Which standpipe system is used in buildings that are not heated and has water activated by a control valve during a fire?

The correct answer is the dry standpipe with a manual control valve. This option refers to a standpipe system specifically designed for use in unheated buildings. In these situations, the standpipe is kept empty to prevent freezing, as freezing conditions can render the system ineffective when needed.

The operation of a dry standpipe system relies on a manual control valve that allows firefighters to activate the system when needed during a fire. This means that when a fire occurs, firefighters must open the control valve to allow water to flow through the standpipe. This design is particularly useful for buildings that may not be in places where heating is constant or reliable, ensuring that the system can be utilized effectively without the risk of freezing.

In contrast, other systems mentioned do not fit this specific description. Wet standpipes contain water at all times and are generally used in heated buildings. The dry standpipe with a pre-action valve involves a different mechanism that combines features of both dry and wet systems, requiring an initial condition to fill with water. Lastly, the yard hydrant system is an outdoor system typically used in conjunction with a fire department's pumper and is not specifically for standpipe applications within a building.