How the hell does that work, wouldn't the pressure equalize almost immediately?you dont need a pump....gravity will move the water if you have a seperate return line coming off the hot side of the shower piping...run it to the drain line at the bottom of the water heater....you can add a valve to the system to throttle it back(and save $$)...it will work so good you can most likely throttle it back 1/2 way.....ive even done this in the basement right under the shower when i couldnt open up the wall....even 8' of gravity was enough to make it work....the higher you can get the return line the better...try to pitch the pipe flat or pitched down/back to the water heater
another thing ive been using alot lately is the Grundfos comfort system...its an after the fact recirc system...it recirc's the hot through the cold water....the valve goes under any sink...it comes with a pump...it needs to push the hot...gravity wont work without a 3rd water pipe(return line)
It's lighter, that's why in a hot water tank the hot water comes out the top. In a shower loop without a recirc pump I think you are just letting convection do the work.I have a well for my water supply, and did it to not waste the pump! I put a return line, "AS" stated above, from the hot line, before the tub/shower valve, returning to the hot water tank. Put a check-valve in it, and remove the plastic crap on the bottom of the tank drain with a "t" to connect return line, with a shut of valve to regulate, with a new valve to drain the tank, and be happy forever! It might take a few to get it to work correct, but it will! My basement is cold in the winter, and to get it started to work, I had to flush the drain for a minute or two, then was fine!
Opps! how does it work? Help me out here if I am wrong, But I was told that HOT water is heavier than Cold??? I have not a clue nor good reason!! LOL It worked for me.