Is it permitted to turn OFF a hot water faucet on shabbos?
The answer to the question depends on how the hot water system works. For instance, if the system works on gas, so that turning off the faucet will cause a fire to be lowered or extinguished, it is prohibited to do so. The same applies to similar systems in which turning off the faucet will cause such a reaction. If, however, the system draws from a hot water reservoir (dud shemesh), there is no halachic problem in turning off the faucet.