HVAC Load Estimating - Detailed Notes

CHAPTER THREE: HVAC Load Estimating

3.1 Introduction to HVAC Load Calculations

• HVAC systems are designed to meet heating and cooling loads based on temperature and humidity.

• Statistical data from worldwide locations is utilized by HVAC designers.

• Outside temperature criteria for design load calculations depend upon the nature of the building.

  • If it is crucial for the system to meet demand, designers may assume the coldest recorded temperature.

  • Most buildings are not designed based on this criterion but rather a median of extremes.

3.2 Temperature Criteria for Load Calculations

• Design temperature criteria include:

  • Heating DB (Dry Bulb)

  • Cooling DB & MCWB (Mean Coincident Wet Bulb)

• Table 3.1 presents climatic conditions for HVAC design for St. Louis, MO, indicating various DB and MCWB values at different percentiles.

Table 3.1: Climatic Conditions for HVAC Design

• 99.6%: 7.5°F DB

• 99%: 12.7°F DB

• 0.4%: 96.2°F DB, 76.7°F MCWB

• 1%: 93.5°F DB, 76.1°F MCWB

• 2%: 91.1°F DB, 75.0°F MCWB

• 0.4%: 79.5°F WB, 90.9°F MCDB

Heating Load Calculation:

• For heating systems, use mean of the coldest recorded temperatures.

• Median value balances annual extremes above and below. Recommended for critical systems (e.g., hospitals).

• General office environments can operate with less stringent criteria; typically using a "percent" concept based on historical frequency.

% criteria for Heating Load Calculations:

• 99.6% value (e.g., 6°F)—statistically 0.4% of the year (35 hours) experiences temperatures below this value.

• 99.0% value (e.g., 10°F)—statistically 1.0% of the year experiences temperature below this value (88 hours).

3.3 Designing HVAC Systems by Calculating Loads

3.3.1 Conducted Loads

• Heat losses include conducted loads through building envelope elements (walls, roofs, windows) and outside air loads (leakage/infiltration and ventilation air).

• Heating load calculations for conduction:

  • Heat transfer is proportional to the temperature difference between warm and cold sides of building envelope elements.

  • Basic formula:
    $Q = \frac{A \times TD}{R}$

  • Where:

    • $Q$ = heat transfer (Btuh)

    • $A$ = area of assembly (ft²)

    • $TD$ = temperature difference (°F)

    • $R$ = resistance (hr-ft²-°F/Btu)

U-Factor Calculation

• The U-factor (or U) is the tendency of an assembly to conduct heat and is the reciprocal of R.

• Using U in heat transfer equations:
  $Q = U \times A \times TD$

3.3.2 Estimating U-Factors for Building Assemblies

• Total resistance of building assemblies can be determined by calculating the resistances of individual layers within the assembly.

• Example: Concrete has a resistance of 0.10 hr-ft²°F per inch, leading to 8 in. of concrete providing a resistance of 0.80. Table 3.2 lists thermal resistances for representative materials.

• The effect of air spaces and surfaces within wall assemblies is significant in estimating thermal resistance.

3.3.3 Infiltration and Ventilation Loads

• Infiltration due to air leakage through windows and doors needs to be included in load calculations:
  $Q = 1.1 \times CFM \times TD$

• Humidification load can be calculated as follows, depending on desired and actual humidity ratios:

  • $Q = 4840 \times CFM \times (W<em>{room} - W</em>{oa})$

  • Where $W<em>{room}$ = desired room humidity ratio, $W</em>{oa}$ = outside air humidity ratio.

3.3.4 Miscellaneous Loads

• Other losses to consider include:

  • Losses through walls below grade.

  • Through slabs on grade.

• Analysis of exhaust systems and fresh air requirements is critical.

3.3.5 Heating Load Example (Sample Problem 3.1)

• Problem 3.1 demonstrates methods for calculating heating and humidification loads for a small office building.

• Given building dimensions and standard U-factors for walls, roof, windows.

Total Heat Loss = 91,700 Btuh
Total Humidification of Outside Air = 21,200 Btuh

3.4 Calculating Cooling Loads

• Heat gains include conduction, solar effects through glazing, outside air loads, internal heat gains, etc.

3.4.1 Conducted and Solar Heat through Glazing

• Total heat gain is represented by the equation:
  $Q = U \times A \times TD + SC \times A \times SHGF$

• Important terms include:

  • SHGF: Solar Heat Gain Factor (Btuh/ft²)

  • SC: Shading Coefficient (dimensionless)

3.4.2 Cooling Load Calculation for Walls and Roofs

• The TETD must also incorporate solar heating effects:
  $Q = U \times A \times TETD$

3.4.3 Internal Heat Gains

• Internal heat from lights, appliances, and occupants should also be included in calculations:

  • General assumptions: 250 Btuh sensible and 250 Btuh latent heat per occupant (ASHRAE standards).

3.4.6 Sample Cooling Loads Calculation (Sample Problems 3.2 and 3.3)

• Shows heat loads in a small office building with variables in lighting, appliance loads, and U-factors.

• Overall air-conditioning load remains largely consistent regardless of design variations.

Conclusion

• HVAC load estimates blending calculations of heating, cooling, humidification, infiltration, and various building properties are paramount to design efficiency and comfort.