Older homes had little insulation, used single pane windows and were very leaky. Since energy was plentiful and cheap, heating and cooling systems were oversized. This was okay back then because the leakiness allowed air to flow through all the assemblies, preventing moisture from building up in walls and in the roof. Today it is a very different story. Houses are tighter, have higher insulation values and allow very little air to pass through them. To match a home to a new heating and cooling system, and to make sure there is adequate ventilation, Manual J, S and D calculate the heating, cooling and ventilation requirements. This allows contractors who read them to properly size HVAC systems.


In addition to just sizing, these documents can specify the make and model of the systems, calculate the duct width, height and length; register and return grill size, and diagram the locations of all these components. Leaving nothing to chance, Manual J, S and D provide close to engineering quality documentation for use by the installer. Modern homes no longer need oversized systems to maintain comfort, safety and durability. These allow proper sizing, saving money on systems and sharply reducing energy costs.