You batch feed a Garn and store the heated water to improve combustion efficiency and store more energy. Unless you have a volunteer, cutting, curing, splitting, stoking and cleaning the boiler, you do not have "free" fuel.

The higher the operating temperature of any boiler, the higher the operating cost. In the case of the Garn, or boiler attached to storage, you must heat more water to a higher temperature if your heating system requires it. But if you emitters satisfy at 120°F instead of 180°F well, let me do a Dana.

1500 gallons of water at 120°F raised and maintained to a minimum 180°F. 60*8.32*1500 gallons stored 748,800 btu divide 8000bth/lb = 93.6 lbs wood.

Now if you are going to load a hundred pounds of wood you want to get your money's worth drawing the tank down until it will now heat the space. If the DWT is 120°F any "over-charge of the tank will result in less stoking and in mild weather the over-charge is unnecessary. The jacket losses lower and "system" efficiency improved.
Depending on design parameters, extruded plates cost less than combination systems, which are rarely needed except in the coldest climates with the poorest construction. Last year we installed plated where suspended PEX failed to satisfy. No wonder, since the house is block with stucco outside, plaster in and no insulation anywhere. Even with the new full-glass "door" and the limited floor space, the kitchen is up to temperature.

if you can take it from 8 cords/year to 6

If the boiler is in living space, the efficiency difference in this case is pretty much zero. If not, then it's likely to be < 5%. On the other hand, stamped aluminum plates are inexpensive and more storage, greater margin for error (or added floor coverings) and faster warm-up might be useful.

garn's are huge. they are installed remote from their buildings.

Posted By NRT.Rob on 01 Aug 2013 04:42 PM
garn's are huge. they are installed remote from their buildings.

Yeah- even the tiny "Junior" has 980 gallons of tank, and may not have all of the necessary ratings to be code-legally installed inside a house in every jurisdiction.  The formerly-smallest 1500 is about 10' x 6' x 6',  and you still need the clearances to the walls, and room to open the door.  The Junior is a more modest 7' 4" long but still over 5' on the other dimensions.