If I turn on a hot water tap on Shabbos by accident, am I allowed to switch it off?
If you accidentally opened a hot water faucet on Shabbos and let cold water enter a tank that heats water, it is forbidden to close the faucet. If you let the water continue running, the cold water that came into the tank will never get hot. If you close the tap, then the cold water that entered the tank will heat up and cook. It is forbidden to cause water to cook on Shabbos. The only recommendation that I can offer in order to conserve water (which is important for Eretz Yisroel and may even be a financial concern) is to stem the flow to a trickle. You must allow enough water to flow from the faucet that would make it impossible for the cold water entering the boiler to reach yad soledes bo