It's really nice when I can recommend a simple solution to solve a problem. But a solution is a solution. A number of people have come seeking a solution to smelly water in cabins and vacation homes, and from what we've seen, even changing to a powered anode might not fix it if a water heater sat idle long enough.
For a long time, this page simply pointed to a possible alternative, that involves a bit more participation from the user, but which works. It makes more sense to leave be whatever anode is already in the tank, and to just rig a plumbing assembly, like the one above, onto the water heater, and pour a couple of pints of drugstore hydrogen peroxide into the tank when you arrive for a visit.
Soon the smell will vanish. You can use and drink the water immediately. The peroxide is already a low concentration and it's then diluted in gallons of hot water. That will solve the problem until you go home. You'll have to do the same thing on each visit.
People kept asking if I sold these parts. I decided maybe it was time I offered them. They are not always available locally, especially PEX-lined steel nipples.
-- Randy Schuyler
Peroxide Fix Plumbing Assembly, consisting of lead-free brass tee, ball valve and nipples, 4-inch plastic-lined steel nipple, pipe-thread seal tape and instructions; requires about 15 inches of overhead clearance:
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