Could it be back-draining out of your house pipes? Open a hot tap and see what happens. There should be suction I'd think.
I'm getting ready to do the same thing with my electric. It takes forever to drain out that little valve due to scale buildup in the bottom. A ball valve will help I think. I have to change the bottom element yearly as the scale builds up to it and ruins it.

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You're a daisy if you do.
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84 Euro 240D 4spd. 220.5k sold
04 Honda Element AWD
1985 F150 XLT 4x4, 351W with 270k miles, hay hauler
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