Is it illegal to turn off the water valve on the first floor and not supply water to the upstairs?

This is an illegal act. It is illegal to turn off the sluice downstairs and prevent water from being used upstairs, which infringes on the legal rights of others and affects their normal life. You can complain to neighborhood committees, village committees, property management units, etc., and you can also file an infringement lawsuit. The ways to bear tort liability mainly include stopping infringement, removing obstruction, eliminating danger, returning property, restoring the original state, compensating losses, apologizing, eliminating influence and restoring reputation.

Tort is an act that infringes on the rights and interests of others, so it can also be called tort, which is a wrong act carried out by the actor. Tort includes not only the responsibility of wrong behavior, but also the responsibility of the actor based on the principle of fairness and no-fault responsibility, which is also produced by the legal system. Tort is a civil illegal act against someone, and the neighbor's act of splashing urine on others is a tort against others, which is suspected of intentional, and the neighbor's act is illegal. Tort fault includes intention and negligence. Usually, we only need to realize our own imputability, and we don't need to realize the actual damage that our actions may bring. General tort refers to the behavior that the actor directly causes damage to others based on his fault, so the general liability clause in civil law applies. Special tort is an act in which the actor is not at fault, but should bear the responsibility according to the special liability clause of civil law or special civil law. If an act is being planned and does not cause damage, it does not constitute infringement. Infringement is objective and damaging. Damage refers to the loss of other people's property and damage to people caused by behavior, and damage is the objective consequence of illegal behavior.

legal ground

People's Republic of China (PRC) Civil Code

Article 120 Where the civil rights and interests are infringed, the infringed has the right to request the infringer to bear the tort liability.

Article 165 Where an actor infringes upon the civil rights and interests of others through fault and causes damage, he shall bear tort liability. If the actor is presumed to be at fault according to the law and cannot prove that he is not at fault, he shall bear tort liability.

Article 166 Where an actor causes damage to the civil rights and interests of others, regardless of whether the actor is at fault or not, if the law stipulates that he should bear tort liability, such provisions shall prevail.