Performing a Load Calculation When Installing or Replacing HVAC Equipment is Crucial
When facing the expense of several thousand dollars for a central air conditioner, heat pump or new air handler, you should take all steps necessary to select an HVAC system that will work efficiently, effectively and will be reliable for many years. Your new equipment must be correctly sized to properly heat or cool your home.
To determine what size HVAC equipment you need, you must know your structure’s heating and cooling loads. An HVAC professional must perform a load calculation to determine those loads.
Load calculations should always be performed by an HVAC professional as the calculations are complex and should be accomplished using the most up-to-date software. When your HVAC professional is performing load calculations, keep these factors in mind:
- The most recent version of Manual J (Residential) and Manual N (Commercial) should be used.
- Heating and cooling equipment recommendations should be based on expert interpretation of the heating and cooling load data.
- Heating and cooling systems should be no more than 15 percent oversized.
- Use the ACCA guidebook, Manual S, “Residential Heating and Cooling Equipment Selection” or Manual CS “Commercial Application, Systems, and Equipment” for industry-standard assistance with choosing the right HVAC equipment to meet your home’s heating and cooling loads.
What is a load calculation?
A load calculation is a detailed and precise method used to determine exactly how much heating and cooling is needed to maintain your structure at the temperatures you prefer.
After an on-site inspection, taking numerous measurements and evaluating the comfort requirements of your family, your HVAC expert will be able to produce an accurate quantitative measure of heating and cooling requirements.
Performing a load calculation considers the unique thermal and structural characteristics of your structure and your geographical area, such as:
- Local weather, temperatures and climate
- Size, shape and directional orientation of your home
- Airtightness of your home
- Air leaks in your home’s seal and duct work that could lead to conditioned air loss
- Amount, type and location of insulation
- Number, location, directional orientation and types of windows
- Number of lights and appliances that produce heat
- Type of construction materials used to build the home
- Landscaping, including trees, shrubs and earthen barriers that could affect how much wind, sunlight and other natural elements reach the home
- Number of occupants in the home and occupant lifestyles
- Energy efficiency of the home and steps taken to increase efficiency, such as sealing ducts and air leaks, adding insulation and weather-stripping or putting in new windows
- Owner and occupant temperature preferences
Given all the variables that must be accounted for to perform a complete and accurate load calculation, they should always be done by an HVAC professional. This will ensure that your system is correctly sized and will run efficiently.
When selecting a mini split system for use in your structure all of the variables listed above must be accounted for, especially local weather, high and low temperatures and the climate in general. These factors are important because in very cold climates the system may require supplemental heating you comfortable.
This is one more reason why an HVAC professional should be consulted to select the proper equipment for your specific area and requirements.
The need for proper equipment sizing
Proper sizing of HVAC equipment means selecting and installing systems that are powerful enough to provide all the heating and cooling you need without being so large that they waste energy by overheating or overcooling.
When you know your structure’s heating and cooling loads, you’ll be able to choose HVAC equipment that’s accurately sized to meet those loads.
Manual J load calculation
The industry-standard procedures for performing a load calculation are contained in Manual J “Residential Load Calculation,” or Manual N “Commercial Load Calculation” guidebooks published by the Air Conditioning Contractors of America (ACCA). The manual contains all the instructions and technical steps required for an accurate load calculation of your home. It gives your HVAC professional tested and proven guidance on how to account for the details and variables in your home’s structure.
After performing a load calculation, your HVAC contractor will have numerical results showing the following:
For heating systems
- Sensible heating load: Sensible heat is how much heat can be felt, or sensed, and measured with a thermometer. This heating load is the amount of heat that your HVAC equipment should be able to generate during the night (when there is no additional heating available from the sun) on the coldest days of the year.
For cooling systems
- Sensible cooling load: The sensible cooling load is the amount of sensible heat that your cooling system should be able eliminate during daylight hours on very warm days.
- Latent cooling load: The amount of moisture the cooling system should be able to remove during hot, humid summer days. It determines how much energy is needed to dehumidify your indoor air on these types of days.
To insure your load calculations are accurate, remember:
- The most recent version of Manual J (Residential) and Manual N (Commercial) should be used.
- Heating and cooling equipment recommendations should be based on expert interpretation of the heating and cooling load data.
- Heating and cooling systems should be no more than 15 percent oversized.
- Use the ACCA guidebook, Manual S, “Residential Heating and Cooling Equipment Selection” or Manual CS “Commercial Application, Systems, and Equipment” for industry-standard assistance with choosing the right HVAC equipment to meet your home’s heating and cooling loads.
Contact an HVAC Professional for more information on the importance of performing a load calculation and installing properly sized heating and cooling equipment.