Hydronic joints between polybutylene plastic pipe and tubing shall be ______________ joints or ______________-fusion joints.

The correct answer emphasizes the importance of using specific types of joints when connecting polybutylene plastic pipe and tubing in hydronic systems. Mechanical joints involve the use of fittings or connectors that are tightened to form a seal, which allows for disassembly and reassembly without damaging the pipes. This is particularly useful in HVAC applications where maintenance or modifications may be necessary.

Heat-fusion joints, on the other hand, involve the process of heating the surfaces of the polybutylene pipes and then pressing them together until they bond as they cool. This method creates a permanent joint that typically offers excellent durability and leak resistance, especially important in hydronic systems that operate under pressure.

The selected answer correctly identifies these two methods as essential practices in ensuring reliable connections that maintain system integrity and performance. Other joint types mentioned in the incorrect options, such as electrical connections or soldered joints, are not compatible with polybutylene materials and therefore are not recommended in this context.