by Phil Rains
A residential load calculation is a way for a contractor (or technician) to determine the envelope loads for a particular residential dwelling.
Every residential structure has these envelope loads which are determined by local weather patterns, features of the structure, and all the building materials and techniques that are or were used in its construction. Other parameters include appliances and the number of occupants in the structure, duct work loads, ventilation loads, and motor heat loads. Effectively, the envelope loads are the total heating and cooling loads for the components that surround the conditioned space (walls, ceilings, roofs, floors, doors, windows, etc.)
The residential heating and cooling system must be selected and designed to provide comfort conditions in all occupied spaces regardless of whether is it winter or summer. The installed system must be able to control temperature, humidity, air movement and ventilation simultaneously.
Load calculations procedures produce improved equipment sizing loads for single-family detached homes, small multi-unit structures, condominiums, town houses and manufactured homes. These procedures are also compatible with different types of comfort systems and applications such as:
.Central single-zone systems
.Central multi-zone systems
.Distributed multi-zone systems
.Dwellings with limited exposure or no exposure diversity
The load calculation is the most important step in determining the size and type of cooling and heating equipment required to maintain comfortable indoor air conditions.
The Air Conditioning Contractors of America (ACCA) is a group of air conditioning contractors who work together to improve the HVAC industry, promote good practices, and keep homes and buildings safe, clean and comfortable. As part of their effort, ACCA has developed the HVAC Quality Installation Specifications (QI) from contribution from contractors and other interested parties. These contributors include original equipment manufacturers (OEMs), public, private, and federal electric utilities, and industry associations. Many contractors now follow the concepts and requirements of the QI, either by desire or requirement.
The QI is designed to assist contractors as they go through the process of determining which approach to follow when they design, install and service a system. Also, the QI requires that contractors perform load calculations for new structures and when dealing with existing structures. The QI states the following concerning load calculations:
“The contractor shall provide evidence that for new residential and commercial buildings, or when adding new ducts to an existing structure, room-by-room heat gain/loss load calculations are completed…”
When discussing load calculations, we need to review equipment sizing considerations. Significantly undersized HVAC equipment will typically be unable to maintain the desired set-point temperature when a design load is imposed on the heating and cooling equipment. However, slightly undersized equipment sometimes will often provide acceptable comfort at a lower cost, but never undersized by more than around 10 % of the design loads.
When someone has oversized the HVAC equipment, short-cycles can occur during the cooling mode. This provides only marginal part-load temperature control for the structure, as well as allowing stagnate air pockets to materialize (unless the indoor blower is operating continuously). There will typically be a degradation of humidity control as well, as the system does not run long enough to condense the moisture out of the air. It is only running basically to meet the sensible load controlled by the indoor thermostat. Oversizing requires larger equipment and larger duct systems, increasing the installed cost and results in increased operating costs as well. Utilities also oppose oversing as it results in increased demand on their systems. And finally, oversizing adds unnecessary stress to the HVAC equipment.
The best way to avoid oversizing or undersizing HVAC equipment is to perform a load calculation. Don’t rely on “rule of thumb” methods or on past experience. At best, these can only provide quick design however they are often not precise and can lead to problems.
Also, remember that a well- insulated house is much tighter than an un-insulated house. Always recommend sufficient levels of insulation in lieu of increased system size. But, if the consumer cannot facilitate more insulation, you will have to calculate the system size with the levels present.
As such, a quality installation begins with a load calculation as part of the professional design process, even for a home.
Year-round, comfort is always the goal when performing a load calculation. In the cooling mode the HVAC system not only cools the indoor air (sensible cooling), it also removes moisture (latent cooling). In the winter, your heating system must keep you comfortable without causing high utility bills.
Concepts and fundamentals of HVAC/R equipment sizing is based on heat gain and losses in a dwelling. You will need to remove the amount of heat gain in the summer and add in the amount of heat loss in the winter with the equipment. Heat gain and loss must be equally balanced by heat removal and addition to get the desired comfort.
Many residential load calculation methods are available in the HVAC/R industry. Two of the most recognizable and frequently utilized are ACCA’s Manual J, Eighth Edition (MJ8), and ACCA’s Manual J, Eighth Abridged Edition (MJ8AE). Regardless of the method, a residential load calculation is always a good idea prior to installing or retrofitting any HVAC/R system.
To learn more about HVACReducation.net's online Hvac Load Calculations course,
Phillip A. Rains
Copyright © Phil Rains
About the Author: Phil Rains is Master Trainer/Technical Developer for HVACReducation.net. He has over 35 years of HVAC and Refrigeration experience in installation, service, and training. He is NATE-certified in 5 areas, a member of ASHRAE and RSES, and ACCA EPIC-Certified in Residential and Commercial Design. He also holds a Universal Classification in EPA 608.