What must an oil supply line that penetrates a foundation wall be?

An oil supply line that penetrates a foundation wall must be sealed to prevent any potential leakage of oil and to protect the line from environmental conditions. Sealing the penetration helps to ensure that the system operates safely and efficiently by preventing moisture and other elements from entering the structure, which could lead to corrosion of the oil line or cause water damage. This sealing is critical for maintaining the integrity of the oil supply system and ensuring compliance with safety regulations.

Other options may not address the necessary protective measures for an oil supply line. For example, while insulation could provide thermal benefits or protection from freezing, it does not directly relate to sealing the penetration point. Painting might offer some level of rust protection if applied to the oil line itself, but it would not serve the purpose of preventing leaks or protecting the line as it passes through the foundation wall. Thus, sealing is the key requirement in this scenario.