Unfortunately we don’t live in an ideal world, at least nothing ever works out ideally for me. You should not have any trouble using a metal fuel storage tank in house. For one thing, if it’s in the house you shouldn’t have water condensation in the tank like if it was outdoors. I know in my house it is ‘dry’ in the winter. Also, biodiesel is used in metal fuel tanks on vehicles with no trouble. If the biodiesel is properly washed and dried I don’t see any problems using a metal tank. I’m going to be using a regular HHO tank in mine. If you are „really” concerned, use a stainless tank or coat the inside of the steel tank with a chemical resistant paint such as por-15.
An alternative to copper fuel lines are PEX tubing. Its an aluminum tubing with a poly liner that is impervious to biodiesel, and complies with the code. Often used for sub-floor heating systems, its durable and comes in a variety of sizes. Use the PEX fittings for a good seal. The storage tank could be a standard 275 gallon steel tank. Steel is compatible with biodiesel. Use a new tank to avoid the loosening of accumulated sludge often present in old fuel tanks. Are you going to burn B-100? That could present some special problems that have been discussed elsewhere on this site and other websites. Maybe better to start with a mix, say B-20, and see how it goes.
All known oil tanks and systems are compatible with Bioheat at blends of 20% or less. For higher biodiesel blends, up to and including 100% biodiesel, compatibility will depend on the materials (metals, plastics, and rubber parts) in your tanks, pumps and fuel lines. For blends higher than 20% biodiesel only steel, mild steel, stainless steel, aluminum, fluorinated polyethylene, fluorinated polypropylene and fiberglass vessels are recommended. Use of tanks or lines made of brass, bronze, and copper or lead, tin, and zinc (i.e. Galvanized) may cause high sediment formation and filter clogging and are not recommended.
Have you looked into using a solar water heater to heat WVO? I have 1056 ft2 available in mid Western Wisconsin and room for a 265 tank in the basement. Solar gain is ok… I commonly have 140*F on sunny winter days. My data says this should be enough area for two collectors, one for hot water and the other WVO. Preferred system would have PV panel and pump to a gravity drain, dewatered, filtered WVO solar collector… WVO in the collector… no sun – no circulation, no additional heat exchanger, auto-magic draining, no freezing issues. Pump head would be roughly 21ft so I’d be pushing the limits on single stage pumps but once it’s running there could be some siphoning. BTW, I have an Ag. Eng. Degree, MWPS… Any ideas?