Built another boiler (hydronic heater) with a similar design as the one I built for myself 3 years ago for my brother-in-law. This boiler is twice the size as mine. The tank is 7 foot diameter by 21 feet long. We used about 88 feet of 6-inch pipe and made the wood box 42-inch diameter by 48-inch long. The exhaust is running a bit cold as our exit temp is about 200 degrees F as it enteres the insulated flue thru the wall. At the start of the last pass it is about 280 degrees F. If I built another one, I'd use about 20 feet less pipe in the heat exchanger. we are going to monitor the pipes closely to make sure that we are not doing any damage to the steel from condensation. The boiler is very effecient, almost condensating or possibly condensating. He is heating a 30x80 garage, house, domestin HW, solarium, and attached garage. He is heating his hot water with a 50 plate heat exchanger and has it plumbed as instintanious hot water system. So far the system works great. He fires at about 140 degrees and brings the water temp up to 195 degress with a burn. Each burn consists of loading the firebox twice, one at lighting and then a refill. His wood is about 16-20 inches long and he makes two stakes in the fire box. He fires every third day right now. He is still working on insulating the tank, but has just a bit of the front to complete. He and I worked on building it about a total of 6 days from start to firing. There is about 3.5 days of stick welding and 1 day of mig welding and about 1.5 days of preping the tank. Cost to build was about 8-9K. I didn't take any pictures, I know I know, no pictures and it didn't happen. I will take some next time I'm at his place. At first I was a little disapointed by the cool exhaust temps. On initial finging with 45 degree water, it was condensating so bad that there was a steady stream out the chimney and the exhaust pipes in the lower passes had to be drained several times. If he refires at or above 130 water temp, he does not get any condensation below the exhaust fan.