Manual J and Manual D load calculations are the way HVAC contractors measure the amount of heat gain and loss that each room undergoes during peak – extremely hot or cold – conditions. These measurements allow contractors to ascertain the size of HVAC equipment a building needs.
Having a properly sized HVAC system is extremely important. Equipment that is too powerful for a building will use more energy than necessary to keep it hot or cold. Weak systems will not be able to keep a building at a comfortable temperature reliably.