I've been reading for a week or so, and I appreciate all the free knowledge available on this site. Looking for comments, concerns, questions, and suggestions to improve my plan. I am planning to build an outdoor wood boiler, relying heavily on my father-in-law who is a retired welder/fabricator with over 40 years of experience, which is a good thing considering I hardly know how to turn my mig welder on, much less use it to weld watertight. Here’s the plan: I have a 300 gallon diesel fuel farm tank that I have had for years that I plan to use as the water jacket (hasn't had fuel in it in over a decade). It’s relatively thin metal (probably 1/8”?) but if it’s designed to hold fuel, I figure it would make an OK water jacket. It measures 38” in diameter and is 60” long. I plan to have a piece of 3/16 steel rolled into a 24” cylinder for the firebox. Not sure on how long to go. I don’t want it to go completely the whole 60” length of the tank, so I’m not sure if I should use a rear exit flue or a top exit. I will have the blower with damper controlled via either aquastat or ranco controller, not sure which I plan to use yet, so I’m open to any suggestions. I would like to install an ash pan in the bottom of the box with some grating, but cannot figure out how I want to build the door or where I can buy a pre-made door. Also, I would like to put the air in from the back side of the firebox rather than at the door if possible, but that would likely mean I would have to run the firebox the full length of the water jacket, so I’m still pondering how to do this. I am planning on buying pre-made insulated pex, so I am trying to make this work with only 1 pair of 1” lines due to cost (rough estimate of about 130-150 feet of run, depending on my exact location of the boiler). My plans are to have a 20-plate 5X12” brazed plate heat exchanger on the water heater (82 gallon electric water heater) first in the circuit, then to the air/water heat exchanger in the ductwork, then returning to the boiler. I also plan to put a “T” in the return line to the boiler and tie into my domestic water supply via a ¼ turn valve for a means to top off the boiler without having to stretch out a garden hose. I’m thinking I should come off the hot water side of the water heater to keep from shocking the boiler so much with cold water during top offs. I do plan to install a thermostatically controlled anti-scald valve to the water heater to prevent kids from getting burned by the hot water in the house. My current setup has a heat pump with 15kw backup electric strips, which have proven to be very costly, not to mention the dual 4500 watt elements in the water heater eating $$$, especially in the winter time. The house is 1800 sq ft of living space, and is fairly well insulated (built in 2003). Since my heat pump has a variable speed blower, from what I have read, I should be able to utilize a second aquastat surface-mounted on my supply line from my boiler and have it hold the contacts open on my yellow wire to my outside unit on the heat pump unless the boiler water temp falls below say 120 degrees (indicating fire went out) whereby it would then close the contacts to allow the heat pump to run normally and allow the backup heat strips to function as normal, and I would be able to continue just using my normal thermostat. Otherwise, I would run into issues of the blower not wanting to spin up to high speed. Since I’m only planning on one set of lines from the boiler for heat and hot water, I would just run my circulator 24/7 as many people do, and I’ve heard that the circulators seem to hold up pretty well like this and don’t use a whole lot of electricity anyway (about similar to a 100 watt bulb). I am open to any and all suggestions, and I have looked at a number of different builds on this site and continue to be impressed with everyone’s ingenuity and the amount of thought and design that go into these builds. I've run various calculations on the size of the firebox, determining how many gallons would be left for the water jacket, etc., just not sure how much of each is enough or too much. Looking forward to some good advice! Thanks in advance.