Chapter 13 : Load Calculations (Advanced HVAC Level)

Load calculation is one of the most critical steps in HVAC design. It determines how much heating or cooling is required to maintain indoor thermal comfort.

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🔹 9.1 What is HVAC Load Calculation?

HVAC Load Calculation refers to the estimation of heating and cooling loads (in BTUs/hr or kW) required to maintain indoor comfort in a building. It includes:

• Sensible Heat Load → Change in air temperature

• Latent Heat Load → Moisture (humidity) control

👉 Total Load = Sensible Load + Latent Load

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🔹 9.2 Types of Loads

Type	Description
Cooling Load	Amount of heat to be removed to keep indoor temperature at setpoint
Heating Load	Amount of heat to be added
Block Load	Total load for entire building
Room-by-Room Load	Individual room-wise calculation for zoning

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🔹 9.3 Heat Gain and Heat Loss

🔥 Heat Gain (Cooling Load):

Occurs due to:

Solar radiation through windows, walls, and roof

Internal loads (people, lights, equipment)
Infiltration (outside air entering)
Ventilation air
Appliances (computers, ovens)

❄️ Heat Loss (Heating Load):

Occurs due to:

Conduction through walls, glass, roof, floor

Infiltration and ventilation of cold outside air
Loss from ducts (uninsulated)

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🔹 9.4 Manual J, Manual S – Residential Design

Developed by ACCA (Air Conditioning Contractors of America), these are standardized methods used in residential HVAC design in the U.S.

Manual J → Heat Load Calculation for homes

Based on area, orientation, insulation, window size, occupancy, etc.

Manual S → Equipment Selection based on Manual J

Ensures proper sizing of air conditioners, furnaces, heat pumps

❗ Oversized systems can cause short cycling, humidity issues, and higher bills.

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🔹 9.5 CLTD Method – Commercial Buildings

CLTD = Cooling Load Temperature Difference Method

Used for commercial buildings, developed by ASHRAE.

Formula:

Q = U × A × CLTD

Where:

Q = heat gain (BTU/hr)

U = overall heat transfer coefficient
A = area of surface (walls, roof, glass)
CLTD = adjusted temperature difference from ASHRAE tables

It accounts for:

Orientation

Time of day
Insulation
Color of surface
Glass type (single/double)

Also includes:

Internal load calculations

Lighting load: 3–5 W/sq. ft.
People load: ~250 BTU/hr per person

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🔹 9.6 Other Load Calculation Methods

Method	Used For	Tool
RTS Method	Hourly loads	Software
Radiant Time Series	Advanced accuracy	eQuest, HAP
ASHRAE Handbook	Engineering reference	Manual
EnergyPlus	Whole building modeling	Simulation tool

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🛠️ Practical: Sample Cooling Load Calculation

Room Info:

Size: 20 ft × 15 ft × 10 ft (L×W×H)

2 occupants
1 computer, 4 lights (60 W each)
Window: 5 ft × 4 ft, west-facing, single-pane
Location: Delhi (Hot, dry summer)

Step-by-step:

1. Sensible Heat Gain from Walls/Glass (Q = U × A × CLTD)
   Use appropriate U-value and CLTD from table.

2. Internal Load:

Occupants: 2 × 250 = 500 BTU/hr

Lights: 4 × 60 = 240 W ≈ 819 BTU/hr
Equipment: 1 computer ≈ 400 BTU/hr

Ventilation Load (Optional for basic)
Total Cooling Load = Sum of all above ≈ ~5,000–6,000 BTU/hr → ~0.5 Ton

✅ Choose a 0.75 or 1 Ton AC for safety margin.

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📄 Assignment/Interview Questions

✅ What factors affect cooling load?

Cooling load is influenced by:

Orientation and size of windows

Solar heat gain (south/west-facing walls)
Wall/roof insulation (R-value)
Internal loads (occupants, lights, computers)
Air leakage (infiltration)
Building materials (glass, concrete, wood)
Outdoor climate (temperature, humidity)
Room usage (kitchen, server room, etc.)

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✅ Define CLTD Method.

CLTD (Cooling Load Temperature Difference) Method is a simplified technique used to estimate the heat gain through walls, roofs, and windows in commercial buildings. It uses:

U-value of the material

Surface area
CLTD values (adjusted for orientation, time, and insulation)

Formula:
Q = U × A × CLTD

It helps engineers size HVAC equipment accurately based on building characteristics and environmental conditions.