I apologize for any confusion. To me it doesn't make any sense to tear out a perfectly good oil boiler and replace it with a wood/oil combination. You're really better off, from a number of different perspectives--most of which we've already discussed--to add a wood-only boiler on to your existing system.
If anybody disagrees with that, I hope they will speak up.
Since you investigated Tarm first, I assume that you want a good wood boiler. When we hear "Tarm" we think wood gasification boilers, since they pioneered that technology in this country, and I believe that's all Tarm sells now. If the Tarm 30 is too big for your house, then you can get an EKO 25, which as the model number suggests, is smaller than the Tarm. My guess is that you only looked at the Tarm combination boiler, which may indeed be too big for your house.
The nice thing about gasifiers is that they don't produce any smoke or creosote, so they're more environmentally friendly than a conventional boiler, require little or no chimney maintenance, and are, as a result, much safer. If you buy a conventional boiler like the Benjamin you are considering, you'll have to deal with smoke and creosote and the resulting 30-40% efficiency (compared to 80-90%) that results from the smoke going up the stack instead of being burned for heat. A gasifier burns the wood in an upper chamber and then pulls the smoke down into a lower chamber and burns it at around 2,000 degrees. A conventional boiler like the Benjamin CC500 has one firebox, where wood is burned and the smoke is allowed to escape up the stack. The smoke is lost efficiency and air pollution. Simple as that. Most--but not all--gasifiers use a blower to supply air to the burning chambers. Look at the diagram on the left side of the top banner for Cozy Heat. That's a cutaway view of a downdraft gasifier. If you go to the website you can watch a video of the thing working (pretty impressive) and see all the relevant specifications.
Here's the link to the manufacturers' website and the page for the Benjamin CC500. It doesn't give any details or specifications, so it's hard to say how big that boiler is, but I'm guessing 75 or 100K btu. The EKO 25, for example, puts out 80K btu per hour. I'm always suspicious of product descriptions that spend a lot of time talking about basically irrelevant product features (i.e., "Dutch Oven" effect) without talking about specifics. What's its overall efficiency when burning wood? How big is the firebox? What's the output?
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And they talk about the "Energy Star" rating, without providing any details, either. My strong hunch is that any Energy Star rating they get will apply to the oil side only--not the wood, despite the strong implication to the contrary.
