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HVAC Calculations and Duct Sizing

Gary D. Beckfeld, P.E.


Course Outline

This course presents the basic procedures for determining the cooling (or heating) unit and distribution system required for a building in order to maintain comfort conditions for people. These procedures include evaluating building heat loads, determining air circulation quantities, use of psychrometric processes, computing the cooling unit and air handler tonnage, and sizing the air distribution ducts.

Building heat loads from external and internal source are evaluated for both sensible and latent heat contributions. Heat conducted through walls and windows and through ceilings and floors from unheated spaces is considered. Walls constructed of layers of different materials, thicknesses and areas are included.

Air flow quantities are found for the conditioned space to maintain the desired temperature and humidity requirements. The effects of outside air infiltration are included and different methods of fresh air ventilation are examined.

A psychrometric chart diagram is used to indicate the processes of heating and humidification in the conditioned space. The air properties due to the mixing of ventilation and room air streams are determined from the chart. The enthalpy of the air entering and leaving the cooling coil is read from the chart as are the coil temperature and re-heater load.

The cooling unit and air handler size are found from the required flow rate and enthalpy change across the coil. Air handler flow and supply pressures are then used with air friction charts to select duct sizes such that all branches of the distribution system have equal pressure drop while having the required circulation quantities.

Finally, the heat load calculation methods using cooling load temperature differences (CLTD) and heating degree days (DD) are briefly reviewed.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

The goal of this course is to teach the following procedures used in HVAC analysis and design:

Intended Audience

The material in this course would be of interest to Mechanical Engineers, Architects, Building Inspectors, Contractors and Project Managers

Benefit to the Audience

This course would be of benefit to those interested in learning basic procedures in HVAC and those wanting to refresh their present understanding of HVAC analysis and design. Benefits to be derived from the course include these: HVAC system design capability, awareness of the importance of material choice and type of construction to minimize heat conduction, arrangement of glass to minimize radiation, ability to select compatible cooling (heating) units and air handlers, and duct design to assure balanced flow rates.

Course Introduction

The analysis and design of an HVAC system depends on the selection of design conditions, location and orientation of the building, and construction characteristics of the building.

Design conditions consist of indoor and outdoor temperatures and humidity or dew point. For this study, the indoor design conditions selected are 80 F and 50% relative humidity. Peak loads are assumed to occur when outdoor conditions are 95 F and 75 F dew point temperature. Recommended design outdoor temperatures can be found in the references for many locations.

Location of the building is important not only for outside design conditions, but also for the amount of solar radiation received, especially on sunlit windows. For this course, a building is assumed facing south in a latitude where there are significant amounts of solar radiation on east, south and west windows. The windows are assumed not externally shaded by overhangs or other building characteristics.

The structure characteristics to be considered include not only window shading features but also materials and type of construction of roofs, walls, ceilings, and floors. Walls for example could be framed with insulation between studs, or post and beam with window walls, or monolithic concrete. Each material has a different thermal conductivity and each type wall has a different thermal resistance. Also, the mass of the concrete as well as glass and interior furnishings absorb and store heat which reduces the peak load and does not appear in the cooling load until later. However, in presenting the basic procedures in this course, these transient effects are neglected and this study starts with a review of steady state heat conduction through walls.

Course Content

The course content is in HVAC Calculations and Duct Sizing (PDF File).

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Content Outline

1. Heat Conduction and Thermal Resistance
Review of one dimensional steady state heat conduction equation, thermal conductivity and thermal resistance

2. Building Walls
Film coefficients and thermal resistance of walls constructed of layers of different materials, areas, and thicknesses

3. Building Attic and Basement
Temperature in unheated spaces and heat conducted through ceiling

4. Building Heat Loads
Sensible and latent heat loads from external and internal sources
Sensible heat ratio

5. Psychrometric chart
Air conditioning cycle diagramed showing temperatures, relative humidity, moisture content, and enthalpy

6. Air Conditioning Calculations
Numerical example of heat loads, air circulation quantities, cooling unit and air handler tonnage

7. Duct Sizing
Sizing air distribution ducts to obtain required flow rates and pressure losses within air handler capabilities

8. Ventilation
Effects of different methods of introducing fresh outdoors air

9. Cooling Load Temperature Difference and Heating Degree Days
Review of the CLTD and DD methods of computing heat loads

Course Summary

This course has presented the basic methods of evaluating building heat gains or losses for air conditioning or heating. Heat conductivity and thermal resistance were reviewed. A numerical example of heat loads to a building was described including external and internal heat sources. Both sensible heat and latent heat loads were discussed. The air conditioning process including ventilation was presented in a diagram of a psychrometric chart. Cooling load tonnage was found and air handler flow and pressures discussed. An air distribution duct sizing method was detailed. Finally, the methods of cooling load temperature difference and heating degree days were reviewed.

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.