I do not have the 17Z, but the 23B is just the big cousin. Are there two of them? Are they under the bathroom sink, at floor level? If so, they most likely are a tee handle valves, which you need to pull up on to open. If you go outside and look really carefully you will locate to pipe stubs sticking out of the floor of the HTT. They are low point drain valves for the water distribution lines. Open the valves, and the faucets and most of the water will drain out of the lines.
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