In which location is it not permissible to install direct-vent water heaters?

Direct-vent water heaters are designed with safety features that allow for the efficient expulsion of combustion gases while drawing fresh air from outside for combustion. However, certain locations are deemed inappropriate for installation due to concerns about safety and the potential for harmful gases, like carbon monoxide, to accumulate.

Bathrooms represent a unique environment because they are often smaller and can have limited ventilation. The presence of moisture from showers and baths can lead to corrosion or other issues with ventilation systems, which are critical for the safe operation of direct-vent appliances. Additionally, the proximity to water sources makes a bathroom more susceptible to leaks and accidents, further risking the safety of a direct-vent water heater installation.

In contrast, living rooms, dining rooms, and garages are typically larger spaces that can provide adequate ventilation and distance from moisture, making them more suitable for such installations. In garages, special precautions are typically taken, such as ensuring that the water heater is properly elevated to prevent damage from potential flooding or spills.

Thus, installing a direct-vent water heater in a bathroom is considered unsafe, which justifies the selection of that location as the correct answer in the context of the International Residential Code (IRC) regulations.