How can water drain and leak like this??!!!

I have a tankless water heater with two valves on each side of it, to isolate it. One vale is on the incoming cold water and one is on the outgoing hot water. I have to replace the unit so I turned both valves off and took it off. No problems.

Now, the valve on the OUTGOING hot side obviously doesnt close because it started leaking back onto my floor. I even drained the lines on the hot water side of my faucet. After the water drains and the faucets drip, if you shut it off, water will eventually start coming out of the outgoing hot side. How is that even possible? The line is completely isolated I thought from the cold. It has no pressure on it, especially since there is a giant hole where my tankless was.

This tankless services one shower, one sink, one kitchen sink and the dishwasher...thats it.

How is it getting backpressure to come out where there is supposedly a break from the main hot water line?

Is it somehow crossing over from the cold side via a slightly leakey mixing valve from the sink, shower, or kitchen? They are all single handle models.

It will come back through your faucets. If it happens when your faucets are closed, then there is a problem with one of your shower/sink valves. If you are using any of the faucets then you are simple allowing the faucet to complete the loop and you will have cold water coming back down the hot water line.

Make sense? You are isolated where the tankless was, but any fixture in your house that is a hot/cold is completing that isolation you thought you had.

OK...so its 'normal' that this backflows even with the hot water side faucets 'on' and 'dripping'? I mean, assuming the valves arent as tight as they were 15 years ago?

The darn ball valves, which I have NEVER had problems with, is for some reason, leaky. Sucks...I have to put my OLD tankless back on and button it up to keep it from leaking since I need water in the house.

just cap the line that is leaking. Is it a sweat valve? It's possible it was overheated during sweating and blew the seal in the valve.

If you have certain brands of single handle faucets and they have become worn, they will transfer cold water into the hot side, and if you open the hot and cold valves on a faucet the water will also backflow into the hot side.

In some situations, there may be a tempering valve external to the tankless (or any) waterheater. This supplies cold water to mix into the hot line, if it gets too hot to minimize the chances of hurting someone. So, there would be cold water being supplied to the output line. Now, should it weep into the line, probably not since it is cold, but it could. Or, as already mentioned, the valve is either damaged, or otherwise not fully closed. Some ball valves take a fair amount of push to turn completely off. Is the handle perfectly perpendicular to the pipe? If not, it may not be fully closed. They'll move fairly easily most of the way, but take a little effort to fully seat so they can seal at the end of their rotation.

Yes, its perpendicular. It was very tough to get it to close though, it was not a smooth operation, so I suspect there was grit or dirt in it or it was damaged somehow. Either way, I just put the old tankless back in and left the valves closed. Ill leave it there until I replace it.