Strategy Guideline: 

Accurate Heating and 

Cooling Load Calculations

Arlan Burdick

IBACOS, Inc.

June 2011
ii

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iii

Strategy Guideline: Accurate Heating and Cooling 

Load Calculations 

Prepared for: 

Building America

Building Technologies Program

Office of Energy Efficiency and Renewable Energy 

U.S. Department of Energy 

Prepared by: 

IBACOS, Inc.

2214 Liberty Avenue

Pittsburgh, Pennsylvania 15222

NREL Technical Monitor: Michael Gestwick 

Prepared under Subcontract No. KNDJ-0-40341-00

June 2011
iv

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v

Forward

The Strategy Guideline: Accurate Heating and Cooling Load Calculations report provides 

information for the following groups:

• Heating Ventilation and Air Conditioning (HVAC) Mechanical Contractors

• HVAC System Designers

• Builders

• House Remodelers.

This guide can be used as a point of entry for collaborative discussions between a builder, 

HVAC contractor, and other trade partners to understand the importance of appropriately 

calculating peak heating and cooling loads, as the first step of HVAC system design.

Accurate load calculations have a direct impact on energy efficiency, occupant comfort, indoor 

air quality, and building durability. The load calculation is the first step of the iterative HVAC 

design procedure, as a full HVAC design involves much more than just the load calculation. The 

loads modeled by the heating and cooling load calculation process will dictate the equipment 

selection and duct design to deliver conditioned air to the rooms of the house. This guide 

references the methodologies of the Air Conditioning Contractors of America (ACCA) 

publication Manual J Residential Load Calculation Eighth Edition (ACCA MJ8), which in turn 

references information provided by the American Society of Heating, Refrigerating and AirConditioning Engineers (ASHRAE). ACCA MJ8 only applies to single family detached 

dwellings, low-rise condominiums, and townhouses.

This guide is not a new method for performing load calculations, a substitute for established 

methods of performing load calculations, or step-by-step instructions on how to perform the load 

calculations. This guide presents the key criteria required to create accurate heating and cooling 

load calculations and offers examples of the implications when inaccurate adjustments are 

applied to the HVAC design process. The guide shows, through realistic examples, how various 

defaults and arbitrary safety factors can lead to significant increases in the load estimate. 

Emphasis is placed on the risks incurred from inaccurate adjustments or ignoring critical inputs 

of the load calculation. 
vi

Contents

Forward ........................................................................................................................................................ v

List of Figures ........................................................................................................................................... vii

List of Tables ............................................................................................................................................. vii

Definitions................................................................................................................................................. viii

Executive Summary................................................................................................................................... ix

Introduction ................................................................................................................................................. 1

Baseline Load Calculation ......................................................................................................................... 4

Load Components....................................................................................................................................... 6

Peak Heating Load.....................................................................................................................7

Peak Cooling Load.....................................................................................................................8

Location – Design Conditions .................................................................................................10

Orientation ...............................................................................................................................13

Building Components ..............................................................................................................15

Heating and Cooling System Location and Duct Leakage ......................................................15

Ventilation/Infiltration .............................................................................................................16

Risks Associated with Oversizing........................................................................................................... 16

First Cost, Energy Costs and Durability ..................................................................................16

Comfort - Space Temperatures................................................................................................16

Comfort - Humidity Control....................................................................................................16

Load Dependencies .................................................................................................................................. 17

Outdoor/Indoor Design Condition Dependencies....................................................................17

Building Components Dependencies.......................................................................................20

Ductwork Conditions Dependencies........................................................................................23

Ventilation/Infiltration Conditions Dependencies...................................................................25

Combined Dependencies .........................................................................................................28

Conclusion................................................................................................................................................. 30

References................................................................................................................................................. 31

Appendix A: Enhancements to Scope for Mechanical Contractor/HVAC Manual J Designer.......... 32

Appendix B: ACCA Manual J8 Input Checklist...................................................................................... 33
vii

List of Figures

Figure 1. Results – Orlando House Manipulated Outdoor/Indoor Design Conditions......................... x

Figure 2. Results – Orlando House Combined Manipulations .............................................................. xi

Figure 3. Residential HVAC Design Process............................................................................................ 1

Figure 4. Typical 2223 ft2 One Story House.............................................................................................. 4

Figure 5. Heat Loss Locations................................................................................................................... 7

Figure 6. Heat Gain Locations ................................................................................................................... 8

Figure 7. ASHRAE Winter and Summer Comfort Zones ....................................................................... 11

Figure 8. Outdoor cooling bin hours in Orlando, FL ............................................................................. 12

Figure 9. Building Orientation.................................................................................................................. 13

Figure 10. Building Orientation Cooling Load ....................................................................................... 13

Figure 11. Test for Adequate Exposure Diversity – Pass ..................................................................... 14

Figure 12. Test for Adequate Exposure Diversity – Fail ....................................................................... 15

Figure 13. Chicago House Change in Loads, Manipulated Outdoor/Indoor Design Conditions...... 18

Figure 14. Orlando House Change in Loads, Manipulated Outdoor/Indoor Design Conditions ...... 19

Figure 15. Chicago House Change in Loads, Building Component Manipulations........................... 21

Figure 16. Orlando House Change in Loads, Building Component Manipulations ........................... 22

Figure 17. Orlando House Change in Loads, Ductwork Conditions Manipulations .......................... 24

Figure 18. Chicago House Change in Loads, Ventilation/Infiltration Manipulations ......................... 26

Figure 19. Orlando House Change in Loads, Ventilation/Infiltration Manipulations.......................... 27

Figure 20. Chicago House Change in Loads, Combined Manipulations............................................. 28

Figure 21. Orlando House Change in Loads, Combined Manipulations ............................................. 29

* Unless otherwise noted, all figures were created by IBACOS.

List of Tables

Table 1. Baseline Model Parameters......................................................................................................... 5

Table 2. Baseline Calculated Loads Using ACCA MJ8 ........................................................................... 6

Table 3. Outdoor Design Conditions for the United States - Manual J Version 8, Table 1A............. 10

Table 4. Chicago House Manipulated Outdoor/Indoor Design Conditions......................................... 17

Table 5. Orlando House Manipulated Outdoor/Indoor Design Conditions ......................................... 17

Table 6. Results – Chicago House Manipulated Outdoor/Indoor Design Conditions........................ 18

Table 7. Results - Orlando House Manipulated Outdoor/Indoor Design Conditions......................... 19

Table 8. Chicago House Building Component Manipulations.............................................................. 20

Table 9. Orlando House Building Component Manipulations.............................................................. 20

Table 10. Results - Chicago House Building Component Manipulations ........................................... 21

Table 11. Results - Orlando House Building Component Manipulations............................................ 22

Table 12. Orlando House Ductwork Conditions Manipulations ........................................................... 23

Table 13. Results - Orlando House Ductwork Conditions Manipulations........................................... 24

Table 14. Chicago House Ventilation/Infiltration Manipulations.......................................................... 25

Table 15. Orlando House Ventilation/Infiltration Manipulations .......................................................... 25

Table 16. Results - Chicago House Ventilation / Infiltration................................................................. 26

Table 17. Results – Orlando House Ventilation/Infiltration................................................................... 27

Table 18. Results - Chicago House Combined Manipulations ............................................................. 28

Table 19. Results - Orlando House Combined Manipulations Results ............................................... 29

* Unless otherwise noted, all tables were created by IBACOS.
viii

Definitions

ACCA Air Conditioning Contractors of America

ACCA MJ8 Air Conditioning Contractors of America publication Manual J 

Residential Load Calculation Eighth Edition

ACH50 Air Changes per Hour at 50 Pascals

ACHn Air Changes per Hour Natural

AED Adequate Exposure Diversity

ALP Average Load Procedure

ASHRAE American Society of Heating, Refrigeration and Air-Conditioning 

Engineers

Btu British Thermal Unit

Btu/h Btu per hour

cfm Cubic Feet per Minute

°Fdb Degree Fahrenheit Dry Bulb

°Fwb Degree Fahrenheit Wet Bulb

HVAC Heating, Ventilation and Air Conditioning

PLP Peak Load Procedure

RESNET Residential Energy Services Network

RH Relative Humidity

SHGC Solar Heat Gain Coefficient
ix

Executive Summary

The heating and cooling load calculation is the first step of the iterative HVAC design procedure;

a full HVAC design involves more than the just the load estimate calculation. Right-sizing the 

HVAC system, selecting HVAC equipment and designing the air distribution system to meet the 

accurate predicted heating and cooling loads, begins with an accurate understanding of the 

heating and cooling loads on a space. The Air Conditioning Contractors of America (ACCA) 

Manual J Version 8 provides the detailed steps required to calculate the heating and cooling 

loads. The accurate heating and cooling loads are used to right-size the equipment with ACCA 

Manual S Residential Equipment Selection, then to design the air distribution system and 

ductwork with ACCA Manual T Air Distribution Basics for Residential and Small Commercial 

Buildings and ACCA Manual D Residential Duct System Procedure.

In the author’s experience several factors have led to a general industry resistance to initially 

perform an accurate load calculation, which is necessary for the design of a right-sized HVAC 

system. Historically, energy codes did not address stringent levels of energy efficiency, and rules 

of thumb were developed for HVAC sizing that worked based on the construction at that time. 

Building enclosures have become more energy efficient as energy codes have become more 

stringent since 2000; however, these rules of thumb have not changed. Full credit should be 

taken for improvements such as better windows, enhanced air tightness strategies, and additional 

insulation.

ACCA MJ8 says of safety factors: 

"Manual J calculations should be aggressive, which means that the designer should take 

full advantage of legitimate opportunities to minimize the size of estimated loads. In this 

regard, the practice of manipulating the outdoor design temperature, not taking full 

credit for efficient construction features, ignoring internal and external window shading 

devices and then applying an arbitrary "safety factor" is indefensible.”

“No additional safety factors are required when load estimates are based on accurate 

information pertaining to the envelope construction and duct system efficiency. Large 

errors are possible if there is uncertainty about insulation levels, fenestration 

performance, envelope tightness or the efficiency of the duct runs installed in the 

unconditioned space.”
x

Examples in this guide show the implications when inaccurate adjustments are applied to the

heating and cooling load calculation process. In order to demonstrate the impact on the loads 

when common “safety factors” are applied to the load calculation process, two houses that meet 

the 2009 International Energy Conservation Code® (IECC) prescriptive path were modeled as 

the accurate baseline. One house was modeled in Chicago, IL, and one in Orlando, FL. Several 

common “safety factors” were applied to the baseline models to create examples of how and 

where load calculations can be inflated leading to system oversizing. The baseline load 

calculations were manipulated for:

• Outdoor/Indoor Design Conditions 

• Building Components

• Ductwork Conditions 

• Ventilation/Infiltration Conditions 

• Worst Case Scenario (combining all the safety factors) 

Seemingly small manipulations such as changing the outdoor/indoor design conditions can result 

in exaggerated loads. The Orlando House example (Figure 1) showed a 9,400 Btu/h (45%) 

increase in the total cooling load, which may increase the system size by 1 ton when the ACCA 

Manual S procedures are applied.

Figure 1. Results – Orlando House Manipulated Outdoor/Indoor Design Conditions
xi

Combining several adjustments only compounds the inaccuracy of the calculation results. The 

results of the combined manipulations to outdoor/indoor design conditions, building components, 

ductwork conditions, and ventilation/infiltration conditions produce significantly oversized

calculated loads. The Orlando House example (Figure 2) showed a 33,300 Btu/h (161%) increase 

in the calculated total cooling load, which may increase the system size by 3 tons (from 2 tons to 

5 tons) when the ACCA Manual S procedures are applied. Not only does this oversizing impact 

the heating and cooling equipment costs, but duct sizes and numbers of runs must also be 

increased to account for the significantly increased system airflow. 

Figure 2. Results – Orlando House Combined Manipulations

Oversizing the HVAC system is detrimental to energy use, comfort, indoor air quality, building

and equipment durability. All of these impacts derive from the fact that the system will be “short 

cycling” in both heating and cooling modes. To reach peak operational efficiency and 

effectiveness, a heating and cooling system should run for as long as possible to address the 

loads. An analogy is that of a car: highway driving at a steady speed will get the best fuel 

economy, while speeding up and slowing down puts undue wear and tear on the engine and 

braking systems, and reduces fuel efficiency.
xii

An oversized HVAC system will have both a higher initial cost and a higher cost of operation. 

The frequent starting and stopping of short cycling can lead to premature failure of the 

equipment. Short cycling limits the total amount of air circulating through each room, and can 

lead to rooms that do not receive adequate duration of airflow. In the cooling season in humid 

climates, cold clammy conditions can occur due to reduced dehumidification caused by the short 

cycling of the equipment. The system must run long enough for the coil to reach the temperature 

for condensation to occur and an oversized system that short cycles may not run long enough to 

sufficiently condense moisture from the air. Excess humidity in the conditioned air delivered to a 

space may lead to mold growth within the house. 
1

Introduction

Heating and cooling loads are the measure of energy needed to be added or removed from a 

space by the HVAC system to provide the desired level of comfort within a space. Right-sizing 

the HVAC system begins with an accurate understanding of the heating and cooling loads on a 

space. Right-sizing is selecting HVAC equipment and designing the air distribution system to 

meet the accurate predicted heating and cooling loads of the house. The values determined by the 

heating and cooling load calculation process will dictate the equipment selection and duct design 

to deliver conditioned air to the rooms of the house, right-sizing the HVAC system. The heating 

and cooling load calculation results will have a direct impact on first construction costs along 

with the operating energy efficiency, occupant comfort, indoor air quality, and building 

durability.

Figure 3. Residential HVAC Design Process

A full HVAC design involves more than the just the load estimate calculation; the load 

calculation is the first step of the iterative HVAC design procedure (Figure 3). The values 

calculated from the ACCA MJ8 procedures are then used to select the size of the mechanical 

equipment. Mechanical equipment selection is done with the aid of the ACCA Manual S 

Residential Equipment Selection. The conditioned air delivery to the space is controlled by the 

type and size of the air outlet. ACCA Manual T Air Distribution Basics for Residential and 

Small Commercial Buildings provides the guidance on selecting the air outlet size and type. The 

ductwork that carries the conditioned air to meet the load requirements of the space from the 
2

mechanical equipment to the air outlet is sized with the aid of the ACCA Manual D Residential 

Duct System Procedure. The iterative nature of the process involves balancing the blower 

performance of the mechanical equipment against the pressure losses of the ductwork and air 

outlets to deliver the conditioned air to the space in as unobtrusive manner as possible. The 

ACCA procedures have been written into commercial software packages to help the designer 

work through the many iterations required for a good design. 

For the purposes of this guide, an energy-efficient house is defined as one that is designed and 

built for decreased energy use and improved occupant comfort through higher levels of 

insulation, more energy-efficient windows, high efficiency space conditioning and water heating 

equipment, energy-efficient lighting and appliances, reduced air infiltration, and controlled 

mechanical ventilation. Specification levels for energy-efficient houses have historically been 

prescribed by beyond code programs that set a percentage better than code for energy use, such 

as ENERGY STAR® that requires houses to be 15% more energy efficient than code. Beyondcode programs continue to set a percentage better than the improved codes for energy use,

raising the bar for whole house energy efficiency. The 2009 International Energy Conservation 

Code® (IECC) establishes an estimated 15% improvement in energy efficiency over the previous 

2006 IECC requirements. Ongoing code cycles are anticipated to incrementally further increase 

the minimum efficiency of a house. For example, the 2012 IECC achieves approximately 30% 

savings over the 2006 version. As the new codes are adopted and implemented, a house that was 

built under an above-code program in 2010 will likely be the code-mandated house in 2015. As 

the energy efficiency of the house is increased under code or above code programs, the peak 

heating and cooling loads are significantly reduced.

ACCA MJ8 says of safety factors: 

"Manual J calculations should be aggressive, which means that the designer should take 

full advantage of legitimate opportunities to minimize the size of estimated loads. In this 

regard, the practice of manipulating the outdoor design temperature, not taking full 

credit for efficient construction features, ignoring internal and external window shading 

devices and then applying an arbitrary "safety factor" is indefensible.”

An accurate evaluation of the heating and cooling loads requires a complete understanding and 

accounting of the building components that make up the thermal enclosure along with the 

outdoor/indoor contributions to the load. The load calculation for a house where the building 

enclosure has been enhanced with added air tightness strategies, better windows, and additional 

insulation will be more sensitive to manipulation of the inputs. In the author’s experience,

several factors have lead to a general industry resistance to initially perform an accurate load 

calculation, which is necessary for the design of a right-sized HVAC system. Historically, energy

codes did not address stringent levels of energy efficiency, and rules of thumb were developed 

for HVAC sizing that worked based on the construction at that time. Building enclosures have 

become more and more energy efficient as energy codes have become more stringent since 2000,

but these rules of thumb have not changed. Full credit should be taken by the HVAC system 

designer for improvements such as better windows, enhanced air tightness strategies, and 

additional insulation. 
3

HVAC contractors are also not 100% convinced that the builder is managing all other trades to 

construct a thermal enclosure that truly achieves targeted performance levels. Typically, the 

HVAC installer will get the comfort complaint if the building enclosure has not met the builder’s 

stated specifications, even though the HVAC system is designed and installed according to the 

stated specifications. Many HVAC companies rely on the “400 square feet per ton” rule for 

sizing systems. Even when a load calculation is required to be performed, the contractor may 

often manipulate the inputs to get a result that is close to the “400 square foot” rule of thumb.

Overcoming this bias requires the builder and their trade base sharing an understanding of the 

importance of achieving the stated building enclosure specifications, and undertaking visual 

inspections and testing during construction to verify the building meets the intended level of 

performance. When these checks and balances are in place, the perceived need to add factors of 

safety during the heating and cooling load calculation process can be avoided. Appendix A 

contains suggested enhancements to the scope of work for the Mechanical Contractor or HVAC 

designer.

The critical inputs and their associated risks discussed in this guide are:

• Design Conditions

o Location

o Latitude 

o Elevation

o Outdoor temperature and relative humidity 

• Orientation

• Internal conditions

o Indoor temperature and relative humidity

• Building Enclosure

o Insulation levels of walls, ceilings, and floors

o Window specification 

o Thermal conductivity

o Solar Heat Gain Coefficient (SHGC)

o Infiltration and ventilation levels

o Interior and exterior shading

• Internal loads

o Number of occupants

o Electronics, lighting and appliances.

A concise checklist to aide in compiling the information required to accurately calculate loads is 

located in Appendix B of this document. 
4

Baseline Load Calculation

In order to demonstrate the impact on the loads when common inaccurate adjustments (also 

known in the industry as “safety factors”) are made to the house, ACCA MJ8 load calculations 

were performed on two houses (Figure 4) that meet the 2009 International Energy Conservation 

Code® (IECC) prescriptive path. The first (“Chicago House”) is a one story 2,223 ft2 (above 

grade) house with a full conditioned basement in Chicago, IL (IECC Climate Zone 5 - CZ5). The 

other (“Orlando House”) is a 2,223 ft2 one story slab-on-grade house in Orlando, FL (IECC 

Climate Zone 2 - CZ2). The input conditions used to model the two houses are listed in Table 1. 

 

Figure 4. Typical 2223 ft2 One Story House
5

Table 1. Baseline Model Parameters

Condition

Chicago House 

Climate Zone 5

Orlando House

Climate Zone 2

Outdoor Design 

Conditions

89°Fdb 73°Fwb Temp Cooling 

2°F Temp Heating

92°F db 76°Fwb Temp Cooling 

42°F Temp Heating

Indoor Design 

Conditions

75°F Temp 50% RH Cooling 

70°F Temp 30% RH Heating

75°F Temp 50% RH Cooling 

70°F Temp 30% RH Heating 

Roof Assembly Vented R-38 Attic, Asphalt 

shingle roofing, 1/2 inch gypsum 

drywall ceiling

Encapsulated R-31 Attic 

(“cathedralized attic”), 

Tile Slate Concrete roofing, 

1/2 inch gypsum drywall ceiling

Wall Assembly 9' height, 2x6 Framed walls, R-19, 

OSB exterior sheathing, Brick 

exterior, 1/2 inch interior drywall

9' height, Light weight 8" concrete 

block 3/4 inch interior XPS, R4.8, one coat light color stucco,

1/2 inch interior drywall

Foundation 

Assembly

9' height, 8" poured concrete wall, 

2" exterior XPS R10 6.5 feet 

below grade depth, Heavy damp 

soil type, concrete slab 8’ below 

grade

Heavy damp soil type, concrete 

slab-on-grade

Windows U = 0.35, SHGC = 0.5 U = 0.65, SHGC = 0.30

Ducts In Conditioned Space In encapsulated R31 Attic 

(“cathedralized attic”), 

Tightness = Supply and Return 

0.06 cfm/ft2, Radial center of 

room outlets, R-8 insulation

Shading Full credit for eaves, 50%exterior 

bug screens, light colored blinds 

at 45 degrees closed

Full credit for eaves, 50% exterior 

bug screens, light colored blinds 

at 45 degrees closed

Infiltration 0.10 ACHn (2.65 ACH50) 

Cooling 

0.19 ACHn (5.03 ACH50) 

Heating

0.10 ACHn (2.65 ACH50) 

Cooling 

0.19 ACHn (5.03 ACH50) 

Heating 

Ventilation Balanced supply and exhaust, 60 

cfm to meet ASHRAE standard 

62.2 with no energy or heat 

recovery

Balanced supply and exhaust, 60 

cfm to meet ASHRAE standard 

62.2 with no energy or heat 

recovery
6

The calculated loads are listed in Table 2. The peak heating and cooling load is in Btu/h (Btu per 

hour). The nominal size of the cooling equipment for these two houses is 2 tons, (1 nominal ton 

= 12,000 Btu/h) based on the calculated cooling loads of 20,600 Btu/h and 20,700 Btu/h for 

Chicago and Orlando respectively. The Load Dependencies section of this Guide shows the 

impact on the calculated loads (and therefore the nominal equipment size) when the baseline 

conditions listed in Table 2 are modified to apply “safety factors” in an effort to come closer to 

the industry standard sizing of 400 square feet per ton. 

Table 2. Baseline Calculated Loads Using ACCA MJ8

Chicago House Orlando House

Heating Load 41,700 Btu/h Heating Load 23,600 Btu/h

Sensible Cooling 17,400 Btu/h Sensible Cooling 16,600 Btu/h

Latent Cooing 3,200 Btu/h Latent Cooing 4,000 Btu/h

Total Cooling 20,600 Btu/h Total Cooling 20,700 Btu/h

Load Components

Heating and cooling load calculations are dependent on the building location, indoor design 

conditions, orientation, and building construction. The use of commercial software to calculate 

the heating and cooling loads greatly simplifies the process. Various software applications that 

provide results certified by ACCA to meet all requirements of MJ8 are available. 
7

Peak Heating Load

The peak heating load represents the amount of heat lost to the outdoor environment at design 

outdoor and indoor conditions, which must be made up by the HVAC system to maintain 

occupant comfort (Figure 5). There is one relatively straightforward and uncomplicated heat loss 

calculation procedure used in ACCA MJ8. The components of the heating load calculation are 

covered in depth in Section 4 of the ACCA MJ8. The total estimated heat loss is a combination 

of the sensible heat loss through conduction, infiltration, and ventilation loads. No credit is taken 

for solar gains or internal loads in calculating the heating load because the peak heat loss occurs 

at night during periods of occupant inactivity.

Figure 5. Heat Loss Locations
8

Peak Cooling Load

Peak cooling loads represent the amount of heat gained by the house from the outdoor

environment at design conditions, which must be removed by the HVAC system to maintain 

occupant comfort. Cooling loads are made up of the sensible and latent heat gains. The 

mechanisms of heat gain are conduction, infiltration, ventilation, and radiation (Figure 6). The 

components of the cooling load calculation are covered in depth in ACCA MJ8.

Figure 6. Heat Gain Locations
9

The cooling load calculation procedure in ACCA MJ8 is more complex than the heating load 

calculation procedure. ACCA MJ8 documents two cooling load calculation procedures in 

Section 5-1, the Average Load Procedure (ALP) and the Peak Load Procedure (PLP).

The ALP uses the conditions that are encountered in late afternoon during midsummer when 

solar gains and temperatures are expected to be highest. This is a simplified method to create 

dependable cooling load estimates for conventional single family detached houses with a central 

single zoned comfort system with Adequate Exposure Diversity (AED). See the Orientation 

section of this Guide for a description of the Test for Adequate Exposure Diversity. The ALP 

procedure minimizes the complexity of the calculations, however, it must be noted the values are 

averaged for room-to-room loads and approximate the peak block load for the structure. 

The PLP looks at time of day sensitivities for midsummer conditions, recognizing that in some 

designs certain rooms may have significant solar gains during midday or the morning. An 

example might be a highly glazed sunroom on the south face of a house with considerable east, 

west and south glass, or a living room with lots of south facing glass and a moderate amount of 

west glass. The PLP is the residential equivalent to a commercial cooling load procedure, and 

should be used when a house does not have AED. It is used to estimate peak room loads for the 

midsummer season and the hour of day in which that room has its maximum solar gain. The 

whole house cooling load is the sum of the peak for each room.
10

Location – Design Conditions

The building location is described by its latitude and elevation. This location describes the values 

for the outdoor design conditions such as the elevation of the location, latitude, winter heating 

dry bulb temperatures, summer cooling dry bulb temperatures and relative humidity (RH)

information. ACCA MJ8 Table 1A Outdoor Design Conditions for the United States has 

ASHRAE tabulated data for various cities throughout the United States. An example for 

Orlando, FL, is shown in Table 3. The outdoor design conditions are determined by choosing the 

city from the table closest to the project city. 

Table 3. Outdoor Design Conditions for the United States - Manual J Version 8, Table 1A

Location Elevation Latitude Winter Summer

Feet Degrees

North

Heating

1%

Dry Bulb

Cooling

1%

Dry Bulb

Coincident

Wet Bulb

Design

Grains

55% RH

Design

Grains

50% RH

Design

Grains

45% RH

Daily

Range

(DR)

Florida

Apalachicola 20 29 35 90 78 57 64 70 M

Belle Glade 17 26 44 91 76 42 49 55 M

Cape Kennedy AP 10 28 42 90 78 57 64 70 L

Daytona Beach AP 31 29 37 90 77 51 58 64 L

Fort Lauderdale 10 26 50 90 78 57 64 70 L

Fort Myers AP 15 26 47 93 77 46 53 59 M

Fort Pierce 25 27 42 90 78 57 64 70 M

Gainsville AP 152 29 33 92 77 47 54 60 M

Housestead, AFB 7 25 52 90 76 57 64 70 L

Jacksonville AP 26 30 32 93 77 46 53 59 M

Jacksonville/Cecil Field NAS 80 30 34 95 76 35 43 49 M

Jacksonville, Mayport Naval 16 30 39 92 78 54 61 67 M

Key West AP 4 24 58 89 79 66 73 79 L

Lakeland CO 214 28 41 91 76 42 49 55 M

Melbourne 15 28 43 91 79 62 69 75 M

Miami AP 11 25 50 90 77 51 58 64 L

Miami Beach CO 8 25 48 89 77 52 59 65 L

Miami, New Tamiami AP 10 25 49 91 78 56 59 65 L

Milton, Whiting Field NAS 200 30 31 93 77 46 53 59 M

Ocala 90 29 34 93 77 46 53 59 M

Orlando AP 100 28 42 93 76 39 46 52 M

Panama City, Tyndall AFB 18 30 37 89 79 66 73 79 L

Pensacola CO 30 30 32 92 78 54 61 67 L
11

Indoor design conditions for the cooling season design conditions of 75 ̊F 50% RH and heating 

season design conditions of 70 ̊F 30% RH are prescribed in MJ8 and based on the ASHRAE 

Comfort Zone Chart (Figure 7). These conditions represent the regions of the ASHRAE comfort 

zone (shaded areas) where, on average, people feel most comfortable. If local codes or 

regulations specify other design conditions, use those instead of the values in ACCA MJ8 and 

the ASHRAE tables.

 

Figure 7. ASHRAE Winter and Summer Comfort Zones

©2009 ASHRAE Handbook Fundamentals. American Society of Heating, Refrigerating and AirConditioning Engineers, Inc., www.ashrae.org. Reprinted with permission.
12

The outdoor design conditions described by the ASHRAE tables represent long-term average 

temperatures that will not be exceeded more than a few hours per season. They do not represent 

the worst weather conditions ever experienced in a given location. Using the Orlando House as 

an example, Figure 8 shows the temperatures above the outdoor design temperature of 93 ̊F will 

occur only 13 hours out of the 8,760 hours in a year. The ASHRAE tabulated temperature data is 

adequate for calculating peak heating and cooling loads, and should not be increased as an 

additional safety factor. 

Figure 8. Outdoor cooling bin hours in Orlando, FL

A right-sized system will run close to 100% of the time at the outdoor design condition and 

proportionally less at temperatures closer to the inside design temperatures. Designing a system 

using peak heating and cooling loads based on extreme weather conditions that occur for only a 

few hours per season, such as the hottest day on record, will result in an oversized system. Oversized systems in humid climates may not run long enough under some part load conditions to 

achieve humidity removal from the air and keep the house within the ASHRAE comfort zone. 
13

Orientation

The orientation of the house must be considered in the cooling load calculation due to changing 

solar heat gains at various times of the day (Figure 9) and the impact of those gains. North, 

Northeast, East, Southeast, South, Southwest, West, Northwest or North are typically the 

orientations used for undertaking load calculations for production housing, although the exact 

cardinal orientation can be used for houses specifically sited on a given lot. The orientation of 

the house can greatly affect the sensible heat gain on the house depending on the ratio of 

windows to opaque walls and the degree of shading from the sun. 

Figure 9. Building Orientation

For production homebuilding, all orientations for given house must be considered. Often times,

the peak cooling load for the worst case orientation is acceptable for system sizing; however, if 

there is a significant difference between loads at various orientations, system sizing may vary for 

the same house. In the case of the Chicago House, the cooling load varies by 4,400 Btu/h

between the worst case and best case orientations of the house (Figure 10). When the HVAC 

design moves to the system sizing step, Manual S, this variance may impact the system size.

Figure 10. Building Orientation Cooling Load
14

The AED limit is an hourly load gain through glazing 30% greater than the average hourly load 

gain. If a dwelling does not have AED, the architectural features of the dwelling, such as a high 

percentage of windows in one orientation, will cause a spike in the block cooling load (heat gain) 

at a particular time of day. The Test for Adequate Exposure Diversity is a procedure that is best 

performed with the aid of software. 

Figure 11 is a software-generated output that shows a house passing the Test for Adequate 

Exposure Diversity by not exceeding that maximum. Figure 12 shows a house that fails by 

slightly exceeding the AED limit late in the day. The results of the Test for Adequate Exposure 

Diversity require appropriate design judgment in the interpretation of the software output. 

Appendix 3 of the ACCA MJ8 describes the procedure to compensate for a house that does not 

pass the Test for Adequate Exposure Diversity by compensating for the load only in those rooms 

with the inadequate exposure diversity without oversizing the whole system. If the variance 

shown in the test is not addressed, there will likely be a comfort issue in the rooms with the high 

glazing load. 

Figure 11. Test for Adequate Exposure Diversity – Pass

Peak within limits
15

Figure 12. Test for Adequate Exposure Diversity – Fail

Building Components

Building construction, proper details, and materials are critical components of the heating and 

cooling load calculations. The R-value of the building wall, roof, and foundation construction 

components can be accurately calculated using the insulation levels specified combined with the 

remainder of the components that make up the construction assembly (i.e. drywall, sheathing, 

exterior siding materials, structural framing system, roofing materials, etc.). The window 

performance, described by the U-value and SHGC, must be known and accurately represented by 

the data input. Shading provided by the overhang of eaves, insect screens, and internal blinds or 

shades will reduce the sensible heat gain. If shading is ignored in the load calculation the cooling 

load will be inflated.

Heating and Cooling System Location and Duct Leakage

Best practice for HVAC design is to keep all ductwork within the conditioned space in order to 

eliminate the duct losses/gains to and from the outside conditions. Scenarios, such as the onestory slab-on-grade Orlando House, present challenges in keeping all ductwork inside 

conditioned spaces. In a slab-on-grade house, it is typical for an installer to put the HVAC 

system completely in the attic. Because it has a basement, the Chicago House does not present 

the same challenges to keeping the ductwork inside conditioned space. In a single-story house 

with a basement, the duct system is typically run in the basement, which is considered 

conditioned space provided the basement walls are insulated or there are supply registers in the 

basement. For ducts outside conditioned space, the heating and cooling loads are more sensitive 

to duct leakage and R-values of the duct insulation.

Peak exceeds limits
16

Ventilation/Infiltration 

Ventilation and infiltration brings outside air into the conditioned space, impacting the heating 

and cooling load. The target ventilation and infiltration rate must be accurately represented in the 

data input of the load calculation. In humid climates, the impact on the latent cooling load added 

by ventilation/infiltration can be significant. 

Risks Associated with Oversizing

Oversizing the HVAC system is detrimental to energy use, comfort, indoor air quality, building, 

and equipment durability. All of these impacts derive from the fact that the system will be “short 

cycling” in both the heating and cooling modes. To reach peak operational efficiency and 

effectiveness, a heating and cooling system should run for as long as possible to meet the loads. 

An analogy is that of a car: highway driving at a steady speed will get the best fuel economy, 

while speeding up and slowing down puts undue wear and tear on the engine and braking 

systems, and reduces fuel efficiency. 

First Cost, Energy Costs and Durability

An oversized HVAC system will have both a higher initial cost and a higher cost of operation. 

The frequent starting and stopping of short cycling can also lead to premature failure of the 

equipment. 

Comfort - Space Temperatures

Short cycling limits the total amount of air circulating through each room, and can lead to rooms 

that do not receive adequate duration of airflow. Short cycling of an oversized system can lead to 

comfort complaints when the spaces located further from the thermostat do not change 

temperature as quickly as spaces near the thermostat. Even in an energy-efficient house with an 

enhanced thermal enclosure, this can lead some rooms being colder during the heating season

and warmer in the cooling season. In attempt to make the spaces further from the thermostat 

more comfortable, the occupant may set the thermostat set point higher, requiring additional 

energy. 

Comfort - Humidity Control

The risks associated with oversizing the cooling system, particularly in more humid climates, are 

also a concern. In the cooling season in humid climates, cold clammy conditions can occur due 

to reduced dehumidification caused by the short cycling of the equipment. The cooling system 

removes moisture from the air by passing the air across a condensing coil. The system must run 

long enough for the coil to reach a temperature where condensation will occur and an oversized 

system that short cycles may not run long enough to sufficiently condense moisture from the air. 

Excess humidity in the conditioned air delivered to a space may lead to mold growth within the 

house. 
17

Load Dependencies

Several common safety factors were applied to the baseline models to create examples of how 

and where load calculations can be inflated leading to system oversizing. 

The baseline load calculations used in this Guide were manipulated for:

• Outdoor/Indoor Design Conditions 

• Building Components

• Ductwork Conditions 

• Ventilation/Infiltration Conditions

• Worst Case Scenario (combining all safety factors).

Outdoor/Indoor Design Condition Dependencies

To illustrate the effects on the load calculation of manipulating the outdoor/indoor design 

conditions, the indoor conditions were reversed and the outdoor conditions manipulated to the 

mean extreme temperatures (Tables 4 and 5). Outdoor temperatures for the manipulated models 

were taken from the Outdoor Mean Extreme temperatures listed in the ASHRAE 2009

Fundamentals handbook. These changes are representative of those the author has observed from

designers wishing to bump the system size up for added cushion. 

Table 4. Chicago House Manipulated Outdoor/Indoor Design Conditions 

Baseline Manipulated

Outdoor Design Conditions 

89°Fdb 73°Fwb Temp Cooling 

2°F Temp Heating

Outdoor Design Conditions 

97°Fdb 78°Fwb Temp Cooling

-11°F Temp Heating

Indoor Design Conditions

75°F Temp 50% RH Cooling 

70°F Temp 30% RH Heating

Indoor Design Conditions

70°F Temp 30% RH Cooling 

75°F Temp 50% RH Heating

Table 5. Orlando House Manipulated Outdoor/Indoor Design Conditions

Baseline Manipulated

Outdoor Design Conditions 

93°Fdb 76°Fwb Temp Cooling 

42°F Temp Heating

Outdoor Design Conditions 

96°Fdb 79°Fwb Temp Cooling 

30°F Heating

Indoor Design Conditions

75°F Temp 50%RH Cooling 

70°F Temp 30% RH Heating 

Indoor Design Conditions

70°F Temp 30% RH Cooling 

75°F Temp 50%RH Heating
18

The results of changing the outdoor/indoor design conditions while keeping all other parameters 

for the Chicago House the same are shown in Table 6 and Figure 13. When the system is 

selected, this manipulation would effectively oversize the cooling system by 1 ton. 

Table 6. Results – Chicago House Manipulated Outdoor/Indoor Design Conditions

Baseline 

Load

Manipulated 

Load

Change In 

Load Btu/h

Change In 

Load %

Heating Load 41,700 Btu/h 52,700 Btu/h 11,000 Btu/h 26 %

Sensible Cooling 17,400 Btu/h 24,100 Btu/h 6,700 Btu/h 39 %

Latent Cooing 3,200 Btu/h 6,800 Btu/h 3,600 Btu/h 113 % 

Total Cooling 20,600 Btu/h 31,000 Btu/h 10,400 Btu/h 51%

Figure 13. Chicago House Change in Loads, Manipulated Outdoor/Indoor Design Conditions 
19

The results of changing the outdoor/indoor design conditions while keeping all other parameters 

for the Orlando House the same are shown in Table 7 and Figure 14. When the system is 

selected, this manipulation would effectively oversize the cooling system by 1 ton.

Table 7. Results - Orlando House Manipulated Outdoor/Indoor Design Conditions

Baseline 

Load

Manipulated 

Load

Change In 

Load Btu/h 

Change In 

Load %

Heating Load 23,600 Btu/h 37,800 Btu/h 14,100 Btu/h 60 %

Sensible Cooling 16,600 Btu/h 22,900 Btu/h 6,300 Btu/h 38 %

Latent Cooing 4,100 Btu/h 7,100 Btu/h 3,000 Btu/h 73 %

Total Cooling 20,700 Btu/h 30,100 Btu/h 9,400 Btu/h 45 %

Figure 14. Orlando House Change in Loads, Manipulated Outdoor/Indoor Design Conditions 
20

Building Components Dependencies 

To illustrate the effects on the load calculation of adding a safety factor by manipulating or 

ignoring known building component values and ignoring shading, the building component 

specifications were changed as shown in Tables 8 and 9. 

Table 8. Chicago House Building Component Manipulations

Baseline Manipulated

Windows U = 0.35, SHGC = 0.5

Walls R-19

Attic R-38 

Full credit for eaves, 50% exterior bug 

screens, light colored blinds at 45 degrees 

closed

Windows U = 0.45 SHGC = 0.5 

Walls R-17

Attic R-30 

No credit for eaves, no bug screens, no blinds

Table 9. Orlando House Building Component Manipulations

Baseline Manipulated

Windows U = 0.65, SHGC = 0.30

Walls Light weight concrete block w/ .75 inch 

XPS, R-4.8

Attic R-31 sealed

Full credit for eaves, 50% exterior bug 

screens, light colored blinds at 45 degrees 

closed

Windows U = 0.65 SHGC = 0.40 

Walls Light weight concrete block no XPS R1

Attic R-30 vented 

No credit for eaves, no bug screens, no blinds

The wall R-value downgrade for the Chicago House is equivalent to the designer attempting to 

account for a poor installation based on the RESNET Grade 3 installation quality level or 

ignoring the XPS for the Orlando House. The attic insulation and window downgrade are 

equivalent to the designer not resetting default values or using values lower that the IECC 2009 

levels. These changes are representative of those the author has observed from designers wishing 

to bump the system size up to for added cushion. 
21

The results of manipulating the building components and shading while keeping all other 

parameters for the Chicago House the same are shown in Table 10 and Figure 15. When the 

system is selected, this manipulation would effectively oversize the cooling system by 1⁄2 ton. 

Table 10. Results - Chicago House Building Component Manipulations

Baseline 

Load

Manipulated 

Load

Change In 

Load Btu/h

Change In 

Load %

Heating Load 41,700 Btu/h 46,300 Btu/h 4,600 Btu/h 11%

Sensible Cooling 17,400 Btu/h 22,400 Btu/h 5,000 Btu/h 28 %

Latent Cooing 3,200 Btu/h 3,200 Btu/h 0 Btu/h 0 %

Total Cooling 20,600 Btu/h 25,700 Btu/h 5,100 Btu/h 24 %

Figure 15. Chicago House Change in Loads, Building Component Manipulations
22

The results of manipulating the building components and shading while keeping all other 

parameters for the Orlando House the same are shown in Table 11 and Figure 16. When the 

system is selected, this manipulation would effectively oversize the cooling system by 1⁄2 ton.

Table 11. Results - Orlando House Building Component Manipulations

Baseline 

Load

Manipulated 

Load

Change In 

Load Btu/h

Change In 

Load %

Heating Load 23,600 Btu/h 26,600 Btu/h 3,000 Btu/h 12 %

Sensible Cooling 16,600 Btu/h 23,000 Btu/h 6,400 Btu/h 39 %

Latent Cooing 4,100 Btu/h 4,200 Btu/h 100 Btu/h 2 %

Total Cooling 20,700 Btu/h 27,100 Btu/h 6,400 Btu/h 31 %

Figure 16. Orlando House Change in Loads, Building Component Manipulations
23

Ductwork Conditions Dependencies

To illustrate the effects on the load calculation of adding a safety factor by manipulating or 

ignoring the ductwork conditions, the specifications were changed as listed in Table 12. This 

manipulation was not performed on the Chicago House since the Chicago House has an insulated 

basement and duct work in conditioned space. As discussed in the Heating and Cooling System 

Location and Duct Leakage section of this Guide, the duct systems in single story houses are 

typically located in conditioned basements. Basements are considered conditioned space 

provided the basement walls are insulated or they have supply registers. The manipulated duct 

tightness values used for the Orlando House are the ACCA MJ8 average duct tightness default 

values. 

Table 12. Orlando House Ductwork Conditions Manipulations

Baseline Manipulated

Tightness level Supply = 0.06 cfm/ft2

Tightness level Return = 0.06 cfm/ft2

Insulation = R-8

Tightness level Supply = 0.12 cfm/ft2

Tightness level Return = 0.24 cfm/ft2

Insulation = R-6
24

The results of manipulating the ductwork characteristics while keeping all other parameters for 

the Orlando house the same are shown in Table 13 and Figure 17. When the system is selected,

this manipulation would effectively oversize the cooling system by 1⁄2 ton.

Table 13. Results - Orlando House Ductwork Conditions Manipulations

Baseline 

Load

Manipulated 

Load

Change In 

Load Btu/h

Change In 

Load %

Heating Load 23,600 Btu/h 26,700 Btu/h 3,100 Btu/h 13 %

Sensible Cooling 16,600 Btu/h 19,100 Btu/h 2,500 Btu/h 15 %

Latent Cooing 4,100 Btu/h 5,100 Btu/h 1,000 Btu/h 24 %

Total Cooling 20,700 Btu/h 24,200 Btu/h 3,500 Btu/h 17 %

Figure 17. Orlando House Change in Loads, Ductwork Conditions Manipulations
25

Ventilation/Infiltration Conditions Dependencies 

To illustrate the effects on the load calculation of adding a safety factor by manipulating or 

ignoring the ventilation/infiltration, Tables 14 and 15 show the specification changes using the 

ACCA MJ8 default infiltration rate of “semi-loose construction” and a ventilation rate of an 

arbitrary continuous 100 cfm that may have been determined by looking at the combined effect 

of simultaneously running all the exhaust fans in the house. 

Table 14. Chicago House Ventilation/Infiltration Manipulations

Baseline Manipulated

Heating season infiltration = 0.19 ACHn (5.04 

ACH50)

Cooling season infiltration = 0.10 ACHn (2.65 

ACH50)

Ventilation balanced 60 cfm to meet ASHRAE 

standard 62.2 without energy or heat recovery

Heating season infiltration = 0.43 ACHn 

(11.39 ACH50)

Cooling season infiltration = 0.23 ACHn 

(6.09 ACH50)

Ventilation exhaust only 100 cfm

Table 15. Orlando House Ventilation/Infiltration Manipulations

Baseline Manipulated

Heating season infiltration = 0.19 ACH (5.03 

ACH50)

Cooling season infiltration = 0.10 ACH (2.65 

ACH50)

Ventilation balanced 60 cfm to meet ASHRAE 

standard 62.2 without energy or heat recovery

Heating season infiltration = 0.43 ACH (11.39 

ACH50)

Cooling season infiltration = 0.23 ACH (6.09 

ACH50)

Ventilation exhaust only 100 cfm
26

The results of manipulating the ventilation/infiltration rates while keeping all other parameters 

for the Chicago House the same are shown in Table 16 and Figure 18. This manipulation alone is 

not enough to increase the cooling system size. 

Table 16. Results - Chicago House Ventilation / Infiltration

Baseline 

Load

Manipulated 

Load

Change In 

Load Btu/h

Change In 

Load %

Heating Load 41,700 Btu/h 46,600 Btu/h 4,900 Btu/h 12 %

Sensible Cooling 17,400 Btu/h 17,900 Btu/h 500 Btu/h 3 %

Latent Cooing 3,200 Btu/h 4,100 Btu/h 900 Btu/h 28 %

Total Cooling 20,600 Btu/h 22,000 Btu/h 1,400 Btu/h 7 %

Figure 18. Chicago House Change in Loads, Ventilation/Infiltration Manipulations
27

The results of manipulating the ventilation/infiltration rates while keeping all other parameters 

for the Orlando House the same are shown in Table 17 and Figure 19. This manipulation alone is 

not enough to increase the cooling system size. 

Table 17. Results – Orlando House Ventilation/Infiltration

Baseline 

Load

Manipulated 

Load

Change In 

Load Btu/h

Change In 

Load %

Heating Load 23,600 Btu/h 25,600 Btu/h 2,000 Btu/h 9 %

Sensible Cooling 16,600 Btu/h 17,300 Btu/h 700 Btu/h 4 %

Latent Cooing 4,100 Btu/h 5,200 Btu/h 1,100 Btu/h 27 %

Total Cooling 20,700 Btu/h 22,500 Btu/h 1,800 Btu/h 9 %

Figure 19. Orlando House Change in Loads, Ventilation/Infiltration Manipulations
28

Combined Dependencies

Each of the safety factors applied to the outdoor/indoor design conditions, building components, 

ductwork conditions, or ventilation/infiltration conditions outlined above has its own impact on 

the heating and cooling loads. But, a more significant impact occurs when the safety factors are 

combined. The compounding of safety factors will result in an inflated load that cannot be relied 

upon for the remaining steps of the HVAC design process. To illustrate the effects on the load 

calculation of the compounded safety factors, all of the individual impacts were applied to the 

baseline models. 

The results of combining the manipulations while keeping all other parameters for the Chicago 

House the same are shown in Table 18 and Figure 20. When the system is selected this 

manipulation would effectively oversize the cooling system by 11⁄2 tons.

Table 18. Results - Chicago House Combined Manipulations

Baseline 

Load

Manipulated 

Load

Change In 

Load Btu/h

Change In 

Load %

Heating Load 41,700 Btu/h 64,700 Btu/h 23,000 Btu/h 55 %

Sensible Cooling 17,400 Btu/h 31,600 Btu/h 14,200 Btu/h 82 %

Latent Cooing 3,200 Btu/h 9,100 Btu/h 5,900 Btu/h 184 %

Total Cooling 20,600 Btu/h 40,600 Btu/h 20,000 Btu/h 97 %

Figure 20. Chicago House Change in Loads, Combined Manipulations
29

The results of combining the manipulations while keeping all other parameters for the Orlando 

House the same are shown in Table 19 and Figure 21. When the system is selected, this 

manipulation would effectively oversize the cooling system by 3 tons.

Table 19. Results - Orlando House Combined Manipulations Results

Baseline 

Load

Manipulated 

Load

Change In 

Load Btu/h 

Change In 

Load % 

Heating Load 23,600 Btu/h 57,200 Btu/h 33,600 Btu/h 142 %

Sensible Cooling 16,600 Btu/h 40,200 Btu/h 23,600 Btu/h 142 %

Latent Cooing 4,100 Btu/h 13,900 Btu/h 9,800 Btu/h 239%

Total Cooling 20,700 Btu/h 54,000 Btu/h 33,300 Btu/h 161 %

Figure 21. Orlando House Change in Loads, Combined Manipulations
30

Conclusion

Right-sizing the HVAC system begins with an accurate understanding of the heating and cooling 

loads on a space. The values determined by the heating and cooling load calculation process 

dictate the equipment selection and the duct design needed to deliver conditioned air to the 

rooms of the house to meet the occupant’s comfort expectations.

Examples in this guide showed the implications when inaccurate or inappropriate adjustments

are applied during the heating and cooling load calculation process. Seemingly small 

manipulations such as changing the outdoor/indoor design conditions can result in exaggerated 

loads. For example, the Orlando House manipulations of outdoor/indoor design conditions

showed a 9,400 Btu/h (45%) increase in the total cooling load, which may increase the system 

size by 1 ton when the ACCA Manual S procedures are applied. 

Combining several adjustments only compounds the inaccuracy of the calculation results. The 

results of the combined manipulations to outdoor/indoor design conditions, building components, 

ductwork conditions, and ventilation/infiltration conditions produce significantly oversized 

calculated loads. The Orlando House example showed a 33,300 Btu/h (161%) increase in the 

calculated total cooling load, which may increase the system size by 3 tons (from 2 tons to 5 

tons) when the ACCA Manual S procedures are applied. Not only does this oversizing impact 

the heating and cooling equipment costs, but duct sizes and numbers of runs must also be 

increased to account for the significantly increased system airflow. 

Again, ACCA MJ8 says of compounding safety factors:

“Research studies and the experience of multiple system designers indicate that 

aggressive use of Manual J procedures provides adequate accuracy. No additional safety 

factors are required when load estimates are based on accurate information pertaining to 

the envelope construction and duct system efficiency. Large errors are possible if there is 

uncertainty about insulation levels, fenestration performance, envelope tightness or the 

efficiency of the duct runs installed in the unconditioned space.”

The compounding of arbitrary safety factors must be avoided in the load calculation process. The 

HVAC design industry’s practice of using extreme outdoor design conditions, de-rating the 

insulation value, and over-estimating the infiltration rate render the calculated load values

meaningless.
31

References

O’Neal, Dennis L. et.al. ASHRAE 2009 Fundamentals Volume. Atlanta, GA: ASHRAE. 2009.

Rutkowowski P.E., Hank. Manual J Residential Load Calculation Eighth Edition Version Two. 

Arlington, VA: ACCA. 2006

National Renewable Energy Laboratory (NREL). Typical Meteorological Year 3 Data Set.

NREL Website. http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/. 2008.
32

Appendix A: Enhancements to Scope for Mechanical 

Contractor/HVAC Manual J Designer

Design Analysis to be completed by Engineer or Contractor, also known as the “Designer”

Calculations to be performed using ACCA Manual J Version 8 Prerequisites:

• All inputs/defaults/building characteristics used for the ACCA Manual J Version 8 

calculations must be verified by the HVAC contractor/Mechanical designer in the field 

and/or must be verified and signed off by the builder that is what is being built 

consistently across all of the builder’s houses in that community or division.

• Design conditions as listed in Table 1A of ACCA Manual J version 8 for the following 

city shall be used <insert city>. Indoor conditions shall be 75°F Temp 50% RH Cooling 

and 70°F Temp 30% RH Heating

The “Designer” shall provide the following information to the builder and HVAC installer:

• Provide room-by-room and building heat gain/loss demand loads based on installed lights 

and equipment, envelope heat transfer, occupants, required ventilation and all other 

factors normally considered in these calculations. Loads are to be determined by analysis 

of actual load calculations performed using ACCA Manual J Version 8 and not by “rules 

of thumb”.

• Determine room supply, return or exhaust air requirements based on the heat gain/loss 

calculations, total and outside air change requirements, and temperature, humidity, 

ventilation, and pressurization criteria for each room or space.
33

Appendix B: ACCA Manual J8 Input Checklist

House Location (city, state)................................................................... _____________________

House Orientation...................................................................................... _____________________

*Complete set of construction drawings including all options are necessary to properly perform a Manual 

J8 analysis. 

Insulation R-Values

Foundation

• Basement Depth below grade .............................................................. _____________________

• Crawlspace Depth below grade ........................................................... _____________________

• Soil Type .............................................................................................. _____________________

• Wall Type (concrete, ICF, CMU, brick, stone, AWW, metal frame) ..... _____________________

• Foundation Wall Thickness .................................................................. _____________________

• Insulation Type ..................................................................................... _____________________

• Insulation Location ............................................................................... _____________________

• Insulation R-Value ................................................................................ _____________________

Walls

Stud Walls

• Stud Spacing ........................................................................................ _____________________

• Stud Depth ........................................................................................... _____________________

• Drywall Thickness ................................................................................ _____________________

• Exterior Sheathing Type ...................................................................... _____________________

• Insulation Type ..................................................................................... _____________________

• Insulation Location ............................................................................... _____________________

• Insulation R-Value ................................................................................ _____________________

Non-Stud Walls

• Wall Thickness ..................................................................................... _____________________

• Drywall Thickness ................................................................................. _____________________

• Insulation Type ..................................................................................... _____________________

• Insulation Location ............................................................................... _____________________

• Insulation R-Value ................................................................................ _____________________

Floors Over Unconditioned Space

• Sheathing Type..................................................................................... _____________________

• Insulation Type ..................................................................................... _____________________

• Insulation Location ............................................................................... _____________________

• Insulation R-Value ................................................................................ _____________________
34

Attic

• Insulation Location ............................................................................... _____________________

• Insulation R-Value ................................................................................ _____________________

• Roofing Type ........................................................................................ _____________________

• Sealed or Ventilated.............................................................................. _____________________

Windows

• U–value ................................................................................................ _____________________

• SHGC ................................................................................................... _____________________

• Low-E or Clear Glazing ........................................................................ _____________________

• Number of Panes ................................................................................. _____________________

Air Infiltration

• Building Envelope Leakage Rate ......................................................... _____________________

Ductwork 

• Insulation Value .................................................................................... _____________________

• Location ................................................................................................ _____________________

• Leakage Rate ....................................................................................... _____________________

Ventilation

• Ventilation Rate .................................................................................... _____________________

• Continuous or Intermittent Ventilation................................................... _____________________

Space Conditioning Zoning

• Multi Zone ............................................................................................ _____________________

• Single Zone .......................................................................................... _____________________

Stop Work – Do Not Proceed If:
DOE/GO-102011-3304 ▪ June 2011

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