“Yeah, I didn’t get that one either. New slang that we haven’t yet picked up on?”
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I think a whole lotta people call it a “hot water heater” - as if you need to heat up hot water, heh.
You’ve got the hot water line and the cold water line. You need to put the hot water heater on the hot water line, else you’ve got like you do at my house, where you have to turn on the cold water in the shower to get hot water, and vice versa.