Do I think the smallest one would work, I think so but I would call central boiler and talk to them. They have been great to work with. As far as leaving the wood shed open, yes I do get rain and snow on the wood but I also get wind to dry it off.The model you are looking at would cover my heating needs now but I hope to build more heated buildings in the future so I went with the next size up (500,000 max BTU).