Residential Hot Water Mystery

[Distantdrummer]

I am puzzled by a water heater circumstance in my residence. Not sure if this might also apply in our Airstreams.
My heater (A.O.Smith gas-50gal) has an intake line (cold water in) with a valve and an outflow line without any valve.Both lines above and into the attic as my plumbing supply is overhead.

Yesterday I needed to replace a hot water spigot behind the washing machine and replace an overflow line out of the water heater pressure release valve.
So instead of going out and shutting off the total water supply at the meter, I thought I'd just shut off the water heater incoming line and extinguish the flame. Doing this I then opened the lowest hot water spigots (bathtubs) and thought this should drain the hot water lines. Well the drainage never stopped.
So I also drained the water heater at it's low spigot.There was not much there.And I opened the lavatory hot water spigots as well. The water never stopped flowing (albeit slowly).And it was long not hot. I assume there was some bleed/blend of the cold and hot water lines.
Of course I finally went out and shut off the total water at meter and completed the repairs.

Why did I have this problem. Doesn't all hot water have to flow from (and through) the water heater? If so where did the water come from? The valve sounded and felt like it had fully shut down the inflow.
This is making me goofy with wonder.
I know you guys can bail me out. (pun intended).

Thanks

Oh and should I have a valve on the outflow (hot) of the water heater. Is there some safety reason not to? If one was there and I shut it, would that eliminate my above mentioned circumstance?

[cammur]

Plumbing, always fun, in the home or trailer.

I have a shut-off valve where the water flows in to the waterheater, and no valve where it flows out. This should normally take care of things.

I have had single-control fixtures in my house that leaked not from the spigot, but from one side to the other, so that some hot would leak to the cold water side, and I suppose vise-versa. That could be your case.

I also had a house with a recirculating pump for the hotwater side. All the hotwater fixtures were linked in a loop, and the return was connected to the coldwater input to the hotwater heater. I believe this also allowed hot water to enter the cold water lines feeding the house. When my waterheater was replaced I had the guy install a check valve so this couldn't happen.

[TG Twinkie]

If you have mixing valves in your plumbing system that could be your problem.

[garry]

Could be the shut off would not seal completely or if you have single handle faucets and opened one of them in anything other than full hot that could be a problem.

[kennethowens]

siphon.....pulling

[Splitrock]

The only water supplies to my water heaters are through the cold water intakes. If I shut off the intake water, and the water continues to come in, I have a bad shut off valve. The reason I have shut offs at the water heater in and out is so I can change one of them out without shutting off the water to the whole building.

I only drain a water heater by opening the clean out on the bottom. That's the only way I know of to drain a water heater. If I open a tap somewhere, the best I can do is take the pressure off as well as drain the whole system. In my home I have bypasses on both heaters as well as shut offs on both the in and out sides. I can isolate one of both heaters from the building system without shutting down the system.

Gary