How much wood will a gassifier save over an OWB?

Everything else being equal, I would expect you to use 1/4 to 1/2 the amount of wood used in you old OWB. The 1/4 could come from storage sized to your system and good system insulation.

Storage is the key to reducing wood consumption, especially in your milder climate. With storage you might save 50% if you had one of the better gasifiers. That's just a guess.

Six cords is nothing in an OWB. There are guys up here that burn 15-20 a year.

Additional storage "might" increase the efficiency of your wood boiler some but no specific knowledge. The OWB inefficiency comes in part from a firebox surrounded by water, thus preventing the fire from ever becoming really hot, forming creosote, smoke, etc. It also comes in part because most OWB's do not have (if at all) a good secondary burn chamber. A figure of about 30% burn efficiency is mentioned for an OWB. Overall efficiency is even less.

Efficiency from adding more storage to an OWB seems to me to come only from longer burns and fewer fire start-ups. I don't know if you would notice a difference in the amount of wood you burn.

Gasification boilers may have a large part of the firebox surrounded by the boiler water, but most importantly all exhaust from the firebox burn is directed into a very efficient secondary burn ceramic chamber where not only secondary burn occurs but extremely high temp burn - gasification - occurs (approx 2000F). This combusts almost everything, resulting in burn efficiency of 98% or more. The very hot exhaust gases now go through heat exchanger tubes (some may have hx water tubes rather than fire tubes) to heat the boiler water, cooling down to as low as 300-600F or so before going out or up a chimney.

... gasifiers shut down completely after a burn if you have storage. (As opposed to my OWB, which burns 24/7 during heating season) Are re-starts difficult without some aux source? I assume they take a while to get up to temp to get the secondary burns to start. Do they form creosote during that period of start-up?

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All wood burners will shut down when the burn is completed. Storage allows you to store excess heat and then draw from heat from storage rather than continuing a burn you don't need. Re-starts may be from existing coals, it they exist, or with kindling, etc. Takes about 4-10 minutes. Time to get up to temp (185F) depends on how cold the boiler is when the burn is started. Creosote forms in the firebox, is self-limiting, and does not accumulate except in unusual conditions.

I would have gone thru close to 8 cords in my Central OWB by now. I've used less than 2.5 cords with my Eko 40 & 500 gallons of storage. If you're considering a gasifier, I would stay away from the outdoor boiler type...

Bob

Let's put it in practical terms.

From my own experience installing many different brands of OWB's, gasifiers of various makes and Garn's I would say conservatively that you will burn 30% less wood with a gasifier than a typical OWB. Simply put, assuming a heating load that demands 10 full cords of wood with an OWB, you would handle, process, split, stack and burn approximately 3 tons less biomass per year with a gasifier than a typical OWB. Depending on the length of your heating season and a few other variables that would equate to handling about 50-60 pounds less wood per day. Well worth it in my humble opinion.

Savings can be a relative term and actually is not finalized until you quit burning wood altogether because what you don't burn this year is slated for next year. But like heaterman says: "Let’s put it in practical terms...you would handle, process, split, stack and burn approximately 3 tons less biomass per year with a gasifier...50-60 pounds less wood per day. Well worth it in my humble opinion." Imagine the pile that would be in 20 years.

This is a hard question to answer with any precision because energy losses vary widely from installation to installation, and some of the variables have more to do with technique than the boiler technology.

That having been said, a gasifier operating under ideal conditions delivers 90% of the theoretical heat energy from the wood that it burns into heating water. It's difficult to find exactly equivalent tests, but most of what I've seen suggests that an OWB operated in a typical / approved fashion with good quality wood is somewhere around 30%.

Most gasifiers are installed indoors, and therefore avoid other big potential energy losses - the buried pipes as well as loss from the boiler to the outside air.

However, few gasifiers are operated under perfectly ideal situations. Calculating my efficiency, I get numbers between 60% and 85% depending how I run the numbers. By the same token, there have been test showing OWBs delivering 25% or less in real-world settings.

The best hard data that I've seen shows a 40% reduction in wood consumption when switching from a conventional indoor wood boiler to a gasifier. The conventional wood boiler had been operated at the highest possible efficiency, and wood consumption was tracked meticulously (the wood was weighed every day).

At the end of the day, I'm confident that a properly operated gasifier will use dramatically less wood than a conventional boiler or OWB.

Everything else being equal, I would expect you to use 1/4 to 1/2 the amount of wood used in you old OWB. The 1/4 could come from storage sized to your system and good system insulation.

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I mis-spoke. I meant to say you will save 1/4 to 1/2 the wood you used in the OWB. Apologize for the misstatement.