Alothough you seldom "see" it, the washer discharge water often backs up into the drain pipe. This is why new Plumbing codes are for 2" drains instead of the old 1-1/2". Newer washers also have higher discharge rates than the older models. Once you secure the heater drain in, you will not likely ever check to see if it is clogged, until it backs up. I agree that heaters seldom, if ever, need the drain pan, but it's that one time that will make you wish you had done it correctly. Many States require the drain pan to exit outside the home.
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