My uncle and I are on the wood boiler train. He purchased a Harman wood/coal boiler and I'm still shopping around. I'm more internet savy so I thought I'd ask around. He's in the midst of plumbing his as a suppliment to an oil boiler. I had mentioned the benefits of adding storage and how this can be achieved with a propane tank. Luckily, he has a friend in the propane business so we can get our hands on 1 or 2. He came up with the idea of burrying a tank under a patio slab outside. His method of constructing would be to encapsulate the tank in thick poly sheathing, sprayfoam the tank and surround with EPS or ISO insulation. I've yet to come across a thread for underground storage so I don't know if its a good idea or not.