The bypass valve is most likely the 'winterizing' valve that allows water to bypass the water heater for winterizing. In the photo, the valve is clearly shown right in the back of the water heater. It's often not so straightforward. In my RV the WH is on the driver's side but the bypass is on the passenger's side next to the water manifold.
To winterize, you'd shut the bypass valve
ON, and shut
OFF the valve in the line supplying water into the tank. At the front of the water heater tank, you'd remove the drain plug and open the pressure relief valve to drain the tank. This would send water from the cold supply line to the hot water line to the rest of the RV, bypassing the tank. If you use RV antifreeze or just air pressure to winterize, no need to fill the WH tank with antifreeze.
For use, the bypass valve is closed (
OFF) and the cold water supply line is opened (
ON), making sure the drain plug is back in and the pressure relief valve is closed.
Note: some set ups use a third valve at the hot outlet of the WH. This photo show no hand valve, it has a one-way check valve at the tank outlet.
Hot water tank
Linear Mode
