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Also apparently it's now against code to put a water heater in the attic b/c in the event of a house fire it can fall on the firefighters heads. I'm dubious; I think the attic is the best place to put them (hot as balls in summer - barely has to run).
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It's the opposite in the winter depending on geography. Depending on access to the attic, it makes monitoring for leaks/failures a little more difficult.