ASHRAE Calculation of Cooling and Heating Loads

2025-03-04

The ASHRAE Heat Balance Method was first defined as the preferred method for load calculation in the 2001 ASHRAE Handbook and is now the most widely adopted method for non-residential load calculation by practicing design engineers. The industry-standard ASHRAE Heat Balance Method has many important concepts, and three of the key ones are as follows.

Include the heat balance of all space surfaces. Figure 1 of the Heat Balance (HB) method shows three heat balances, two of which are applied to each surface of a space or room.
From the perspective of design engineering, there are two important implications:
Accurate model geometry is necessary, and all surfaces of the space or room should be taken into account, including interior walls, ceilings, and floors.
In some cases, a high thermal mass floor in contact with the ground may even release heat from the space during the cooling load calculation.
Solar tracking should be considered in all spaces, including interior spaces. When the sun's angle is low, interior spaces may receive solar radiation in the morning or afternoon. Calculate the heat conduction, convection, and radiation heat balance directly for each surface in the room. Therefore, tracking the incident solar radiation is crucial for accurately calculating the solar heat gain in the perimeter and interior spaces. The cooling load report for the interior area shows that 11.5% of the load comes from solar radiation heat gain.
The sum of heat gains ≠ cooling load
The ASHRAE Heat Balance Method states: "At any given time, the sum of the instantaneous heat gains within all spaces is not necessarily (and often not) equal to the cooling load of that space within the same time period." Figure 2 attempts to convey this phenomenon by demonstrating the time delay related to the discussion of "heat gains and loads".
From the perspective of engineering design, there are three important implications:
Designers should calculate the cooling load for rooms and areas considering the full utilization of all internal heat gains (such as the maximum occupant capacity) to account for this design condition, regardless of how frequently this situation occurs. We call this practice the design cooling load calculation with "saturated" internal heat gains.
Note that when sizing central air conditioning equipment (such as AHU fans and cooling coils), some load differences should be considered. Depending on the space function and operation, typical values may be 90% for occupants, 80% for lighting, and 50% for plug-in equipment.
Some exceptions may include laboratories, healthcare, or pharmaceutical applications, which may have constant ACH requirements.
When predicting annual building energy consumption/cost/carbon emissions, we do not recommend using this method but should use the hourly operation scenarios.
Typical load calculations are for the "design day", but since the load peak does not necessarily occur in the month of the peak external dry bulb temperature, in order to consider all influencing factors, hourly calculations should be carried out by month. The ASHRAE Design Weather Data Bank provides these data for thousands of locations around the world. The design data include the highest external dry bulb temperature conditions for each month and the corresponding monthly simultaneous wet bulb temperature conditions, in case potential loads or lower sun angles become influencing factors for the peak conditions.
All building materials in a building have heat capacity.
Therefore, the thermal mass of each building component is included in the cooling load calculation, including internal building components. The review of any given building component characteristics (total U-value, thermal resistance R-value) should also include the thermal mass (light, heavy) of the building component.
Review the results according to the rule of thumb. Although the latest ASHRAE Fundamentals Handbook (Chapter 18) provides special information about the Heat Balance Method, it does not contain information about the load results and the comparison of these results with the rule of thumb. There are multiple options available for communicating and checking the load results.
From the perspective of engineering design, this review has three common outcomes:
Comparison with the rule of thumb: General rules of thumb vary depending on the climate and space function (such as corridors and laboratories).