The Alphabet Soup of HVAC Design

What are Manuals J, S, D and T?

Manuals J, S, T and D all describe technical specifications for designing and installing an efficient Heating, Ventilation and Air Conditioning (HVAC) System. Improper design and installation can decrease the energy efficiency of a building by as much as 30%. It is something many building permitting departments request, and something your HVAC contractor should be using as a framework.

Explanation of the Manual J, S, D and T Specifications

This page provides a basic introduction and description of the various technical specifications i.e. the purpose of each Manual. It is not intended to be a primer for doing the calculation or installing the components of an HVAC system. All of the manuals are published and sold by the Air Conditioning Contractors of America (ACCA).

Manual J – Heating and Cooling Load Calculations

This specification outlines the requirements for conducting a "load calculation" on the home. It covers the measuring of insulation and ventilation levels and includes aspects of air duct tightness, the number of heat-producing appliances and people.

Each room is measured for how many BTUs are lost in the winter and how many are gained in the summer. A BTU (British Thermal Unit) is the unit used to measure thermal energy (heat energy). These measurements are used determine how much conditioned air a room requires.

Manual S – Heating and Cooling Sizing and Selection

Once you have the load calculation using Manual J, Manual S helps with the selection of the proper equipment. It contains information on equipment sizing, ventilation and airflow, taking into consideration the amount of airflow, static pressures and sensible and latent heat. It also includes sizing requirements for different types of cooling and heating equipment, as well as detailed explanations and examples of how to use the manufacturer's performance data.

Combined with the load calculations from Manual J, Manual S gives the technician the technical information for selecting the proper system for your home such an air conditioner, a heat pump, furnace or boiler.

Manual D – Heating and Cooling Duct Design

Manual D addresses the proper design, sizing, and installation of residential duct systems. Ductwork design is extremely important in the energy efficiency of your home. A good duct system prevents air and energy leaks, whereas one that is poorly installed can cause humidity and moisture problems.

Manual D includes information for determining the best duct design for the available space, including how to properly size, seal, and insulate air ducts.

Manual T – Air Distribution

Manual T has to do with air distribution. Once you have the duct installed, Manual T addresses where the supply and return grille should go, and the type of grill, vent or diffuser that should be used. This specification is intended to improve the airflow, to minimize getting drafts or inadequate airflow. If the airflow is weak, the air will feel stagnant and uncomfortable.

It covers aspects of the selection, sizing, and location of the supply air diffusers, grilles, registers (grilles with moving parts), and the return grilles. Not putting an air register in the right spot or making the opening too small or too big can cause air not to mix properly.