I thought readers might enjoy exploring the issues raised in a recent e-mail. At first glance, it seems like a fairly simple straight-forward exercise, until you realize the variables and pitfalls are numerous and that the math doesn’t support the running-out-of-hot-water issue. Here’s the e-mail:   Hi Dave: I’m shopping for a water heater for a small condo building. We have six units, each has one bathroom with a shower/standard tub. I'm assuming a 2.5 GPM ...

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