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Edit: Based on response, I needed to clarify a few things...

I was recently trying to drain my hot water tank. I turned off the cold supply, opened up a few hot faucets, started draining, but it seemed as though it was taking forever, much longer than the 40-gallon capacity would need.

Convinced something was wrong, I detached the hot line, and then turned the house supply on - WITH THE TANK SUPPLY OFF - and cold water came gushing through the normally hot outbound side! Turned it back off, no more water. For a time, I became convinced the builder's plumber put the tank's supply cutoff on the wrong line.

With the house water turned off, the tank drained in just a few minutes. When I detached the cold line, then restarted the water, the cutoff line held (confirming that valve was not leaking), and cold inbound water continued to come out the normally hot outbound side.

I'm no plumbing expert, but the only thing I could infer is that supply water is jumping onto the hot side, and that would presumably only occur where the two would physically mix, such as in a single-handle faucet cartridge/pressure balancer. Is there some other possibility I'm missing?

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TL;DR I think your setup is perfectly normal and functioning properly

The description is a bit confusing, but I have a theory (and if it is wrong, someone who knows more than me, like a professional plumber, will come up with the correct explanation):

Hot water can only come out of the tank if there is something else pushing out. In a normal setup, that is the cold water. But that doesn't work very well if you are trying to empty the tank. So the other possibility is air. But air needs a way to get into the tank, and normally the tank is airtight.

  • Turn off cold water, open hot water faucets

This is what you tried first. Some hot water came out, but very slowly. That's because the vacuum created by draining some water out was only very slowly replaced by a little bit of air making its way back through the hot water faucets.

  • Detach hot line, turn on house supply

When you did this, the cold water was now flowing back into the tank and immediately flowing out the hot line. That is to be expected, as that is how the hot water tank normally works! Except normally it is full of hot water and the cold water goes in one side, gets heated (unless you are using hot water faster than the tank can heat it up) and hot water comes out the other side. Since you had already drained some of the hot water, you got a mix of hot & cold and probably more mixed up than usual - and therefore relatively cold.

  • With the house water turned off, the tank drained in just a few minutes.

If this was with the hot line detached then it makes sense. The tank would now be able to drain quickly because with an open hot line, air can get in easily, unlike the situation with the hot line connected over many feet of pipe (filled with water) going to the open faucets.

  • When I detached the cold line, then restarted the water, the cutoff line held, and water continued to come out the hot side.

Also normal. As long as the cutoff is between the "detached cold line" and the tank, there is no real difference whether that cold line is connected or not. So water continued to come out the hot side just fine.

  • supply water is jumping onto the hot side

Actually, that happens normally inside the hot water tank.

  • Based on this answer, I realize my question WAS very confusing and muddled. I was referring to cold water coming back from what would be the "outbound" hot connection, as if it were feeding the tank. The supply side at the tank was turned off; the house supply was on. I'm not understanding how house supply backflows such that it is coming in what would normally be the hot outflow side. Hence, my inference it is coming from a place where the two physically mix, creating a path from home house supply. Separate faucets are necessarily eliminated. – David W Feb 10 at 19:56
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    I think I follow what you're saying (though not 100%). One possibility would be a single-handle faucet turned "all hot" but that was really mixing. In that case, cold would have the advantage because it was under pressure where the hot was not. – manassehkatz Feb 10 at 20:00
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    Bingo. Sorry for the lack of clarity! I was up a little too late wrestling with this last night :) I think I have a bad shower cartridge in one bathroom. Thanks! – David W Feb 10 at 20:05

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