Load Calculations

Manual J is used to determine the required heating and cooling loads for each room of the house. That’s just the starting point, though. Once you know the loads, you need to find equipment that will supply that load. It’s not simply selecting a unit rated for the loads. It’s what a unit can actually deliver once installed at a location. This is where Manual S comes in. Manual D is used to size the ducts and Manual T is used for the terminals (registers).

Many units that are installed are oversized. This can lead to other problems. For example, if a unit has a capacity that is too great, it will not operate as long as it should so it will not have time to remove enough moisture from the air. The water vapor in the air can cause a host of other problems in the home.