ASHRAE Journal - October 2022 - 58

TECHNICAL FEATURE
in the ASHRAE Handbook procedure contains the term
∆T, the difference between the desired optimum indoor
temperature of 72°F (22°C) and the estimated minimum
dry-bulb outside air temperature of 21.9°F (-5.6°C).
Complete details of the DHL calculations for the singleroom
building are presented below.
Single-Room Base Design Heating Load (DHL) Data
The 2017 ASHRAE Handbook method for estimating the
base DHL for a single-room building in Atlanta can be
represented by the following formula:
Q U i=×Ai T I H T
()×∆ × ×∆
n
∑ () ()
i=1
where
Q = Base design heating load, Btu/h
U = Heating load factor for various exterior surfaces
of the building, Btu/h·ft2·°F
A = Area of various exterior surfaces of the building, ft2
n = Number of exterior building surface subareas
with different thermal transmission
characteristics
H = Heating load factor of 1.1 Btu/h·cfm·°F for outside
air infiltration into the building
I = Hourly air infiltration into the building at the rate
of 19.5 cfm (9.2 L/s), or one building volume per
hour
T = Difference between selected optimum indoor
temperature of 72°F (22°C) and the selected
low dry-bulb outside air temperature of 21.9°F
(-5.6°C) for Atlanta.
The summary of 2017 ASHRAE Handbook-Fundamentals
calculations for estimated base DHL, for a single-room
building in Atlanta is shown in Table 1.
Calculated Base Hourly Heating Load (HHL) for A Single-Room Building,
Using the U.S. National Weather Service's Reported Hourly Outside Air
Temperature Data for Atlanta Hartsfield-Jackson International Airport
This section provides a comparison between the calculated
ASHRAE base design heating load and the base
hourly heating load for maintaining the indoor temperature
at 72°F (22°C) in a single-room building. The HHL
was calculated by using hourly outside dry-bulb air temperature
data published by the U.S. National Weather
Service for the 2,184 hours of the winter (December 20,
2020, through March 22, 2021), for Atlanta HartsfieldJackson
International Airport, as shown in Table 2. The
58
ASHRAE JOURNAL ashrae.o rg O CTO B E R 2022
+
(1)
TABLE 1 Estimated HVAC base design heating load (DHL) for a single room in
Atlanta.
SINGLE ROOM
EXTERIOR
ELEMENTS
Windows
Spandrel
Wall
Brick Wall
Roof
INFILTRATION
ELEMENTS
Infiltration
AREA, ft2
80
120
100
130
AIR
INFILTRATION
(CFM)
19.5
HEATING LOAD
FACTOR,
Btu/h·ft2·°F
0.56
0.077
0.08
0.032
HEATING LOAD
FACTOR,
Btu/h·cfm·°F
1.1
BASE DESIGN
∆T, °F
(72 - 21.9)
(72 - 21.9)
(72 - 21.9)
(72 - 21.9)
∆T, °F
(72 - 21.9)
TOTAL
Source: 2017 ASHRAE Handbook-Fundamentals (Chapter 18, Section 9.2)
∆T required for heating load calculation was derived by
subtracting the reported hourly dry-bulb outside air
temperature data from 72°F (22°C). The heating load
estimate for the winter period was then calculated by
using the following formula:
Q = ∆× ×() ()
∑ ()
,
i=1
Ti U j A j I H Ti
∑
j
n
=1
 ()+ × ×∆ ()
2 184


(2)
All the terms in Equation 2 are as described for
Equation 1. For each hourly time interval the term (U × A)
as well as the air infiltration factor (19.5 × 1.1) remained
constant and ∆T(i) was calculated based on the reported
hourly outside air temperature data. For the ASHRAE
base design heating load calculation, ∆T(i) also remained
constant at (72minus21.9) degrees Fahrenheit, which is
(22 minus -5.6) degrees Celsius. The summary of ∆T(i)
calculations, for both HHL and DHL methods, for various
outside surface subareas of the building plus the air
infiltration rate, along with their associated heat transfer
coefficients, are shown in Table 2.
Table 2 shows the summary of calculations performed
for deriving the estimated operational heating load by
using the reported hourly local air temperature data and
the DHL by using the ASHRAE formula. These data were
calculated by assuming that the building would be in full
operation during both daytime and nighttime hours.
Based on this data, it is apparent that the calculated DHL
is about 214% of the operational heating load, which was
calculated by using the measured hourly outside air temperature
for the three-month winter period in Atlanta.
It is probable that the calculated operational heating
HEATING LOAD,
Btu/h
2,244
463
401
208
BASE DESIGN
HEATING LOAD,
Btu/h
1,075
4,391
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