What component must always be included with a bottom-fed water heater?

A bottom-fed water heater requires a vacuum relief valve as a critical component. The purpose of the vacuum relief valve is to prevent negative pressure from forming within the tank. This can occur when hot water is drawn from the heater, which can create a vacuum if the water supply is suddenly interrupted. The vacuum relief valve allows air to enter the system when such a vacuum occurs, thereby preventing the tank from collapsing and ensuring the safety and integrity of the water heater.

Including this valve is essential for the proper operation and safety of a bottom-fed water heater, particularly in systems where the water is drawn from the bottom of the tank. This contrasts with other components, which may serve important functions but are not strictly necessary for the operation of a bottom-fed system.