/hour modulating boiler would actually be capable of producing variable heat output, effectively making the boiler many boilers in one. I know this is what the premium gas-fired boilers being sold today can do, but I guess I don't understand why. Presumably there's some advantage in running a gas boiler for a longer period at a lower output, but I don't what that would be. When it comes to wood gasification, I can see some real potential advantages to modulation, namely lessening the need for hot water storage and allowing the boiler to run for longer periods with the same amount of fuel during periods of low demand, without idling. The Veissmann literature says that the Vitolig 200 achieves modulation by varying the draft blower speed, which doesn't sound like a big deal, but I assume there's a lot more to it than that. I also notice that the blower on that boiler is mounted on the exhaust side so that it's pulling instead of pushing, like the fans do on the more common gassifiers like many of us have. Somebody explained this the other day and their explanation made sense to me. Any thoughts or clarifications would be more than welcome.