Max height above the tank for a side arm?

[uglydukwling]

I also have a sidearm with an outlet that's too high. I just replaced my water heater (on Christmas day) and the old one was considerably taller than the new one, even though they're the same capacity. I don't know how old "old" is, but it had a sticker on it saying that the gas company inspected it in 1969.
The result is that the outlet of the sidearm is 18'' above the tank. I reconnected it because it was easier than blocking it off, but I didn't really expect much of the hot water coming out of it to find its way back down to the tank. I was going to start looking for a shorter heat exchanger, but if other people have them working with the outlet in this position, maybe I won't have to.
Next time I'm going to be home alone all day, I'll have to try to remember to shut the gas off and see if the water in the tank gets hot.

 

[Crofter]

If you have ever siphoned water with a hose you will remember that the more rise there is to the hose the more you have to suck to get it primed but after it is going as long as the surface of the supply side is higher than the end of the hose or the surface of the receiving container it will continue to flow. You could actually have the hose rise up and back down over a vertical distance of nearly thirty feet and it would work undiminished aside from line loss.

That is gravity siphon with no temperature change. It depends on unequal weight of the two columns of water and you have a potential pressure differential of a bit less than half a pound psi for each foot of height difference; thermo siphon is much weaker and will give you a pressure differential of only about 1/400 of a psi per 100 deg F. of average temperature difference in the rising and falling (hot and cold) water columns.

The loop up, over, and back down wont destroy the action but because the driving force is weak it is sensitive to the line loss and air locks and is harder to keep vented.

 

[oneoldbanjo]

When the sidearm goes up above the water heater you will also need some way to purge the air out of the line - or the thermosiphon will not work.