Latent Capacity in Air Conditioners

HVAC systems are available in various designs, capacities, and have different energy efficiencies. The main task of an air conditioner is to remove heat and moisture content from air. The cooling capacity of an AC is measured in tons.

Most people believe that bigger the AC tonnage, higher the cooling capacity, or nameplate capacity as it is also called. While this may be true in theory, the actual sensible (cooling capacity) and latent (moisture removing capacity) capacities of an AC depend on many factors. Some are intrinsic to design while some are external variants such as ambient temperature and indoor humidity.

Manual J and Manual S are load calculation manuals created by the Air Conditioning Contractors of America (ACCA). Homeowners and HVAC contractors can make use of these standards to determine the right sized HVAC system for a home.

Each standard is important in the HVAC designing and installation process. It is usually advised to do them in a prescribed order for best results.

Manual J

Every house is different, and using the rule of thumb to calculate heating and cooling load required for a house of a particular size may not always give you accurate results. The HVAC system installed after calculating in such a way, may be under or over-sized for the house, causing performance issues later.

Manual J is used to determine how much heat a house loses in winter or gains in summer. By checking this, one can determine how much heating or cooling will be required for each room. The Manual J standard includes factoring in all surfaces of the house or building, along with their areas and insulation levels. Internal loads, ductwork and entry of outdoor air are also checked.

But the capacity of an HVAC system depends on 3 main natural variables- indoor temperature, outdoor temperature and indoor humidity, along with one system variable- airflow at the evaporator coil.

Cooling load for the same house turned in a different direction, for instance, can be largely different depending on insulation, entry of air and certain other factors. So, it is not just enough to calculate load capacity but to choose the right equipment.

Manual S

Manual S refers to Selection. Based on design loads, Manual S is a simple process to select the right kind of equipment for heating or cooling your home. Studying heating is easy but cooling is a bit more complicated. To know the right equipment needed for cooling your home, one needs to determine the sensible and latent capacity of the equipment at the design conditions particular to the current project. The Manual S calculation is extremely important because it helps you decide what type of air conditioner, heat furnace or boiler you should install for maximum output and Bay Area comfort.

Manual T and D

Manual T helps determine the distribution of air to various rooms. And Manual D stands for Design. Once you know the capacity required, the type of equipment needed and the air distribution for every room, you can accordingly design the ductwork.