Illinois Plumbing Code Practice

The requirement for installing water heaters emphasizes the importance of following the manufacturer's instructions as well as adhering to the state plumbing code. This is essential because manufacturers provide specific guidelines that ensure safety, efficiency, and reliability of their products. These guidelines take into account factors such as venting, clearances, and connections that are critical to the safe operation of the water heater.

Additionally, complying with the state code ensures that installations meet established safety standards and regulations that safeguard public health and welfare. The Illinois Plumbing Code, like many plumbing codes, is designed to create uniformity and provide guidelines for safe practice in plumbing installations, which is why it is crucial to follow both the manufacturer's guidelines and the applicable state regulations.

Following only local custom or disregarding professional standards could potentially lead to unsafe installations that do not perform as intended, putting residential safety at risk. Similarly, installing water heaters without the requisite qualifications or certification can result in substandard installations that could be unsafe or non-compliant with the law. Thus, option C accurately reflects the legal and safety requirements for the installation of water heaters.