The heel inlet of a heel inlet one quarter bend shall be installed only in the _________ position.

The heel inlet of a heel inlet one quarter bend must be installed in the vertical position to ensure proper function and flow within the plumbing system. When positioned vertically, it allows gravity to assist in the drainage and prevents potential clogging that can occur if water and waste are not flowing efficiently through the bend.

This orientation also aligns with plumbing codes and best practices, which stipulate that certain fittings, especially those involving inlets for drainage, must be oriented appropriately to maintain clear paths for movement and minimize the risk of backflow or stagnation.

Other positions such as horizontal, horizontal to vertical, or diagonal could lead to various issues including inadequate drainage or increased chances of blockages due to the slope and angle of the fitting. Thus, the vertical position is critical for optimal system performance and compliance with established plumbing standards.