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Comment by tomasien | Hacker News Reader

Why are these illegal?

They aren't anymore. This article is from 2014; the rule was amended a year later to make an exception for grid-enabled electric water heaters.[0]

[0] https://www.phcppros.com/articles/5312-water-heater-regulati...

Great - and as I understand it the original ban was for efficiency reasons that don't apply here

They are illegal above a certain capacity. I assume that's because to large storage is wasteful: heat will leak without being used efficiently.

In general, the bigger the tank, the better the 'skin to volume ratio'. Hence I'd expect less % per second of loss. The total wattage of loss would still be higher though.

But that's not how heat works. A larger water tank has a higher volume-to-surface-area ratio. This makes larger tanks more efficient than smaller ones, not less.

This is why elephants need such large ears. Their bodies are otherwise incredibly efficient at retaining heat. Those massive, thin, flappy ears provide them with a way to expose their blood to a much larger surface area of skin.

Your average household uses somewhere between 30 and 60 gallons of hot water a day [0], and a bigger tank will just have water sitting in it, going cold. Any extra surface area costs you extra heat, even if the marginal loss decreases with growing tank volumes.

[0] http://www.fsec.ucf.edu/en/publications/pdf/FSEC-PF-464-15.p...

You can use warm water to power your heating in winter. So it might be less efficient in summer, but the amount of energy you’d save in winter would exceed the losses in summer.

Perhaps legionnaires' disease has something to do with it instead?

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