The Implied Warranty of Habitability

Except in Arkansas, every state recognizes an implied warranty that residential rentals meet basic health and safety standards: weatherproofing, working plumbing, heat, hot water, functional electrical, and freedom from pests and structural hazards.

The warranty cannot be waived in the lease. Attempting to waive it can void other lease terms and can trigger local penalty statutes.

Once the tenant gives notice of a problem, you generally have a 'reasonable' time to fix it. Reasonable depends on the severity: no heat in January is hours, a dripping faucet is weeks.