There are several places that you may be losing energy in your OWB system. One is the boiler itself. If it is not well insulated it will radiate the heat out into space. Next is the PEX lines to the house. If they are well insulated and buried deeper than 2 feet in a cold area (18 inches around here where the ground rarely freezes more than an inch), and kept dry, they should not lose too much heat. From what I know, the biggest place to lose heat on an OWB is through ground water contact with the PEX lines. For that reason I insulated my one inch PEX lines (I used one red line and one blue line for hot out and cool return) with foam and then shoved them into 4 inch corrugated plastic drain pipe. Then I sealed up both ends with that spray foam stuff. The R value is not that great, but the fact that they are away from any water that can rob heat is important. Also the ground is a pretty good insulator.
Another issue can be the wood itself. If you are using wet or green wood it will not heat nearly as well as seasoned dry wood will. Also wood heat value will vary by species. Oak and madrone have 2x the heat value of cottonwood or light pine. That being said, OWBs tend to be wood eaters, and we go through about 5 cords here in fall/winter using ours. That is a mix of medium and hard heavy wood (oak, madrone, doug fir, alder, grand fir, and hard and soft maple). But that is better than the ten cords that the neighbor burns in his fireplace for heating. Milder climate here in the west; temps lately here have been high 20's at night and high 30's during the day. Snowed already here this year two weeks ago. We do not get much snow here. Maybe half a foot of snow a year total. Rain is another matter... we get lots of rain. 6+ feet a year on average.
