Design for Typical Building Use. In general, residential systems should be designed to meet representative maximum-load conditions, not extreme conditions. Normal occupancy should be assumed, not the maximum that might occur during an occasional social function. Intermittently operated ventilation fans should be assumed to be off. These considerations are especially important for cooling-system sizing.

Building Codes and Standards. This chapter presentation is necessarily general. Codes and regulations take precedence; consult local authorities to determine applicable requirements.

Designer Judgment. Designer experience with local conditions, building practices, and prior projects should be considered when applying the procedures in this chapter. For equipment-replacement projects, occupant knowledge concerning performance of the existing system can often provide useful guidance for achieving a successful design.

Verification. Postconstruction commissioning and verification are important steps in achieving design performance. Designers should encourage pressurization testing and other procedures that allow identification and repair of construction shortcomings.

Uncertainty and Safety Allowances. Residential load calculations are inherently approximate. Many building characteristics are estimated during design and ultimately determined by construction quality and occupant behavior. These uncertainties apply to all calculation methods, including first-principles procedures such as RHB. It is therefore tempting to include safety allowances for each aspect of a calculation. However, this practice has a compounding effect and often produces oversized results. Typical conditions should be assumed; safety allowances, if applied at all, should be added to the final calculated loads rather than to intermediate components. In addition, temperature swing provides a built-in safety factor for sensible cooling: a 20% capacity shortfall typically results in a temperature excursion of at most about one or two degrees.

Common air-conditioning processes involve transferring heat via air transport or leakage. The sensible, latent, and total heat conveyed by air on a volumetric basis is

where

qs, ql, qt	=	sensible, latent, total heat transfer rates, Btu/h
Cs	=	air sensible heat factor, Btu/h · °F · cfm (1.1 at sea level)
Cl	=	air latent heat factor, Btu/h · cfm (4840 at sea level)
Ct	=	air total heat factor, Btu/h · cfm per Btu/lb enthalpy h (4.5 at sea level)
Q	=	air volumetric flow rate, cfm
Δt	=	air temperature difference across process, °F
ΔW	=	air humidity ratio difference across process, lbw/lbda
Δh	=	air enthalpy difference across process, Btu/lb

The heat factors C_s, C_l, and C_t are elevation dependent. The sea-level values in the preceding definitions are appropriate for elevations up to about 1000 ft. Procedures are provided in Chapter 18 for calculating adjusted values for higher elevations.

The initial step in the load calculation is selecting indoor and outdoor design conditions.

Indoor Conditions. Indoor conditions assumed for design purposes depend on building use, type of occupancy, and/or code requirements. Chapter 9 and ASHRAE Standard 55 define the relationship between indoor conditions and comfort.

Typical practice for cooling is to design for indoor conditions of 75°F db and a maximum of 50 to 65% rh. For heating, 68°F db and 30% rh are common design values. These conditions are the default values used throughout this chapter.

Outdoor Conditions. Outdoor design conditions for load calculations should be selected from location-specific climate data in Chapter 14, or according to local code requirements as applicable.

Cooling. The 1% design dry-bulb temperature and mean coincident wet bulb temperature from Chapter 14 climate data are generally appropriate. As previously emphasized, oversized cooling equipment results in poor system performance. Extremely hot events are necessarily of short duration (conditions always moderate each night); therefore, sacrificing comfort under typical conditions to meet occasional extremes is not recommended.

Load calculations also require the hottest-month dry-bulb temperature daily range, and wind speed. These values can also be found in Chapter 14, although wind speed is commonly assumed to be 7.5 mph.

Typical buildings in middle latitudes generally experience maximum cooling requirements in midsummer (July in the northern hemisphere and January in the southern hemisphere). For this reason, the RLF method is based on midsummer solar gains. However, this pattern does not always hold. Buildings at low latitudes or with significant south-facing glazing (north-facing in the southern hemisphere) should be analyzed at several times of the year using the RHB method. Local experience can provide guidance as to when maximum cooling is probable. For example, it is common for south-facing buildings in mild northern-hemisphere climates to have peak cooling loads in the fall because of low sun angles. Chapter 14 contains monthly temperature data to support calculations for any time of year.

Heating. General practice is to use the 99% design dry-bulb temperature from Chapter 14. Heating load calculations ignore solar and internal gains, providing a built-in safety factor. However, the designer should consider two additional factors:

Depending on the application and system type, the designer should consider using the 99.6% value or the mean minimum extreme as the heating design temperature. Alternatively, the heating load can be calculated at the 99% condition and a safety factor applied when equipment is selected. This additional capacity can also serve to meet pickup loads under nonextreme conditions.

Adjacent Buffer Spaces. Residential buildings often include unconditioned buffer spaces such as garages, attics, crawlspaces, basements, or enclosed porches. Accurate load calculations require the adjacent air temperature.

In many cases, a simple, conservative estimate is adequate, especially for heating calculations. For example, it is generally reasonable to assume that, under heating design conditions, adjacent uninsulated garages, porches, and attics are at outdoor temperature. Another reasonable assumption is that the temperature in an adjacent, unheated, insulated room is the mean of the indoor and outdoor temperatures.

In cases where a temperature estimate is required, a steady-state heat balance analysis yields the following:

where

tb	=	buffer space temperature, °F
Q	=	buffer space infiltration/ventilation flow rate, cfm
to	=	outdoor air temperature, °F
Ax	=	area of xth buffer space surface, ft2
Ux	=	U-factor of xth buffer space surface, Btu/h · ft2 · °F
tx	=	air temperature at outside of xth buffer space surface, °F (typically, outdoor air temperature for exterior surfaces, conditioned space temperature for surfaces between buffer space and house, or ground temperature for below-grade surfaces)
q	=	additional buffer space heat gains, Btu/h (e.g., solar gains or distribution system losses)

Component Areas. To perform load calculations efficiently and reliably, standard methods must be used for determining building surface areas. For fenestration, the definition of component area must be consistent with associated ratings.

Gross area. It is both efficient and conservative to derive gross surface areas from outer building dimensions, ignoring wall and floor thicknesses. Thus, floor areas should be measured to the outside of adjacent exterior walls or to the centerline of adjacent partitions. When apportioning to rooms, façade area should be divided at partition centerlines. Wall height should be taken as floor-to-floor.

Using outer dimensions avoids separate accounting of floor edge and wall corner conditions. Further, it is standard practice in residential construction to define floor area in terms of outer dimensions, so outer-dimension takeoffs yield areas that can be readily checked against building plans (e.g., the sum of room areas should equal the plan floor area). Although outer-dimension procedures are recommended as expedient for load calculations, they are not consistent with rigorous definitions used in building-related standards (e.g., ASTM Standard E631). However, the inconsistencies are not significant in the load calculation context.

Fenestration area. Fenestration includes exterior windows, skylights, and doors. Fenestration U-factor and SHGC ratings (see Table 2) are based on the entire product area, including frames. Thus, for load calculations, fenestration area is the area of the rough opening in the wall or roof, less installation clearances (projected product area A_pf). Installation clearances can be neglected; it is acceptable to use the rough opening as an approximation of A_pf.

Net area. Net surface area is the gross surface area less fenestration area (rough opening or A_pf) contained within the surface.

Volume. Building volume is expediently calculated by multiplying floor area by floor-to-floor height. This produces a conservative estimate of enclosed air volume, because wall and floor volumes are included in the total. More precise calculations are possible but are generally not justified in this context.

Construction Characteristics.

U-factors. Except for fenestration, construction U-factors should be calculated using procedures in Chapter 27, or taken from manufacturer’s data, if available. U-factors should be evaluated under heating (winter) conditions.

Fenestration. Fenestration is characterized by U-factor and solar heat gain coefficient (SHGC), which apply to the entire assembly (including frames). If available, rated values should be used, determined according to procedures set forth by National Fenestration Rating Council (NFRC), Canadian Standards Association (CSA), or other specifying body (see Chapter 15). Ratings can be obtained from product literature, product label, or online listings (NFRC 2017). For unrated products (e.g., in existing construction), the U-factor and SHGC can be estimated using Table 2 or tables in Chapter 15. Note that fenestration U-factors are evaluated under heating (winter) design conditions but are used in this chapter for both heating and cooling calculations.

Relatively few types of glazing are encountered in residential applications. Single-glazed clear, double-glazed clear, and double-glazed low-emissivity (“low-e”) glass predominate. Single-glazed is now rare in new construction but common in older homes. Triple-glazing, reflective glass, and heat-absorbing glass are encountered occasionally. Acrylic or glass skylights are common. Multipane low-e insulated glazing is available in high- and low-solar-gain variants, as discussed in Chapter 15. Low-solar is now the more common for new construction in all parts of the United States.

Properties of windows equipped with storm windows should be estimated from data for a similar configuration with an additional pane. For example, data for clear, double-glazed should be used for a clear single-glazed window with a storm window.

Fenestration interior and exterior shading must be included in cooling load calculations, as discussed in the Cooling Load section.

Table 2 shows representative window U-factor and SHGC values for common glazing and frame combinations. Consult Chapter 15 for skylight characteristics.

Below-Grade Surfaces. For cooling calculations, heat flow into the ground is usually ignored because it is difficult to quantify. Surfaces adjacent to the ground are modeled as if well insulated on the outside, so there is no overall heat transfer, but diurnal heat storage effects are included. Heating calculations must include loss via slabs and basement walls and floors, as discussed in the Heating Load section.

Infiltration. Infiltration is generally a significant component of both cooling and heating loads. Refer to Chapter 16 for a detailed discussion of residential air leakage. The simplified residential models found in that chapter can be used to calculate infiltration rates for load calculations. Infiltration should be evaluated for the entire building, not individual rooms or zones.

Natural infiltration leakage rates are modified by mechanical pressurization caused by unbalanced ventilation or duct leakage. These effects are discussed in the section on Combined Ventilation and Infiltration Airflow.

Leakage rate. Air leakage rates are specified either as airflow rate Q_i, or air exchanges per hour (ACH), related as follows:

where

Qi	=	infiltration airflow rate, cfm
ACH	=	air exchange rate, changes/h
V	=	building volume, ft3

Infiltration airflow rate depends on two factors:

Using the simplifying assumptions presented in Chapter 16, these factors can be evaluated separately and combined using Equation (8).

where

AL	=	building effective leakage area (including flue) at reference pressure difference = 0.016 in. of water, assuming discharge coefficient CD = 1, in2
IDF	=	infiltration driving force, cfm/in2

The following sections provide procedures for determining A_L and IDF.

Leakage area. As discussed in Chapter 16, there are several interconvertible ways to characterize building leakage, depending on reference pressure differences and assumed discharge coefficient. This formulation uses the effective leakage area at 0.016 in. of water, assuming C_D = 1, designated A_L (Sherman and Grimsrud 1980).

The only accurate procedure for determining A_L is by measurement using a pressurization test (commonly called a blower door test). Numerous field studies have shown that visual inspection is not adequate for obtaining even a crude estimate of leakage.

For buildings in design, a pressurization test is not possible and leakage area must be assumed for design purposes. Leakage can be estimated using tabulated component leakage areas found in Chapter 16. A simpler approach is based on an assumed average leakage per unit of building surface area:

where

Aes	=	building exposed surface area, ft2
Aul	=	unit leakage area, in2/ft2 (from Table 3)

A_ul is the leakage area per unit surface area; suitable design values are found in Table 3. Field experience indicates that the level of care applied to reducing leakage often depends on winter conditions, because cold-air leakage is readily detected. Thus, lower A_ul values are expected in colder climates. Note that the A_ul value doubles at each reduced construction quality step in Table 3; very high infiltration loads are typical in older houses.

In Equation (9), A_es is the total building surface area at the envelope pressure boundary, defined as all above-grade surface area that separates the outdoors from conditioned or semiconditioned space. Table 4 provides guidance for evaluating A_es.

IDF. To determine IDF, use the Chapter 16 methods cited previously. As a further simplification, Barnaby and Spitler (2005) derived the following relationship that yields results approximately equal to the AIM-2 model (Walker and Wilson 1990, 1998; Chapter 16’s enhanced model) at design conditions:

where

I0, I1, I2

=

coefficients, as follows:

H	=	building average stack height, ft (typically 8 to 10 ft per story)
Δt	=	difference between indoor and outdoor temperatures, °F
AL, flue	=	flue effective leakage area at reference pressure difference = 0.016 in. of water, assuming CD = 1, in2 (total for flues serving furnaces, domestic water heaters, fireplaces, or other vented equipment, evaluated assuming associated equipment is not operating and with dampers in closed position; see Chapter 16)

Building stack height H is the average height difference between the ceiling and floor (or grade, if the floor is below grade). Thus, for buildings with vented crawlspaces, the crawlspace height is not included. For basement or slab-on-grade construction, H is the average height of the ceiling above grade. Generally, there is significant leakage between basements and spaces above, so above-grade basement height should be included whether or not the basement is fully conditioned. With suitable adjustments for grade level, H can also be estimated as V/A_cf (conditioned floor area).

Equation (10) is valid for typical suburban residential wind sheltering, A_{L, flue} < A_L/2, and at any elevation. Table 5 shows IDF values derived with Equation (10), assuming A_{L, flue} = 0.

Verification of leakage. A postconstruction pressurization test is strongly recommended to verify that design leakage assumptions are actually achieved. Excess leaks should be located and repaired.

Allocation of infiltration to rooms. Total building infiltration should typically be allocated to rooms according to room volume; that is, it should be assumed that each room has the same air exchange rate as the whole building. In reality, leakage varies by room and over time, depending on outdoor temperature and wind conditions. These effects can either increase or decrease room leakage. In addition, system air mixing tends to redistribute localized leakage to all rooms. Thus, in most cases, there is no reasonable way to assign more or less leakage to specific rooms.

An exception is leaky, multistory houses. The preferable and cost-effective response is mitigation of the leakage. If repair is not possible, then for heating load calculation purposes, some leakage can be differentially assigned to lower story and/or windward rooms in proportion to exposed surface area (i.e., adjustment using an “exposure factor”).

Multifamily buildings. Usually, the simplified methods in Chapter 16 and this section do not apply to multifamily residences. However, they can be used for row houses that are full building height and have more than one exposed façade. For apartment units subdivided within a former detached residence, the entire building should be analyzed and the resulting exchange rate applied to the apartment volume. In other multifamily structures, infiltration is determined by many factors, including overall building height and degree of sealing between apartments. For low-rise construction, an upper bound for the infiltration rate can be found by evaluating the entire building. As building height increases, leakage problems can be magnified, as discussed in Chapter 16. Estimating leakage rates may require advice from a high-rise infiltration specialist.

Ventilation.

Whole-building ventilation. Because of energy efficiency concerns, residential construction has become significantly tighter over the last several decades. Natural leakage rates are often insufficient to maintain acceptable indoor air quality. ASHRAE Standard 62.2 specifies the required minimum whole-building ventilation rate as

where

Qv	=	required ventilation flow rate, cfm
Ac f	=	building conditioned floor area, ft2
Nb r	=	number of bedrooms (not less than 1)

Certain mild climates are exempted from this standard; local building authorities ultimately dictate actual requirements. In addition, Standard 62.2 specifies alternative methods for determining ventilation requirements that may result in smaller Q_v values. Whole-building ventilation is expected to become more common because of a combination of regulation and consumer demand. The load effect of Q_v must be included in both cooling and heating calculations.

Heat recovery. Heat recovery devices should be considered part of mechanical ventilation systems. These appliances are variously called heat recovery ventilators (HRVs) or energy recovery ventilators (ERVs) and integrate with residential distribution systems, as described in Chapter 26 of the 2020 ASHRAE Handbook—HVAC Systems and Equipment. Either sensible heat or total heat (enthalpy) can be exchanged between the exhaust and intake airstreams. ERV/HRV units are characterized by their sensible and total effectiveness.

Local mechanical exhaust. Kitchen and bathroom exhaust fans are required by Standard 62.2 and are typically present. Exhaust fans that operate intermittently by manual control are generally not included in load calculations. Continuously operating ventilation should be included. Note that exhaust fans induce load only through enhanced infiltration because of building depressurization (see the section on Combined Ventilation and Infiltration Airflow for further discussion).

Combustion Air. Fuel-fired boilers, furnaces, and domestic water heaters require combustion air. If the combustion air source is within the building envelope (including in semiconditioned basements), additional infiltration and heating load are induced. Locating the equipment outside of conditioned space (e.g., in a garage or vented mechanical closet) or using sealed-combustion equipment eliminates this load.

Combustion air requirements for new forced-draft equipment can be estimated at 0.25 cfm per 1000 Btu/h or about 25 cfm for a 100,000 Btu/h heating appliance. The requirements for existing natural draft equipment should be estimated at twice that amount. In many cases, these quantities are relatively small and can be neglected.

For cooling load calculations, heating equipment is assumed to be not operating, leaving only any domestic water heaters, for which the combustion air requirements are generally neglected.

Combined Ventilation and Infiltration Airflow. Mechanical pressurization modifies the infiltration leakage rate. To assess this effect, overall supply and exhaust flow rates must be determined and then divided into “balanced” and “unbalanced” components.

where

Qbal	=	balanced airflow rate, cfm
Qsup	=	total ventilation supply airflow rate, cfm
Qexh	=	total ventilation exhaust airflow rate (including any combustion air requirements), cfm
Qunbal	=	unbalanced airflow rate, cfm

Note that unbalanced duct leakage can produce additional pressurization or depressurization. This effect is discussed in the section on Distribution Losses.

Airflow components can be combined with infiltration leakage as follows (Palmiter and Bond 1991; Sherman 1992):

where

Qvi	=	combined infiltration/ventilation flow rate (not including balanced component), cfm
Qi	=	infiltration leakage rate assuming no mechanical pressurization, cfm

Ventilation/infiltration load. The cooling or heating load from ventilation and infiltration is calculated as follows:

where

qvi, s	=	sensible ventilation/infiltration load, Btu/h
εs	=	HRV/ERV sensible effectiveness
Qbal, hr	=	balanced ventilation flow rate via HRV/ERV equipment, cfm
Qbal, oth	=	other balanced ventilation supply airflow rate, cfm
Δt	=	indoor/outdoor temperature difference, °F
ΔW	=	indoor/outdoor humidity ratio difference
qvi,t	=	total ventilation/infiltration load, Btu/h
εt	=	HRV/ERV total effectiveness
Δh	=	indoor/outdoor enthalpy difference, Btu/lb
qvi,l	=	latent ventilation/infiltration load, Btu/h

Distribution Losses. Air leakage and heat losses from duct systems frequently impose substantial equipment loads in excess of building requirements. The magnitude of losses depends on the location of duct runs, their surface areas, surrounding temperatures, duct wall insulation, and duct airtightness. These values are usually difficult to accurately determine at the time of preconstruction load calculations, and must be estimated using assumed values, so that selected equipment capacity is sufficient.

Good design and workmanship both reduce duct losses. In particular, locating duct runs within the conditioned envelope (above dropped hallway ceilings, for example) substantially eliminates duct losses. Specific recommendations are found in Chapter 10 of the 2020 ASHRAE Handbook—HVAC Systems and Equipment. Good workmanship and correct materials are essential to achieve low leakage. Many common sealing techniques, notably duct tape, have been shown to fail in a few years. Well-constructed duct systems show leakage rates of 5% of fan flow from supply and return runs, whereas 11% or more on each side is more typical. Because of the potentially large load impact of duct leakage, postconstruction verification of airtightness is strongly recommended.

Duct losses can be estimated using models specified in ASHRAE Standard 152, Francisco and Palmiter (1999), and Palmiter and Francisco (1997). The allowance for distribution losses is calculated as follows:

where

qd	=	distribution loss, Btu/h
Fdl	=	duct loss/gain factor, from Table 6 or ASHRAE Standard 152 design efficiencies or a detailed model
qbl	=	total building load, Btu/h

Table 6 shows typical duct loss/gain factors calculated for the conditions indicated. These values can provide guidance for hand estimates, and illustrate the need for achieving low duct leakage. To the extent conditions differ from those shown, specific calculations should be made using a method cited previously. Note also that Table 6 cooling factors represent sensible gain only. Duct leakage may also introduce significant latent gain; see ASHRAE Standard 152.

To select a properly sized cooling unit, the peak or maximum load (block load) for each zone must be computed. The block load for a single-family detached house with one central system is the sum of all the room loads. If the house has a separate system for each zone, each zone block load is required. When a house is zoned with one central cooling system, the system size is based on the entire house block load, whereas zone components, such as distribution ducts, are sized using zone block loads.

In multifamily structures, each living unit has a zone load that equals the sum of the room loads. For apartments with separate systems, the block load for each unit establishes the system size. Apartment buildings having a central cooling system with fan-coils in each apartment require a block load calculation for the complete structure to size the central system; each unit load establishes the size of the fan-coil and air distribution system for each apartment. One of the methods for nonresidential buildings discussed in Chapter 18 may be used to calculate the block load.

Heat gain through walls, floors, ceilings, and doors is caused by (1) the air temperature difference across such surfaces and (2) solar gains incident on the surfaces. The heat capacity of typical construction moderates and delays building heat gain. This effect is modeled in detail in the computerized RHB method, resulting in accurate simultaneous load estimates.

The RLF method uses the following to estimate cooling load:

where

qopq	=	opaque surface cooling load, Btu/h
A	=	net surface area, ft2
CF	=	surface cooling factor, Btu/h · ft2
U	=	construction U-factor, Btu/h · ft2 · °F
Δt	=	cooling design temperature difference, °F
OFt, OFb, OFr	=	opaque-surface cooling factors (see Table 7)
DR	=	cooling daily range, °F

OF factors, found in Table 7, represent construction-specific physical characteristics. OF_t values less than 1 capture the buffering effect of attics and crawlspaces, OF_b represents incident solar gain, and OF_r captures heat storage effects by reducing the effective temperature difference. Note also that CF can be viewed as CF = U × CLTD, the formulation used in prior residential and nonresidential methods.

Table 7 factors for walls are simplified in two ways. First, the values do not depend on wall orientation. This has minimal effect on total load, because residences typically have a mix of exposures. Second, only wood frame construction is included. The wood frame values can be used for heavier construction (e.g., masonry), but this overpredicts the wall’s contribution to cooling load and is thus conservative.

Slab floors produce a slight reduction in cooling load, as follows:

where

A	=	area of slab, ft2
CFslab	=	slab cooling factor, Btu/h · ft2
hsrf	=	ffective surface conductance, including resistance of slab covering material such as carpet =1/(Rcvr + 0.68), Btu/h · ft2 · °F. Representative Rcvr values are found in Chapter 6 of the 2020 ASHRAE Handbook—HVAC Systems and Equipment.
0.59	=	constant, Btu/h · ft2
2.5	=	factor, °F

Heat gain from adjacent unconditioned or semiconditioned spaces can be calculated based on the partition U-factor and temperature difference. Buffer space air temperature t_b can be estimated using procedures discussed in the section on Adjacent Buffer Spaces. Generally, simple approximations are sufficient. If the partition surface is large and/or poorly insulated, buffer temperature should be calculated with more care.

Cooling load associated with nondoor fenestration is calculated as follows:

where

qfen	=	fenestration cooling load, Btu/h
A	=	fenestration area (including frame), ft2
CFfen	=	surface cooling factor, Btu/h · ft2
U	=	fenestration NFRC heating U-factor, Btu/h · ft2 · °F
Δt	=	cooling design temperature difference, °F
PXI	=	peak exterior irradiance, including shading modifications, Btu/h · ft2 [see Equations (26) or (27)]
SHGC	=	fenestration rated or estimated NFRC solar heat gain coefficient
IAC	=	interior shading attenuation coefficient, Equation (29)
FFs	=	fenestration solar load factor, Table 13

Peak Exterior Irradiance (PXI). Although solar gain occurs throughout the day, RP-1199 regression studies (Barnaby et al. 2004) showed that the cooling load contribution of fenestration correlates well with the peak-hour irradiance incident on the fenestration exterior. PXI is calculated as follows:

where

PXI	=	peak exterior irradiance, Btu/h · ft2
Et, Ed, ED	=	peak total, diffuse, and direct irradiance (Table 9 or 10), Btu/h · ft2
Tx	=	transmission of exterior attachment (insect screen or shade screen)
Fshd	=	fraction of fenestration shaded by permanent overhangs, fins, or environmental obstacles

For horizontal or vertical surfaces, peak irradiance values can be obtained from Table 10 for primary exposures, or from Table 9 equations for any exposure. Skylights with slope less than 30° from horizontal should be treated as horizontal. Steeper, nonvertical slopes are not supported by the RLF method.

Exterior Attachments. Common window coverings can significantly reduce fenestration solar gain. Table 11 shows transmission values for typical attachments.

Permanent Shading. The shaded fraction F_shd can be taken as 1 for any fenestration shaded by adjacent structures during peak hours. Simple overhang shading can be estimated using the following:

where

SLF	=	shade line factor from Table 12
Doh	=	depth of overhang (from plane of fenestration), ft
Xoh	=	vertical distance from top of fenestration to overhang, ft
h	=	height of fenestration, ft

The shade line factor (SLF) is the ratio of the vertical distance a shadow falls beneath the edge of an overhang to the depth of the overhang, so the shade line equals the SLF times the overhang depth. Table 12 shows SLFs for July 21 averaged over the hours of greatest solar intensity on each exposure.

More complex shading situations should be analyzed with the RHB method.

Fenestration Solar Load Factors. Fenestration solar load factors FF_s depend on fenestration exposure and are found in Table 13. The values represent the fraction of transmitted solar gain that contributes to peak cooling load. It is thus understandable that morning (east) values are lower than afternoon (west) values. Higher values are included for multifamily buildings with limited exposure.

Interior Shading. Interior shading significantly reduces solar gain and is ubiquitous in residential buildings. Field studies show that a large fraction of windows feature some sort of shading; for example, James et al. (1997) studied 368 houses and found interior shading in 80% of audited windows. Therefore, in all but special circumstances, interior shading should be assumed when calculating cooling loads. In the RLF method, the interior attenuation coefficient (IAC) model is used, as described in Chapter 15. Residential values from that chapter are consolidated in Table 14. IAC values for many other configurations are found in Chapter 15, Tables 14A to 14G.

In some cases, it is reasonable to assume that a shade is partially open. For example, drapes are often partially open to admit daylight. IAC values are computed as follows:

where

IAC	=	interior attenuation coefficient of fenestration with partially closed shade
Fcl	=	shade fraction closed (0 to 1)
IACcl	=	interior attenuation coefficient of fully closed configuration (from Table 14 or Chapter 15, Tables 14A to 14G)

See the Common Data and Procedures section.

The contributions of occupants, lighting, and appliance gains to peak sensible and latent loads can be estimated as

where

qig,s	=	sensible cooling load from internal gains, Btu/h
qig,l	=	latent cooling load from internal gains, Btu/h
Acf	=	conditioned floor area of building, ft2
Noc	=	number of occupants (if unknown, estimate as Nbr + 1)

Equations (30) and (31) and their coefficients are derived from Building America (2004) load profiles evaluated at 4:00 pm, as documented by Barnaby and Spitler (2005). Predicted gains are typical for U.S. homes. Further allowances should be considered when unusual lighting intensities or other equipment are in continuous use during peak cooling hours. In critical situations where intermittent high occupant density or other internal gains are expected, a parallel cooling system should be considered.

For room-by-room calculations, q_ig,s should be evaluated for the entire conditioned area, and allocated to kitchen and living spaces.

See the Common Data and Procedures section.

The latent cooling load is the result of three predominant moisture sources: outdoor air (infiltration and ventilation), occupants, and miscellaneous sources, such as cooking, laundry, and bathing. These components, discussed in previous sections, combine to yield the total latent load:

where

ql	=	total latent load, Btu/h
qvi,l	=	ventilation/infiltration latent gain, Btu/h, from Equation (16) or (18)
qig,l	=	internal latent gain, Btu/h, from Equation (31)

Additional latent gains may be introduced through return duct leakage and specific atypical sources. These may be estimated and included. Lstiburek and Carmody (1993) provide data for household moisture sources; however, again note that Equation (31) adequately accounts for normal gains.

Because air conditioning systems are usually controlled by a thermostat, latent cooling is a side effect of equipment operation. During periods of significant latent gain but mild temperatures, there is little cooling operation, resulting in unacceptable indoor humidity. Multispeed equipment, combined temperature/humidity control, and dedicated dehumidification should be considered to address this condition.

Table 15 contains a brief list of equations used in the cooling load calculation procedure described in this chapter.

All above-grade surfaces exposed to outdoor conditions (walls, doors, ceilings, fenestration, and raised floors) are treated identically, as follows:

where HF is the heating load factor in Btu/h · ft²

Two ceiling configurations are common:

The Heating Load Calculations section of Chapter 18 includes simplified procedures for estimating heat loss through below-grade walls and below- and on-grade floors. Those procedures are applicable to residential buildings. In more detailed work, Bahnfleth and Pedersen (1990) show a significant effect of the area-to-perimeter ratio. For additional generality and accuracy, see also methods described or cited in Beausoleil-Morrison and Mitalas (1997), CAN/CSA Standard F280, HRAI (2014), and Krarti and Choi (1996).

Heat loss to adjacent unconditioned or semiconditioned spaces can be calculated using a heating factor based on the partition temperature difference:

Buffer space air temperature t_b can be estimated using procedures discussed in the section on Adjacent Buffer Spaces. Generally, simple approximations are sufficient except where the partition surface is poorly insulated.

Crawlspaces and basements are cases where the partition (the house floor) is often poorly insulated; they also involve heat transfer to the ground. Most codes require crawlspaces to be adequately vented year round. However, work highlighting problems with venting crawlspaces (DeWitt 2003) has led to application of sealed crawlspaces with insulated perimeter walls. Equation (5) may be applied to basements and crawlspace by including appropriate ground-related terms in the heat balance formulation. For example, when including below-grade walls, A_x = A_bw, U_x = U_{avg, bw}, and t_x = t_gr should be included as applicable in the summations in Equation (5). Losses from piping or ducting should be included as additional buffer space heat gain. Determining the ventilation or infiltration rate for crawlspaces and basements is difficult. Latta and Boileau (1969) estimated the air exchange rate for an uninsulated basement at 0.67 ach under winter conditions. Field measurements of eight ventilated crawlspaces summarized in Palmiter and Francisco (1996) yielded a median flow rate of 4.6 ach. Clearly, crawlspace infiltration rates vary widely, depending on vent configuration and operation.

Infiltration of outdoor air causes both sensible and latent heat loss. The energy required to raise the temperature of outdoor infiltrating air to indoor air temperature is the sensible component; energy associated with net loss of moisture from the space is the latent component. Determining the volumetric flow Q of outdoor air entering the building is discussed in the Common Data and Procedures section and in Chapter 16. Determining the resulting sensible and latent loads is discussed in the Ventilation/Infiltration Load subsection.

In many climates, humidification is required to maintain comfortable indoor relative humidity under heating conditions. The latent ventilation and infiltration load calculated, assuming desired indoor humidity conditions, equals the sensible heat needed to evaporate water at a rate sufficient to balance moisture losses from air leakage. Self-contained humidifiers provide this heat from internal sources. If the heat of evaporation is taken from occupied space or the distribution system, the heating capacity should be increased accordingly.

For intermittently heated buildings and night thermostat setback, additional heat is required to raise the temperature of air, building materials, and material contents to the specified temperature. The rate at which this additional heat must be supplied is the pickup load, which depends on the structure’s heat capacity, its material contents, and the time in which these are to be heated.

Because the design outdoor temperature is generally much lower than typical winter temperatures, under most conditions excess heating capacity is available for pickup. Therefore, many engineers make no pickup allowance except for demanding situations. If pickup capacity is justified, the following guidance can be used to estimate the requirement.

Relatively little rigorous information on pickup load exists. Building simulation programs can predict recovery times and required equipment capacities, but a detailed simulation study is rarely practical. Armstrong et al. (1992a, 1992b) developed a model for predicting recovery from setback and validated it for a church and two office buildings. Nelson and MacArthur (1978) studied the relationship between thermostat setback, furnace capacity, and recovery time. Hedrick et al. (1992) compared Nelson and MacArthur’s results to tests for two test houses and found that the furnace oversizing required for a 2 h recovery time ranges from 20 to 120%, depending on size of setback, building mass, and heating Δt (colder locations require less oversizing on a percentage basis).

The designer should be aware that there are trade-offs between energy savings from thermostat setback and energy penalties incurred by oversizing equipment. Koenig (1978) studied a range of locations and suggested that 30% oversizing allows recovery times less than 4 h for nearly the entire heating season and is close to optimum from an energy standpoint.

The preceding guidance applies to residential buildings with fuel-fired furnaces. Additional considerations may be important for other types of heating systems. For air-source heat pumps with electric resistance auxiliary heat, thermostat setback may be undesirable (Bullock 1978).

Thermostats with optimum-start algorithms, designed to allow both energy savings and timely recovery to the daytime set point, are becoming routinely available and should be considered in all cases.

Table 16 lists equations used in the heating load calculation procedures described in this chapter.

Design Conditions. Table 18 summarizes design conditions. Typical indoor conditions are assumed. Outdoor conditions are determined from Chapter 14.

Component Quantities. Areas and lengths required for load calculations are derived from plan dimensions (Figure 1). Table 19 summarizes these quantities.

Opaque Surface Factors. Heating and cooling factors are derived for each component condition. Table 20 shows the resulting factors and their sources.

Window Factors. Deriving cooling factors for windows requires identifying all unique glazing configurations in the house. Equation (25) input items indicate that the variations for this case are exposure, window height (with overhang shading), and frame type (which determines U-factor, SHGC, and the presence of insect screen). CF derivation for all configurations is summarized in Table 21.

For example, CF for operable 3 ft high windows facing west (the second row in Table 21) is derived as follows:

Envelope Loads. Given the load factors and component quantities, heating and cooling loads are calculated for each envelope element, as shown in Table 22.

Infiltration and Ventilation. From Table 3, A_ul for this house is 0.02 in²/ft² of exposed surface area. Applying Equation (9) yields A_L = A_es × A_ul = 3848 × 0.02 = 77 in². Using Table 5, estimate heating and cooling IDF to be 1.0 and 0.48 cfm/in², respectively [alternatively, Equation (10) could be used to find IDF values]. Apply Equation (8) to find the infiltration leakage rates and Equation (7) to convert the rate to air changes per hour:

Calculate the ventilation outdoor air requirement with Equation (11) using A_cf = 2088 ft² and N_br = 3, resulting in Q_v = 93 cfm. For design purposes, assume that this requirement is met by a mechanical system with balanced supply and exhaust flow rates (Q_unbal = 0).

Find the combined infiltration/ventilation flow rates by summing the balanced ventilation flow with net infiltration flow derived with Equation (14):

At Atlanta’s elevation of 1027 ft, elevation adjustment of heat factors results in a small (4%) reduction in air heat transfer; thus, adjustment is unnecessary, resulting in C_s = 1.10 Btu/h · °F · cfm. Use Equation (15) with Q_bal,hr = 0 and Q_bal,oth = 0 to calculate the sensible infiltration/ventilation loads:

Internal Gain. Apply Equation (30) to find the sensible cooling load from internal gain:

Distribution Losses and Total Sensible Load. Table 23 summarizes the sensible load components. Distribution loss factors F_dl are estimated (from Table 6) at 0.13 for heating and 0.27 for cooling.

Latent Load. Use Equation (16) with C_l = 4840 Btu/h · cfm, Q_vi,c = 129 cfm, Q_bal,oth = 0, and ΔW = 0.0050 to calculate the infiltration/ventilation latent load = 3122 Btu/h. Use Equation (31) to find the latent load from internal gains = 378 Btu/h. Therefore, the total latent cooling load is 2565 Btu/h.