ASHRAE Heating & Cooling Load Calculations

ASHRAE Heating & Cooling Load Calculations

The ASHRAE Heat Balance Method was first defined as the preferred method for Load Calculations in the 2001 ASHRAE Handbook—Fundamentals, and it is now the most widely adopted non-residential load calculation method by practicing design engineers.  The industry-standard ASHRAE Heat Balance Method has a number of important concepts, three of which are highlighted below. 

(1) Include all Space Surfaces 

There are three “Heat Balances” shown in Figure 1 of the Heat Balance (HB) Method and two of those “Heat Balances” are applied to each surface of a space or room.

From a design-engineering perspective, there are two important implications:

• Accurate model geometry is necessary and should account for all surfaces of a space or room including the internal walls, ceilings and floors. On some occasions, a ground-contact floor with high thermal mass may even remove heat from a space during a cooling load calculation. 
• Solar tracking should be accounted for in all spaces, including interior spaces which may receive solar radiation in the morning or late afternoon when the sun angle is lower.  Conductive, convective, and radiative heat balance is calculated directly for each surface within a room, so tracking the incident solar radiation is critical to accurate calculations of solar gains in perimeter and internal spaces. In Figure 1, the cooling load report for an internal zone shows 11.5% of the load is due to solar gains. 

(2) Sum of Gains ≠ Cooling Load  

The ASHRAE Heat Balance Method states that the “sum of all space instantaneous heat gains at any given time does not necessarily (or even frequently) equal the cooling load for the space at that same time”. Figure 2 attempts to convey this phenomenon by demonstrating the time delay associated with the ‘Gains vs Loads’ discussion.

From a design-engineering perspective, there are three important implications:

• Designers should consider performing cooling load calculations for rooms and zones with all of the internal gains fully on (e.g. maximum occupant capacity) in order to account for this design condition, regardless of how infrequent that scenario may occur.  We refer to this practice as “saturating” the internal gains for the design cooling load calculations. 
  • Note, when sizing central HVAC equipment (e.g. AHU fan & cooling coil) some load diversity should be considered. Typical values may be 90% for occupants, 80% for lighting and 50% for plug load equipment, depending on the space function and operation.  Some exceptions may include a laboratory, healthcare or pharmaceutical application which may have a constant ACH requirement. 
  • When predicting annual building energy/cost/carbon performance, we do not encourage this approach and use hourly operational profiles instead. 
• While the typical load calculation is for the “design day”, hourly calculations for each month should be calculated in order to account for all influential factors because the peak load may not necessarily occur on the month of the peak external dry-bulb temperature. The ASHRAE Design Weather Database provides this data for thousands of worldwide locations.  The design data includes maximum external dry-bulb temperature conditions for each month and corresponding monthly coincident wet-bulb temperature conditions, should the latent load or lower sun angle be the influential cause of the peak condition.   
• All construction materials in buildings have a thermal capacitance and as such, the thermal mass of every construction assembly is included in the cooling load calculations, including internal construction assemblies. A review of any given construction assembly characteristics (overall U-value, insulation R-value) should also include the thermal mass of the construction assembly (lightweight, heavyweight). 

(3) Review Results against Rules-of-Thumb 

While the most current version of the ASHRAE Handbook—Fundamentals (Chapter 18) provide exceptional details about the Heat Balance Method, it does not include much information about the loads results and how those results compare against rules of thumb. There are various options available to communicate and review loads results. 

From a design-engineering perspective, there are three common outcomes from such review:

• Compare against rules-of-thumb. The common rules of thumb will vary across climates and space-functions (e.g. corridor vs laboratory). For example, typical published values based on the ASHRAE Handbook are:
  • Heating: ~ 10 Btu/h.ft² [31.5 W/m²]
  • Sensible Cooling
    • ~ 15 Btu/h.ft² [47 W/m²] 
    • ~ 1.0 cfm/ft² [4.5 l/sec/m²]
• Add safety factors (oversizing margins). The two IESVE spreadsheet reports shown automatically include 10% for sensible cooling loads and 10% for heating loads. This can vary from company to company and even from engineer-to-engineer within the same company. Many factors can influence the safety factors, including distribution losses, regional construction quality, space operation and start-up capacity.
• Select & Size the HVAC system types. For example, a typical radiant floor can provide ~13 Btu/h.ft2 [~40 W/m2] of sensible cooling, assuming no solar gains and 30 Btu/h.ft2 [~90 W/m2] of sensible heating depending on the finished floor covering. Alternatively, if an all-air system is to be selected, diffusers can be selected and ducts can be sized. The next step is to calculate central HVAC system capacities. 

IESVE Software uses the Heat Balance (HB) Method to calculate cooling and heating loads of rooms, zones & buildings, in order to comply with ANSI/ASHRAE/ACCA Standard 183. To learn more about the calculation software, see our free hands-on training video ASHRAE Heating & Cooling Loads and HVAC Equipment Sizing, or email me at liam.buckley@iesve.com.