Some potential disadvantages of using malleable iron black fittings in piping systems include:
Weight
Malleable iron is dense, making fittings heavier than plastic or copper options to handle.
Threading Skill Required
Properly taping threads is important for leak-free joints, requiring some training.
Thermal Conductivity
Transfers heat readily, so insulated when used in heating/cooling systems to prevent condensation.
Susceptibility to Rust
Although more rust resistant than cast iron, malleable iron can still rust without protective coatings in moist environments.
Nominal Pipe Size Restrictions
May only be suitable for smaller diameter piping depending on structural needs of system.
Paint/Coatings Needed
Exterior usually requires painting/coating for full corrosion protection at exposed joints.
Difficult Joint Disassembly
Threaded joins can corrode together over time, making repairs more complex than slip fittings.
Limited Chemical Compatibility
Not as chemically resistant as other metals or piping materials like plastics.
Cost
Generally more expensive initially than some other piping options due to material and jointing.
Proper application and installation can prevent many issues, but malleable iron may not suit some specialized high-temperature, chemical or large-bore piping needs.