Introduction to Energy Management

Lesson 4 Determining the Loads on the HVAC System

After completing this chapter, you will be able to:  Explain how heat transfer applies to buildings  Describe how HVAC load calculations are affected by: facility location site orientation weather, building and indoor conditions  Explain how to determine heating/cooling loads Determining the Loads on the HVAC System

Heat Transfer Coefficients  Create a mathematical model of heat transfer  R-value — Thermal resistance value The higher the value, the better the insulator  U-factor — Heat transmission coefficient High U-factor = High heat transfer rate U = 1/R Determining the Loads on the HVAC System

Example 4-1  R1 = 0.44  R2 = 1.11  R3 =  R4 = 0.32  RT = =  U = 1/RT = 1/12.87 = Btu/hr/sq ft/ºF Determining the Loads on the HVAC System

Establishing HVAC Design Conditions  Design for the worst case conditions  Ensures proper operation in extreme conditions  Designers add a safety margin to calculations Effect of Facility Location  Location, Latitude, Elevation Determining the Loads on the HVAC System

Effect of Site Orientation  Northern exposures vs. Southern exposures  Radiant transfer rates depend on orientation Selecting Design Weather Conditions  Defined for both summer and winter  Occurrence values Determining the Loads on the HVAC System

Determining Building Characteristics  Physical dimensions of conditioned space  Construction materials used  Orientation/Dimensions of transfer surfaces  Occupancy levels, use and scheduling  Interior heat sources (lighting, etc.)  Ventilation requirements Determining the Loads on the HVAC System

Determining Heat Loads  Peak heating load/heat loss calculation Heat transmissions through walls, floors, etc. Transmission through partitions to unconditioned spaces Infiltration of outside air Ventilation Transmission from floors to the ground Determining the Loads on the HVAC System

Determining Cooling Loads  Transmission through walls, floors, etc.  Transmissions through partitions from unconditioned spaces  Solar radiation, ventilation and outside air  Latent and sensible heat losses from people  Lighting, appliances and equipment heat gains Determining the Loads on the HVAC System

Degree Days  Helps to estimate heating/cooling needs  Heating degree days 65 - Avg. of day’s high and low temps.  Cooling degree days 65 + Avg. of day’s high and low temps. Determining the Loads on the HVAC System

Example 4-3 – lets do several examples  High temperature for January 26 = 34ºF  Low temperature for January 26 = 0ºF  Number of heating degree days = 65 – (34 – 0)/2 = 65 – 17 Number of heating degree days = 48 Data on file at the National Weather Service Determining the Loads on the HVAC System

Btu/DD Why not use Btu/square foott? Why is this a more accurate measure of the efficiency of a commercial facility?