Why is cold water coming out of my HW OUT-TAKE pipe?

Question

I'm replacing our water heater.

I disconnected the cold intake and hot out-take. I'm replacing the valve on the cold intake.

I got to the point where I had both the cold in and hot out disconnected, and a new valve on the cold intake (so I can turn it off and restore at least cold water to the house).

Here's my huge problem:

If I leave the cold water intake valve open and turn on the water to the house again, cold water comes pouring out of the cold intake pipe as you'd expect.

If I then close the cold water intake, water then starts coming out of my hot water out take!?

Normal?

If not, what could be going on here?

UPDATE:

I'm trying to run this through in my head and I can't think of any situation where the hot water lines would be connected to the cold water lines...except possibly at a mixing valve somewhere? One of the showers, perhaps?

If that's the case, is there any harm in letting it be? Or am I looking at some major plumbing tear-outs to figure out why this is happening?

Answer 1

If this happens when all of the faucets in the house are closed, it would have to be a mixing valve of some sort.

If any single spout faucets are open (perhaps you had them open to bleed air out of the lines), they will allow water to move between the hot and cold sides if both valves are open (normally, this doesn't happen, because both sides are pressurized equally, so all the water comes out the spout).

A smaller possibility would be a recirculating pump that is meant to make sure there's ready hot water at a distant fixture -- these usually work by pumping water from the hot pipe near the fixture back into the cold near the tank. It's possible that water could move backward through that in your situation.

I would check that all valves are closed (including making sure the clothes washer is off, since it could be trying to mix hot and cold) and then test again.