Strategy Guideline: 

HVAC Equipment Sizing

Arlan Burdick

IBACOS, Inc.

February 2012
This report received minimal editorial review at NREL

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i

Strategy Guideline: 

HVAC Equipment Sizing

Prepared for: 

Building America

Building Technologies Program

Office of Energy Efficiency and Renewable Energy 

U.S. Department of Energy 

Prepared by: 

Arlan Burdick

IBACOS, Inc.

2214 Liberty Avenue

Pittsburgh, Pennsylvania 15222

NREL Technical Monitor: Michael Gestwick

Prepared under Subcontract No. KNDJ-0-40341-02

February 2012
ii

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iii

Contents

List of Figures ............................................................................................................................................ iv

List of Tables ............................................................................................................................................... v

Definitions................................................................................................................................................... vi

Executive Summary.................................................................................................................................. vii

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

2 Overview................................................................................................................................................ 3

2.1 Design Conditions................................................................................................................6

2.2 Preliminary Equipment Selection ........................................................................................7

2.3 Manufacturer Data Tables....................................................................................................8

2.4 Cooling Equipment Capacity Sensitivities..........................................................................9

2.5 Estimated Target Cubic Feet per Minute ...........................................................................12

2.6 Efficiency Considerations..................................................................................................13

3 Chicago House Example.................................................................................................................... 15

3.1 Cooling Equipment Selection ............................................................................................15

3.2 Furnace Selection...............................................................................................................20

4 Orlando House Example .................................................................................................................... 24

4.1 Air Source Heat Pump Selection .......................................................................................26

4.2 Heat Pump Cooling Capacity Selection.............................................................................27

4.3 Heat Pump Heating Capacity Selection.............................................................................30

5 Conclusion .......................................................................................................................................... 35

References................................................................................................................................................. 36
iv

List of Figures

Figure 1. Design information from ACCA Manual J—Residential Load Calculation. .......................... 1

Figure 2. Upflow furnace scenario as in the Chicago House. ................................................................ 4

Figure 3. Heat pump scenario as in the Orlando House. ........................................................................ 5

Figure 4. General steps to equipment selection...................................................................................... 6

Figure 5. Design information needed........................................................................................................ 7

Figure 6. Fictitious example: detailed cooling capacities table............................................................. 8

Figure 7. Airflow sensitivity graph (from Manual S, Figure 3-1)........................................................... 10

Figure 8. OAT sensitivity graph (from Manual S, Figure 3-1). .............................................................. 10

Figure 9. Indoor EWB sensitivity graph (from Manual S, Figure 3-1). ................................................. 11

Figure 10. Indoor EDB temperature sensitivity graph (from Manual S, Figure 3-1)........................... 11

Figure 11. TD value table (from Manual S, Table 1-4)............................................................................ 12

Figure 12. Chicago House BEopt 1.1 output.......................................................................................... 13

Figure 13. Orlando House BEopt 1.1 output. ......................................................................................... 14

Figure 14. Cooling equipment selection step 1. .................................................................................... 15

Figure 15. Chicago House design information. ..................................................................................... 15

Figure 16. Cooling equipment selection step 2. .................................................................................... 16

Figure 17. Cooling equipment selection step 3. .................................................................................... 16

Figure 18. Wet-bulb temperature on sea-level psychometric chart..................................................... 17

Figure 19. Cooling equipment selection step 4. .................................................................................... 17

Figure 20. Fictitious example: detailed cooling capacities table, 800 cfm. ........................................ 18

Figure 21. Fictitious example: detailed cooling capacities table, 715 cfm and 750 cfm. .................. 19

Figure 22. Interpolation table for capacities at 89°F outdoor design temperature. ........................... 19

Figure 23. Furnace selection steps. ........................................................................................................ 20

Figure 24. Furnace specification chart. .................................................................................................. 21

Figure 25. Air delivery performance table. ............................................................................................. 22

Figure 26. Fictitious furnace performance example.............................................................................. 22

Figure 27. Air delivery performance table for the Chicago House. ..................................................... 23

Figure 28. Air source heat pump selection steps.................................................................................. 24

Figure 29. Development of balance-point diagram. .............................................................................. 25

Figure 30. Air source heat pump selection step 1................................................................................. 26

Figure 31. Orlando House design information....................................................................................... 26

Figure 32. Air source heat pump selection step 2................................................................................. 27

Figure 33. Air source heat pump selection step 3................................................................................. 28

Figure 34. Wet-bulb temperature on sea-level psychometric chart..................................................... 28

Figure 35. Cooling equipment selection step 4. .................................................................................... 29

Figure 36. Fictitious example: detailed heat pump cooling capacities table, 700 cfm. ..................... 30

Figure 37. Interpolation table for cooling capacities at 92°F outdoor design temperature. ............. 30

Figure 38. Cooling equipment selection step 5. .................................................................................... 31

Figure 39. Heat pump heating performance table at 700 cfm............................................................... 31

Figure 40. Interpolation table for heating capacities at 38°F outdoor design temperature. ............. 32

Figure 41. Cooling equipment selection step 6. .................................................................................... 32

Figure 42. Balance-point diagram for the Orlando House. ................................................................... 33

Figure 43. Fan coil nomenclature............................................................................................................ 34

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

List of Tables

Table 1. ACCA Manual J Heating and Cooling Loads. ............................................................................ 6
vi

Definitions

ACCA Air Conditioning Contractors of America

AFUE Annual Fuel Utilization Efficiency

AHRI Air-Conditioning, Heating, and Refrigeration Institute

ARI Air-Conditioning and Refrigeration Institute

ASHRAE American Society of Heating, Refrigerating, and Air 

Conditioning Engineers

BEopt Building Energy Optimization (software)

cfm Cubic Feet per Minute

EDB Entering Dry-Bulb

EWB Entering Wet-Bulb

HSPF Heating Seasonal Performance Factor

HVAC Heating, Ventilation, and Air Conditioning

IECC International Energy Conservation Code

LAT Temperature of Air Leaving the Cooling Coil

OAT Outdoor Air Temperature

SEER Seasonal Energy Efficiency Ratio

SHR Sensible Heat Ratio

TD Room-Air to Supply-Air Temperature Difference
vii

Executive Summary

Right-sizing of a residential heating, ventilation, and air conditioning (HVAC) system involves 

the selection of equipment and the design of the air distribution system to meet the accurate 

predicted heating and cooling loads of a house. This Strategy Guideline follows the Air 

Conditioning Contractors of America (ACCA) Manual S—Residential Equipment Selection

(Manual S) (Rutkowski 1995) to describe what information is needed and how to use that 

information to initially select the equipment for a properly designed HVAC system. Equipment 

selection and duct design are iterative processes where the capacity of the equipment is balanced 

against the design of the distribution system. 

This Strategy Guide describes the equipment selection of a split system air conditioner and 

furnace for an example house in Chicago, Illinois, as well as a heat pump system for an example 

house in Orlando, Florida. The required heating and cooling load information for the two 

example houses was developed by Burdick (2011).

Selection of the equipment can have a substantial impact on the efficiency and operating costs of 

the system. For the example houses presented in this Strategy Guide, a 92.5% AFUE (annual fuel 

utilization efficiency) furnace with a 16 SEER (seasonal energy efficiency ratio) air conditioner 

was the system chosen for the Chicago House and a 9.2 HSPF (heating seasonal performance 

factor)/16 SEER heat pump for the Orlando House.

With an accurate estimate of the heating and cooling loads completed, the HVAC system 

designer can begin the equipment selection procedure. It is the system designer’s responsibility 

to select the heating and cooling equipment and to verify that the selected pieces of equipment 

meet the following:

• The total cooling load, composed of both the sensible load (temperature) and the latent

load (humidity)

• The heating load

• The blower capacity (in cubic feet per minute [cfm]) to meet the volume of air cfm range 

needed.

The general steps to equipment selection are:

1. Determine accurate heating and cooling loads.

2. Select cooling equipment.

3. Select heating equipment.

4. Design the ducts.

It is worth noting that the design of a space conditioning system is an iterative process. Once a 

duct system configuration and preliminary design are completed, different equipment may need 

to be selected to achieve the proper airflow to satisfy the room-by-room heating and cooling 

loads.
1

1 Introduction

The heating, ventilation, and air conditioning (HVAC) system is arguably the most complex 

system installed in a house and is a substantial component of the total house energy use. A rightsized HVAC system will provide the desired occupant comfort and will run efficiently. This 

Strategy Guideline discusses the information needed to initially select the equipment for a 

properly designed HVAC system. Right-sizing of an HVAC system involves the selection of 

equipment and the design of the air distribution system to meet the accurate predicted heating 

and cooling loads of the house. Right-sizing the HVAC system begins with an accurate 

understanding of the heating and cooling loads on a space; however, a full HVAC design 

involves more than just the load estimate calculation—the load calculation is the first step of the 

iterative HVAC design procedure. 

The iterative nature of HVAC design is illustrated in Figure 1, along with the Strategy 

Guidelines that cover the various aspects of the design process. The heating and cooling loads 

depend on the building location and construction, whereas the equipment selection and 

distribution design depend on the loads and each other. After initial equipment selection, several 

adjustments to duct design and equipment selection may be needed to optimize the design.

Figure 1. Design information from ACCA Manual J—Residential Load Calculation.
2

The procedures of residential HVAC design are covered in detail by a series of publications 

produced by the Air Conditioning Contractors of America (ACCA), which in turn references 

information provided by the American Society of Heating, Refrigerating and Air-Conditioning 

Engineers (ASHRAE). The heat loss and gain values are estimated using the procedures from the

ACCA Manual J—Residential Load Calculation (Manual J) (Rutkowski 2006). Manual J applies 

only to single-family detached dwellings, low-rise condominiums, and townhouses. Mechanical 

equipment selection is done with the aid of the ACCA Manual S—Residential Equipment 

Selection (Manual S) (Rutkowski 1995). The ductwork to convey the proper amount of 

conditioned air to meet the load requirements of the space is designed with the aid of the ACCA 

Manual D—Residential Duct System Procedure (Manual D) (Rutkowski 2009a). 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 (Manual T) (Rutkowski 

2009b) provides the guidance on selecting the air outlet size and type. The ACCA procedures 

have been written into commercial software packages to help the designer work through the 

iterations required for a good design. Although commercial software is an important tool for 

design, it should be operated with a solid understanding of the procedures of good design.
3

2 Overview

For the purposes of this Strategy Guide, an energy-efficient house is defined as one that is 

designed and built for decreased energy use and improved 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. For 

example, ENERGY STAR® requires houses to be 15% more energy efficient than code. Beyond 

code 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 2009) establishes an estimated 15% improvement in energy efficiency over the 

previous 2006 IECC requirements. Ongoing code cycles incrementally 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 a 

beyond 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 beyond code programs, the peak heating and 

cooling loads are significantly reduced.

To illustrate the procedures of HVAC equipment selection, two theoretical houses that meet the 

2009 IECC prescriptive path were modeled. The first (the “Chicago House”) is a one-story,

2,223-ft2 (above-grade) house that has a full conditioned basement and is located in Chicago, 

Illinois (IECC Climate Zone 5 - CZ5). The other (the “Orlando House”) is a one-story 2,223-ft2

slab-on-grade house in Orlando, Florida (IECC Climate Zone 2 - CZ2).

In addition to the climatic differences between the two house locations, the Chicago House is a 

house over a full basement, whereas the Orlando House is a slab-on-grade foundation, allowing 

for the discussion of different mechanical and ductwork layouts. 

Figure 2 shows the Chicago House example split system with an upflow furnace and evaporator 

coil located in the basement and an outdoor condenser coil. Figure 3 shows the Orlando House

example air source heat pump with outdoor coil and horizontal fan coil in the conditioned attic. 
4

Figure 2. Upflow furnace scenario as in the Chicago House.
5

Figure 3. Heat pump scenario as in the Orlando House.
6

2.1 Design Conditions

The impacts and importance of accurate calculation of heating and cooling loads are covered in 

depth by Burdick (2011). The equipment selection discussion in this Strategy Guide builds upon 

the information developed in that heating and cooling loads guide. The Manual J loads from 

Burdick (2011) are listed in Table 1.

Table 1. ACCA Manual J Heating and Cooling Loads

Chicago House Orlando House

Heating Load 41,660 Btu/h Heating Load 23,640 Btu/h

Sensible Cooling 17,370 Btu/h Sensible Cooling 16,630 Btu/h

Latent Cooling 3,240 Btu/h Latent Cooling 4,060 Btu/h

Total Cooling 20,610 Btu/h Total Cooling 20,690 Btu/h

The general steps to equipment selection are shown in Figure 4, and the design information 

needed to select equipment is outlined in Figure 5.

Figure 4. General steps to equipment selection.

Note that the design of a space conditioning system is an iterative process, and once a duct 

system configuration and preliminary design are completed, different equipment may need to be 

selected to achieve the proper airflow to satisfy the room-by-room heating and cooling loads.
7

Figure 5. Design information needed.

The design loads along with the indoor and outdoor conditions information needed for 

equipment selection are all taken from the inputs used in the Manual J procedure. Faithful 

adherence to the conditions used in Manual J is required for accurate equipment selection.

2.2 Preliminary Equipment Selection

With an accurate estimate of the heating and cooling loads completed, the designer can begin the 

equipment selection procedure. It is the system designer’s responsibility to select the heating and 

cooling equipment and to verify that the selected pieces of equipment meet the following:

• The total cooling load, composed of both the sensible load (temperature) and the latent 

load (humidity)

• The heating load

• The blower capacity (in cfm) to meet the volume of air cfm range needed.

In most areas of the lower 48 United States, there will be both a cooling load and a heating load,

where the sizing of the cooling equipment will dominate the equipment selection process. A 

system designed to satisfy only the heating load will not have enough blower power if cooling is 

added after installation, because of the greater pressure loss across the cooling coil. The blower 

must be capable of supplying sufficient cfm airflow across the cooling coil.

Selecting equipment based on the cooling load may lead to a furnace that is oversized for the 

heating load. Manual S recommends that the furnace size remains within 140% of the peak 

heating load estimate and that cooling equipment capacity not exceed the total load by more than 

15%. Manual S recommends that a heat pump in a cooling dominated climate not exceed the 
8

load by more than 15% in either the cooling or the heating mode; however, in a heating 

dominated climate, a heat pump may exceed the heating load by as much as 25%. 

It should be recognized that in energy-efficient houses, the loads may be so small that the 

standard manufacturer’s range of capacities exceeds these percentages. For example, if a house 

has a 19,000 Btu/h cooling load and manufacturer’s equipment is available in only 17,500 Btu/h 

and 23,400 Btu/h, the larger unit is more than 15% oversized. However, the larger unit should be 

the equipment selected to ensure the peak load will be met. Designers may want to consider 

multispeed equipment in these cases, where the system runs more frequently at a lower capacity

to more closely meet the load for more times of the year.

2.3 Manufacturer Data Tables

HVAC equipment manufacturers make available tabulated performance data that are used to 

select the equipment based on the loads and design conditions. A fictitious example of a detailed

cooling capacities table is shown in Figure 6.

Figure 6. Fictitious example: detailed cooling capacities table.

It is imperative that actual manufacturer’s performance data are used to select equipment rather 

than data from the Air-Conditioning, Heating, and Refrigeration Institute (AHRI), formerly 

known as Air Conditioning and Refrigeration Institute (ARI) data. AHRI certification data are

tested at one set of conditions to offer an “apples-to-apples” comparison of equipment 
9

performance; this is not the same set of conditions as the Manual J indoor design conditions. 

Also, AHRI certification data list only total capacity and not sensible or latent capacity. 

Therefore, AHRI does not provide enough information to accurately select a piece of equipment. 

Details about the limitations of AHRI data are provided in Section 8 of Manual S.

Each manufacturer may tabulate the equipment performance data differently. Taking time to 

understand how the equipment data are formatted is important to making the correct selection. In 

the example detailed cooling capacities table shown in Figure 6, the manufacturer lists the total 

and sensible capacities of the equipment in MBtu/h. Design temperatures for outdoor air entering 

the condenser are listed in a 10-degree spread, requiring interpolation to find performance at a 

design temperature between the listed temperatures. Interpolation of data is acceptable to 

determine the performance standards of specific design temperatures; extrapolation of non-linear 

data is not an acceptable practice. The conditions for indoor air entering the evaporator coil are 

listed as the entering wet-bulb (EWB) temperature. 

The outside condenser and indoor coil package are selected from the manufacturer’s expanded 

product performance data tables. The outside condenser and indoor coil package will determine 

the system size and operating airflows. The initial cfm airflow estimate and operating cfm will 

likely be different values.

2.4 Cooling Equipment Capacity Sensitivities

Capacities of cooling equipment are conditional based on the following operating conditions:

• Cubic feet per minute airflow across the coil (Figure 7)

• Outdoor air temperature (OAT) at the condenser (Figure 8)

• Indoor wet-bulb temperature at the coil (Figure 9)

• Indoor dry-bulb temperature at the coil (Figure 10).

Figures 7 through 10 show the conditional nature of direct expansion cooling equipment and that 

the available latent capacity is a direct relationship to the total and sensible capacities. 

It is helpful to consider these relationships while searching the manufacturer’s detailed cooling 

capacity data. This conditional nature among total, sensible, and latent capacities causes an 

imbalance between latent load and latent capacity, where latent capacity slightly exceeds the 

latent load. This can be somewhat self-correcting. Manual S, Section 3-6, explains that with a 

latent capacity that slightly exceeds the latent load, about half the excess latent capacity will be 

converted to sensible capacity, aiding in equipment selection.

Figure 7 shows that increasing the airflow across the coil (cfm) will increase the total and 

sensible capacities of the equipment. However, because the sensible capacity increases at a 

higher rate than the total capacity, the latent capacity available is reduced as the cfm increases. 

This relationship between cfm and latent capacity of the equipment is an important consideration 

when selecting equipment to meet the latent load. 
10

Figure 7. Airflow sensitivity graph (from Manual S, Figure 3-1). 

(Reprinted with permission from Air Conditioning Contractors of America)

Figure 8 shows that as the OAT at the condenser increases, the total sensible and latent capacities 

decrease. Because the total capacity decreases at a faster rate than the sensible capacity, the 

available latent capacity of the equipment is lower. Because of this effect of OAT on equipment 

capacity, it is critical that the same temperature used for load calculations is used for equipment 

selection.

Figure 8. OAT sensitivity graph (from Manual S, Figure 3-1). 

(Reprinted with permission from Air Conditioning Contractors of America)
11

As shown in Figure 9, higher EWB temperatures at the indoor coil will result in a higher total 

capacity but a lower sensible capacity. This will result in a greater available latent capacity as the 

difference between the total and sensible capacities increases. 

Figure 9. Indoor EWB sensitivity graph (from Manual S, Figure 3-1). 

(Reprinted with permission from Air Conditioning Contractors of America)

Figure 10 shows that a higher entering dry-bulb (EDB) temperature at the indoor coil will have 

relatively little effect on the total capacity but will increase the sensible capacity of the 

equipment, resulting in a lower available latent capacity. 

Figure 10. Indoor EDB temperature sensitivity graph (from Manual S, Figure 3-1). 

(Reprinted with permission from Air Conditioning Contractors of America)
12

Sensible Heat Ratio (SHR) –

The sensible load divided by 

the total load. 

Temperature Difference 

(TD) – The difference 

between the room air 

temperature and the supply 

air temperature.

2.5 Estimated Target Cubic Feet per Minute

To search the manufacturer’s expanded performance data, an initial estimate of the required cfm

airflow is needed. The estimated cfm airflow is used only to narrow down the equipment choices 

from the manufacturer’s data and will likely be higher than the actual cfm used for the design. 

The detailed procedure to estimate a target cfm is described in 

Manual S. Section 1.3 of Manual S instructs the designer to use 

the sensible heat ratio (SHR) value to determine the desired 

room-air to supply-air temperature difference (TD) in degrees 

Fahrenheit. The SHR is the ratio of the sensible load 

determined from the Manual J procedures to the Manual J total 

load shown in Equation 1.

Total Load

Sensible Load SHR =

(1)

Figure 11, taken from Table 1-4 in Manual S, shows the relationship of an SHR range to TD

value that is compatible with the sensible and latent loads. The column “LAT” is the temperature 

of air leaving the coil, and the column “Room db” is the indoor conditions dry-bulb temperature 

used in the Manual J calculations. The difference between the two is the TD value. 

Figure 11. TD value table (from Manual S, Table 1-4). 

(Reprinted with permission from Air Conditioning Contractors of America)

Determining the SHR early is helpful because the SHR can also be used as a double check on the 

accuracy of the Manual J loads. A relatively high SHR (e.g., above 0.85) indicates a relatively 

small latent load compared to the sensible load. Conversely, a relatively low SHR (e.g., below 

0.75) indicates the latent load is relatively large compared to the sensible load.

Once the TD value is known, the estimated cfm target can be calculated using Equation 2. 

1.1 TD

ManualJ Sensible Load cfm Target

× = (2)

The number 1.1 in the denominator of Equation 2 represents an air properties constant associated 

with elevations lower than 1,500 feet and is appropriate for both the Chicago House and the 
13

Orlando House examples. The value of the air properties constant will decrease with higher 

elevations. Correction factor information can be found in Appendix 6 of Manual S.

2.6 Efficiency Considerations

The selection of the efficiency rating (AFUE or SEER) of the equipment will affect the longterm energy consumption and system operating costs. Although the decision-making process for 

selecting an efficiency level is a cost/benefit analysis of installed first cost versus the operational 

savings of the more efficient equipment, it is anticipated that future legislation will tighten the 

efficiency requirements of HVAC equipment, making more efficient equipment mandatory. To 

illustrate the impact of equipment efficiency on operating energy use and costs, two performance 

levels were compared using Building Energy Optimization software, version 1.1 (BEopt 1.1).

For the Chicago House example, an 80% AFUE furnace with a 13 SEER air conditioner was 

compared to a 92.5% AFUE furnace with a 16 SEER air conditioner. The results show a total 

annual source energy use reduction of 37.7 MBtu/yr or 11%. The bulk of the reduction, 31.6 

MBtu/yr, was in the heating mode with 1.7 MBtu/yr in cooling and the remaining 4.4 MBtu/yr 

reduction coming from fan energy savings. Figure 12 shows that, on an annualized utility bill 

basis (dollars/year), the more efficient equipment in the Chicago House resulted in an estimated 

$273 reduction.

Figure 12. Chicago House BEopt 1.1 output.
14

For the Orlando House, an 8.0 HSPF/13 SEER heat pump was compared with a 9.2 HSPF/16 

SEER heat pump. The results show a total annual source energy use reduction of 9.3 MBtu/yr or 

13%. The bulk of the reduction, 7.5 MBtu/yr, was in the cooling mode with 0.7 MBtu/yr in 

heating and the remaining 1.1 MBtu/yr reduction coming from fan energy savings. Figure 13

shows that, on an annualized utility bill basis (dollars/year), the more efficient equipment in the 

Orlando House resulted in an estimated $65 reduction.

Figure 13. Orlando House BEopt 1.1 output.

For the example houses in this Strategy Guide, a 92.5% AFUE furnace with a 16 SEER air 

conditioner was the system chosen for the Chicago House and a 9.2 HSPF/16 SEER heat pump 

for the Orlando House.
15

3 Chicago House Example

The following is an example selection process of a split system for the Chicago House with an 

upflow gas furnace located in the basement and an air conditioner. The Chicago House has both 

a cooling load and a heating load; therefore, the cooling equipment will be selected first. 

3.1 Cooling Equipment Selection

Split system cooling equipment consisting of an outdoor condensing unit and an indoor 

evaporator coil needs the two pieces to be matched for efficiency and performance. The example 

here shows a manufacturer matched equipment package. Although it is possible to select the two 

pieces separately, it is typically not necessary. The manufacturer matched package greatly 

simplifies the cooling equipment selection.

The first step highlighted in Figure 14 instructs the designer to define the cooling design 

parameters from the cooling loads of the Manual J procedure. The cooling design information 

developed by Burdick (2011) from the Manual J procedures is shown in Figure 15. 

Figure 14. Cooling equipment selection step 1.

Figure 15. Chicago House design information.
16

Step 2, highlighted in Figure 16, instructs the designer to estimate the cooling cfm. Utilizing the 

methods outlined in Section 1.3 of Manual S, the initial cfm airflow is estimated. 

Figure 16. Cooling equipment selection step 2.

Equation 3 applies the cooling design parameters from Figure 15 to Equation 1, resulting in an

SHR of 0.84. 

0.84

h

MBtu 20.61

h

MBtu 17.37

Total Load

Sensible Load SHR = = = (3)

From the TD value table (Figure 11), the TD value for SHR = 0.84 is 19. These values can now 

be applied to the sensible heat equation (Equation 2) as shown in Equation 4.

831

1.1 19

17,370

1.1 TD

ManualJ Sensible Load cfm Target = × = × = (4)

Step 3, highlighted in Figure 17, is to search the manufacturer’s data for an adequate cooling 

equipment package. 

Figure 17. Cooling equipment selection step 3.

To search the fictitious example of a cooling equipment package, detailed cooling capacities, as 

shown in Figure 6, a nominal size must be determined. Considering that 12.00 MBtu/h is 

equivalent to 1 nominal ton of cooling, the 20,610 Btu/h total cooling load for the Chicago 

House leads to a 2-ton nominal size cooling system as the target size to begin the equipment 

search. 
17

The EWB temperature for the indoor coil can be determined on the sea-level psychometric chart 

from the Manual J conditions of 75°F indoor design dry-bulb temperature and 50% relative 

humidity, as shown in Figure 18.

Figure 18. Wet-bulb temperature on sea-level psychometric chart.

The detailed cooling capacities tables give the data required for step 4 of Figure 19.

Figure 19. Cooling equipment selection step 4.

The outside design temperature for the Chicago House used in the Manual J load calculation was 

89°F. The 89°F outdoor design temperature will require interpolation of the manufacturer’s 

published data between the 85°F and 95°F condenser entering air temperature points.

The fictitious example cooling equipment package detailed cooling capacities table is searched at 

the closest published cfm data to the initial cfm airflow estimate of 831 cfm. Figure 20 highlights 

the 800 cfm and 63°F EWT temperature row, depicting a total capacity of 24.94 MBtu/h at 85°F 

and 23.73 MBtu/h at 95°F. 
18

Figure 20. Fictitious example: detailed cooling capacities table, 800 cfm.

It is clear without going through the interpolation exercise that this cooling equipment total 

capacity at 800 cfm exceeds the sizing limits for the total Manual J cooling load of 15% larger 

than the peak total load (see Equation 5).

20.61MBtu/h × 1.15 = 23.70 MBtu/h (5)

This is not an unexpected result because the initial 831 cfm airflow estimate is in alignment with 

the 400 cfm per 12.00 MBtu/h conventional methodologies of lower performing equipment and 

less efficient houses. It is not uncommon to find the target cfm range for a higher efficiency 

house with higher efficiency equipment to fall closer to 350 cfm per 12.00 MBtu/h.

Evaluating the same piece of equipment at lower cfm is the appropriate next step. The 715 cfm 

data row in Figure 21 shows the total capacity to be closer to the total load requirement. 

However, the sensible capacity values of 17.57 MBtu/h at 85°F and 16.88 MBtu/h at 95°F show,

without interpolating between the values, that the package will likely not meet the sensible 

portion of the load (17.37 MBtu/h) at 89°F.
19

Figure 21. Fictitious example: detailed cooling capacities table, 715 cfm and 750 cfm.

The published 750 cfm data in Figure 21 for both total and sensible capacities appear to be 

within the range of the load requirements. Figure 22 shows the linear interpolation of the values 

to determine the capacity at 89°F. This analysis shows that the capacity of this package at 89°F 

will meet the total, sensible, and latent loads at 750 cfm. Utilizing a spreadsheet to handle the 

multiple interpolation calculations makes the procedure more manageable (see Figure 23).

Figure 22. Interpolation table for capacities at 89°F outdoor design temperature.

The fictitious example cooling equipment package will exceed our cooling load requirements by 

17% at 750 cfm. As previously stated, in energy efficient houses the loads may be so small that 

the standard manufacturer’s range of capacities could exceed the 15% sizing limits. Equipment 

must be selected to meet the peak load without grossly oversizing the equipment, and the 

selection should be based on the selection procedures backed up by the data.
20

The target 750 cfm value will be carried through the remainder of the system design procedures;

however, this cfm value may be revisited and changed when the duct design is done. A furnace 

for the Chicago House can now be selected based on the cooling equipment selection and the 

Manual J heating loads.

3.2 Furnace Selection

With the cooling equipment package selected, a furnace must be selected that meets the heating 

load. The furnace capacity must meet the Manual J heating load while not exceeding the heating 

load by more than 40%. From Manual S, Section 2-5, the steps to selecting a furnace are shown 

in Figure 23.

Figure 23. Furnace selection steps.

Figure 24 shows that furnace capacities span a much larger range than cooling equipment 

capacities. A particular manufacturer may size a furnace line from 60,000 Btu/h to 120,000 Btu/h 

with 80,000 Btu/h as the intermediate size. This can be alleviated somewhat with multiple-stage 

equipment. A two-stage furnace in high stage may have a rating of 60,000 Btu/h but can be 

39,000 Btu/h in low stage. Furnaces are rated as the input Btu/h, but the capacity is derated by 

the efficiency level. For example, a 60,000 Btu/h furnace with a 92.5% efficiency rating will 

yield around 55,500 Btu/h output capacity. 
21

Figure 24. Furnace specification chart.

Each manufacturer may tabulate the furnace performance data differently. The furnace must also 

dimensionally match the size of the indoor coil. The example equipment manufacturer furnace is 

matched to the indoor cooling coil size and type, but not all manufacturers treat coil matching the 

same way. Some may require that the designers verify the furnace and coil are dimensionally 

compatible. Taking time to understand how the furnace data are formatted is important to 

making the correct selection.

Figure 25 is an example of an air delivery performance table. The blower performance of the 

furnace and the required cooling cfm must be compared. While comparing the blower 

performance of the furnace and the required cooling cfm, the blower must be able to meet the 

cooling cfm within 10%. The assumption at the start of the equipment selection process is 0.50 

inch water column external static pressure for the system. The equipment should have the 

capability of field setting the blower speed to match the cooling cfm per the manufacturer’s 

instructions.
22

Figure 25. Air delivery performance table.

To select a furnace for the Chicago House, the heating load estimate from the Manual J 

procedure of 41,660 Btu/h must be satisfied. The two parameters that must be considered for 

selection are the heating capacity and the cfm airflow. Equation 6 sets the maximum target 

capacity of a furnace for the Chicago House. 

41,660 Btu/h × 1.4 = 58,324 Btu/h (6)

The furnace with specifications as shown in Figure 26 will meet the required load without 

exceeding the maximum target capacity in the upflow configuration at the high setting. The 

cooling airflow of 1250 cfm almost doubles the 750 cfm design airflow determined in the 

cooling equipment selection; however, Figure 27 shows that the equipment is field adjustable to 

within the airflow margin constraint of 0.5 inch water column external static pressure. 

Figure 26. Fictitious furnace performance example.
23

Figure 27. Air delivery performance table for the Chicago House.

It is possible that conditions will arise from the duct design that may change the external static 

pressure and may require a different piece of equipment be chosen. The iterative nature of the 

equipment selection process is in balancing the capacity of the equipment to meet the load with 

the blower performance.
24

4 Orlando House Example

The Orlando House example equipment choice is a 9.2 HSPF/16 SEER air source heat pump. 

The selection of an air source heat pump is driven by climatic conditions, the size of the cooling 

load compared to the heating load, and the economic factors of gas prices compared to the cost 

of electricity. Air source heat pump systems have not traditionally been a common choice in cold 

climates with a large heating load; however, an air source heat pump is a likely choice for 

climates where the heating load is much smaller than the cooling load. Dual fuel options can 

make an air source heat pump a viable choice for a cold climate by providing an economical 

solution of a gas-fired burner to meet the larger heating load in cold climates. This evaluation 

will require an in-depth cost comparison. Details of the economic balance-point evaluation for a 

dual fuel system are covered in Manual S, Section 6.

Initially, heat pump equipment selection is based on the cooling load. The selection of heat pump 

equipment to meet the cooling capacity follows the selection of cooling equipment in the air

conditioner selection in Section 3 of Manual S and is sensitive to the same four operating 

parameters as the split system cooling equipment. In warm climates, the sizing limitations of an 

air source heat pump are 15% greater than the Manual J loads for heating and total cooling loads, 

whereas in cold climates, it is permissible to exceed the Manual J heating load by 25%. 

Because no furnace is used in this particular heat pump system, an indoor fan coil air handler 

with electric resistance heat coils will be selected to match the performance requirements and 

design parameters for airflow and heating load. The additional information required for heat 

pump selection is the amount of supplemental heat and the emergency heat capacity.

Based on Section 5 of Manual S, the steps to selecting a heat pump are shown in Figure 28. 

Figure 28. Air source heat pump selection steps.
25

Supplemental Heat –

The amount of electric 

resistance heat beyond the heat 

pump capacity required to meet 

the peak heating load. 

Emergency Heat –

The amount of electric 

resistance heat required to meet 

the peak heating load should the 

heat pump fail.

Once the cooling equipment selection has been made, a 

balance-point diagram can be created from the 

manufacturer’s published performance data and the 

heating season outdoor design temperatures. The balancepoint diagram is used to determine the amount of 

supplemental and emergency heat required. Figure 29

shows the development of the balance-point diagram. The 

capacity line (A) is provided by the manufacturer in the 

detailed equipment specifications. The load line (B) is 

created by drawing a straight line from the design heating 

load at the outdoor design temperature to 0 Btu/h load at 

65°F outdoor temperature. Emergency heat (C) is the total design heating load. Supplemental 

heat (D) is the difference between the capacity line and the design heating load.

Figure 29. Development of balance-point diagram.
26

4.1 Air Source Heat Pump Selection

The first step highlighted in Figure 30 instructs the designer to define the cooling design 

parameters from the cooling loads of the Manual J procedure. The cooling design information 

developed by Burdick (2011) from the Manual J procedures is shown in Figure 31.

Figure 30. Air source heat pump selection step 1.

Figure 31. Orlando House design information.
27

4.2 Heat Pump Cooling Capacity Selection

Step 2 of the selection process, highlighted in Figure 32, instructs the designer to estimate the 

cooling cfm. The methods outlined in Section 1-3 of Manual S are used to estimate the initial 

cfm airflow. 

Figure 32. Air source heat pump selection step 2.

Equation 7 applies the cooling design parameters from Figure 31 to Equation 1, resulting in an

SHR of 0.80. 

0.80

h

MBtu 20.69

h

MBtu 16.63

Total Load

Sensible Load SHR = = = (7)

From the TD value table (Figure 11), the SHR value of 0.80 is the break point between the TD 

values of 19 and 21. Note that a higher TD value will lead to a lower target cfm value, and a 

lower cfm value is desirable for a humid climate. Therefore, a TD of 21 was selected for this 

Orlando House example in Florida. This is the type of design consideration that can be applied to 

the equipment selection procedure.

With the TD value for SHR = 0.80 of 21 selected, these values can now be applied to the 

sensible heat equation (Equation 2) as shown in Equation 8.

719

1.1 21

16,630

1.1 TD

ManualJ Sensible Load cfm Target = × = × = (8)
28

Step 3, highlighted in Figure 33, is to search the manufacturer’s data for an adequate cooling 

equipment package. To search the fictitious example of a cooling equipment package detailed 

cooling capacities, a nominal size must be determined. Considering that 12.00 MBtu/h is 

equivalent to 1 nominal ton of cooling, the 20,690 Btu/h total cooling load for the Orlando House 

leads to a 2-ton nominal cooling system as the target size to begin the equipment search. 

Figure 33. Air source heat pump selection step 3.

The EWB temperature for the indoor coil can be determined on the psychometric chart from the 

Manual J conditions of 75°F indoor design dry-bulb temperature and 50% relative humidity, as 

shown in Figure 34.

Figure 34. Wet-bulb temperature on sea-level psychometric chart.
29

The detailed cooling capacities tables give the data required for step 4 (see Figure 35). The 

outside design temperature for the Orlando House used in the Manual J load calculation was 

92°F. The 92°F outdoor design temperature will require interpolation of the manufacturer’s 

published data between the 85°F and 95°F condenser entering air temperature points.

Figure 35. Cooling equipment selection step 4.

The fictitious example heat pump detailed cooling capacities table (Figure 36) is searched at the

closest published cfm data to the initial cfm airflow estimate of 719 cfm. Figure 36 highlights the 

700 cfm and 63°F entering evaporator wet-bulb temperature row. The published 700 cfm data in 

Figure 36 for both total and sensible capacities appear to be within the range of the load 

requirements. Figure 37 shows the linear interpolation of the values to determine the capacity at 

92°F. This analysis shows that the capacity of this package at 92°F will meet the total, sensible, 

and latent loads at 700 cfm. Utilizing a spreadsheet to handle the multiple interpolation 

calculations makes the procedure more manageable, as shown in Figure 37.
30

Figure 36. Fictitious example: detailed heat pump cooling capacities table, 700 cfm.

Figure 37. Interpolation table for cooling capacities at 92°F outdoor design temperature.

4.3 Heat Pump Heating Capacity Selection

To complete step 5 highlighted in Figure 38 and to evaluate the heating season performance of

the heat pump, the heating capacity must be determined from the heat pump heating performance 

table as shown in Figure 39, and interpolation to the 38°F heating design temperature will be 

required. The indoor heating design temperature from Manual J is 70°F, and the design cfm

determined in the cooling selection is 700 cfm.
31

Figure 38. Cooling equipment selection step 5.

Figure 39. Heat pump heating performance table at 700 cfm.

Interpolation of the capacity at 38°F is shown in Figure 40. This equipment’s capacity of 

22.73 MBtu/h closely matches the heating load of 23.64 MBtu/h, and the balance will be made 

up with supplemental heat as determined in step 6 (Figure 41). Step 6 is to determine the 

supplemental and emergency heat requirements from the balance-point diagram. 
32

Figure 40. Interpolation table for heating capacities at 38°F outdoor design temperature.

Figure 41. Cooling equipment selection step 6.

The balance-point diagram, Figure 42, for the selected outdoor heat exchanger and fan coil 

shows a small 3,000 Btu/h supplemental heat requirement. 
33

Figure 42. Balance-point diagram for the Orlando House.
34

Supplemental and emergency heat are supplied by electric resistance heat and are sized in 

kilowatts, requiring a conversion from Btu/h. The 23.64 MBtu/h total load requirement converts 

to 6.9 kW emergency heat, and the supplemental heat requirement is less than 1 kW. This 

supplemental heating capacity will be specified in the sizing of the fan coil.

The electric resistance coils will be specified with the fan coil sizing shown in Figure 43. The 

available coils are 5, 8, and 10 kW. The 8 kW is larger than the required 6.9 kW, but this is 

resolved by staged resistance heat providing resistance heat in eight individual 1-kW increments.

Figure 43. Fan coil nomenclature.

We now have a heat pump and fan coil package that will meet the heating and cooling load needs 

of the Orlando House. The next step in right-sizing the HVAC system is to use the blower 

performance data of the fan coil to create the air delivery duct design. 
35

5 Conclusion

Right-sizing of an HVAC system is the selection of equipment and the design of the air 

distribution system to meet the accurate predicted heating and cooling loads of the house. The 

equipment effective capacity should match the loads generated in the load calculation procedure 

within the sizing guidelines. Selection of the equipment can have a substantial impact on the 

overall efficiency and operating costs of the installed system.

Note that the design of a space conditioning system is an iterative process, and once a duct 

system configuration and preliminary design are completed, different equipment may need to be 

selected to achieve the proper airflow to satisfy the room-by-room heating and cooling loads. 

The design loads along with the indoor and outdoor conditions information needed for 

equipment selection are all taken from the inputs used in the Manual J procedure. Faithful 

adherence to the conditions used in Manual J is required for accurate equipment selection. 

The selection of heating and cooling equipment is dependent on accurate heating and cooling 

load calculations. From this point onward, in the design process, the actual performance data of 

the equipment selections and room-by-room loads from the Manual J calculations should be used 

to design the duct system. 
36

References

Burdick, A. (2011). Strategy Guideline: Accurate Heating and Cooling Load Calculations. 

Pittsburgh: IBACOS. Prepared for the U.S. Department of Energy Building America Program. 

www.nrel.gov/docs/fy11osti/51603.pdf.

IECC (2009). International Energy Conservation Code. Washington, DC: International Code 

Council.

Rutkowski, H. (1995). Manual S—Residential Equipment Selection. Arlington, VA: Air 

Conditioning Contractors of America. 

Rutkowski, H. (2006). Manual J—Residential Load Calculation, 8th edition, Version 2. 

Arlington, VA: Air Conditioning Contractors of America. 

Rutkowski, H. (2009a). Manual D—Residential Duct Systems, 3rd edition, Version 1.00. 

Arlington, VA: Air Conditioning Contractors of America. 

Rutkowski, H. (2009b). Manual T—Air Distribution Basics for Residential and Small 

Commercial Buildings. Arlington, VA: Air Conditioning Contractors of America. 
DOE/GO-102012-3442 ▪ February 2012

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