Load Calculations

A load calculation is a measurement to determine how much heating and cooling a home requires.  This information is used to accurately size heating and cooling equipment in order to maximize efficiency and optimize performance.  A load calculation takes into consideration anything that causes heat loss and solar gain.  This includes many detailed measurements such as:

Dimension of each individual room; window and door quality, size, and location (including shading from screens and overhangs); construction materials, insulation, and finish of floors, ceilings, and walls; ductwork location, configuration, insulation, and quality (including supplies and returns); and infiltration from building construction and fireplace quality.
Room Dimensions DrewRoom Measurement
Door Measurement DrewDoor Measurement
Window Measure 3Window Measurement
Window Shading MeasurementShading Measurement
Room Dimensions DaveRoom Dimensions
Ductwork InspectionDuctwork Inspection
Fireplace Inspection DaveFireplace Inspection

InspectorNorth Carolina State Building Code Mechanical Section 312.1 mandates “heating and cooling equipment shall be sized based on building loads calculated in accordance with ACCA Manual J.” Do not expose yourself to the liability of using a contractor who risks installing a system without performing a load calculation based on industry approved standards.  Not only is it ILLEGAL, but most importantly a disservice to the customer and can result in delays in completing the project, failed inspections, fines and loss of license for contractors. 

Mountaineer Heating and Cooling Comfort Specialists are specially trained and certified to perform assessments on your home including a Manual-J Load Calculation and to custom engineer ductwork using Manual D designs that meet or exceed standards set forth by ASHRAE and ACCA

So why doesn’t everyone perform a load calculation? 

Wrightsoft picPerforming a load calculation in years past required engineers using paper, pencil, calculators or even a slide ruler and a lot of math and this was very time consuming.  Because of the time needed to complete the proper assessment, this process was often omitted in favor of a less accurate estimate using a rule of thumb “guesstimate”.  Using technological advances, these calculations can now be performed with software programs by specialists who are trained in Manual-J calculations and Manual-D designs using ASHRAE and ACCA standards.  So, with the proper training and tools available, a contractor has no excuse for failing to performing a complete assessment and load calculation. Unfortunately, some less reputable contractors still use an old “rule of thumb” and base the selection of equipment only on the total square footage of a home or office.  This usually results in incorrectly sizing of the system because they have not accounted for many of the vital details unique to a particular home or office.

Sholder ShrugThis presents a number of problems discussed further in the next section. Every structure is different.  Even identical dwellings, rotated a few degrees to face a slightly different solar orientation, may require equipment with a capacity difference of 25% or more!

load calc pie chartBe very cautious of any contractor who fails to perform and explain a detailed load calculation of your project.

The Air Conditioning Contractors of America (ACCA) says “Determining heating/ cooling loads based on the building’s square footage is inaccurate and inadequate.  Also, basing replacement equipment on the size of the original system could lead to problems since the original equipment size may have been incorrect.” REMEMBER, IT IS ILLEGAL FOR A CONTRACTOR TO INSTALL OR REPLACE A HEATING OR COOLING SYSTEM WITHOUT PERFORMING A LOAD CALCULATION.

What if my system IS under or oversized?  Why is this a problem?

Thermostat -1Understanding why an undersized system is a problem is easy.  An undersized system will struggle to keep a home comfortable and may never obtain the desired temperature. 

This will be true especially on hot or cold days when heating or cooling is needed the most, and will cause a system to be overworked. However, over sizing a system is just as bad, if not worse than under sizing.  An improperly sized system will result in higher utility costs, reduced comfort from excessive or lack of humidity and shorter equipment life.

An oversized system will cost more to install (upfront cost) and more to operate (long term cost).
Oversized equipment will “short cycle” or start and stop too frequently.  This will result in more recurrent repair costs and shorter equipment life.
Due to short cycling, some rooms will not receive adequate air flow. This may cause hot or cold spots in the house, particularly those areas further away from the thermostat. This will also lead to more thermostat adjustments and reduce efficiency which results in higher utility costs.
Moldy Ductwork
Short cycling also diminishes a systems ability to sufficiently condense moisture from the air during cooling mode.  The result is high humidity and a cold, clammy atmosphere that is not only uncomfortable but causes mold growth.