Why do a Manual J Load Calculation - Enser Consulting - Residential HVAC Manual J, D & Energy Calculations

Oversizing

The most common mistake made in sizing a residential heating and cooling system is oversizing. Issues with an oversized heating and cooling system include:

Higher initial cost of the heating and cooling equipment
Inefficient operation yielding higher operation costs and equipment breaking down more often
Oversized air conditioners do not operate long enough to dehumidify the air, which results in moist air and possible mold growth
Oversized furnaces create large temperature swings in the home

Using a "rule of thumb" estimate based on the square footage of the home without taking into account the construction of the home and other factors can lead to an oversized system.

Correctly Sizing a Heating & Cooling System

A correctly sized heating and cooling system will be sized using the Manual J load calculation procedure. The Manual J load calculation procedure takes into account the following items:

Local climate and actual orientation of the home
Insulation R-values
Roof and attic construction
Wall and floor construction
Basement and crawlspace construction
Window U-values, shading, insect screens & overhangs
Door U-values
Tightness of construction and infiltration
Number of occupants
Major appliances
Several other factors

For new homes, the Manual D duct design procedure should be followed to properly size the ducts to ensure proper airflow to each room. The Manual J load calculation procedure sizes each individual room instead of sizing the home as a whole to ensure that each room receives the proper airflow. Without a properly sized duct system, there may be issues with the airflow. New heating and cooling equipment should be selected following the Manual S equipment selection procedure to ensure that the equipment is properly selected for the home.

Even if your heating & cooling contractor or home builder is not required by your local building department to perform a load calculation on your new or existing home, home buyers and home owners should insist that a Manual J load calculation be performed for the reasons above. Heating & cooling contractors and builders should have the Manual J, D and S procedures performed to ensure that they are sizing the heating and cooling system correctly and offering the best product/service to their clients possible. Please contact us today with any questions.