When can combination heating systems employing a water heater be attached to baseboards or radiators that were previously used for boiler heating?

The correct choice indicates that combination heating systems using a water heater can be connected to baseboards or radiators that were originally employed for boiler heating when additional capacity is needed. This means that if the existing heating system is insufficient for the heating demands of the space, a combination system can be a viable solution to enhance the overall heating capacity.

It is essential to ensure that the water heater used in the combination system is capable of providing the necessary temperature and flow rates to meet the requirements set forth by the heating system's design and the space it serves. This allows for an efficient and effective integration of the new heating method with the existing infrastructure.

The context around the other options highlights that while local codes and special approvals are important factors in heating system installations, they are not the primary determining criteria for when a combination system can be added. The focus is more appropriately on capacity requirements and ensuring that the system can perform effectively for user needs.