Energy Efficient Buildings
Design Heating & Cooling Loads
Design Heating and Cooling Loads
Heating and cooling equipment must be sized to maintain interior conditions at
comfortable levels during most the most extreme weather and occupancy conditions.
Thus for “design” purposes, calculate the maximum hourly heating and cooling loads,
and select equipment large enough to meet these loads.
Design Heating Load
Major sensible heat flows into a building during winter are shown below.
Toa

Q ceil


Q sol

Q elec

Q people
Q wall

Q win

Q inf
Tia

Q grnd
Using the sign convention defined in the figure above, the net heating load out is:









Q net, out  Q ceil  Q wall  Q win  Q grnd  Q inf  Q sol  Q elec  Qpeop
For most extreme case:



Q sol  Q elec  Q peop  0 and Tsa  Toa
Therefore,






Q heat, des  Q ceil  Q wall  Q win  Q grnd  Q inf
 UA ceil  UA wall  UA win  UA grnd  UA inf Tia  Toa, des 
And choose Toa = Toa,min = Toa,heating des

Use Q heat, des to size furnaces and boilers. The method to size heat pumps is explained in
the chapter on Heat Pumps. Heating equipment is rated by output capacity. For non-
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
 Btu 
standard sizes, round up. For example, if Q heat, des  58,000   , then specify a furnace
 hr 
 Btu 
with an output capacity of 60,000 

 hr 
Design Cooling Load
Major sensible heat flows into a building during summer are shown below.
Toa

Q ceil


Q sol

Q elec

Q people
Q wall

Q win

Q inf
Tia

Q grnd
Using the sign convention defined in the figure above, the net cooling load in is:









Q net,in  Q ceil  Q wall  Q win  Q inf  Q sol  Q elec  Q peop  Q grnd
To create the design heat load, consider the most extreme cases when:

 I  Imax
Q sol,max

Q people, max

Q elec, max
Use Tsa  Toa 



Q inf  Q sen  Q lat

Iα
h
ω
I  Imax 
oa
 ωoa, max 

Then Q net, in  Q cool,des
And choose: Toa  Toa,max  Toa,cooling des
oa  oa,max  oa,cooling des
I  I max
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Most cooling equipment is rated in “tons” of cooling capacity, where:
 Btu 
1 ton  12,000 
.
 hr 

Use Q cool,des to specify the size of cooling equipment. For non-standard sizes, round up.
For example, if

1  ton h 
 Btu 
 Btu 
Q cool,des  34,000 

34,000


 hr  12,000  Btu   2.8 tons
 hr 




then specify a “3.0-ton” air conditioner.
Design Weather Conditions
Ambient Temperature and Humidity
For calculating peak heating and cooling loads, ASHRAE tabulates the minimum and
maximum outdoor air temperatures and coincident outdoor humidity that is likely to
occur for hundreds of U.S. cities.
For peak heating load calculations, ASHRAE publishes the “99.6%” and “99.0%” design
conditions, meaning that the actual hourly temperatures were greater (warmer) than
the design temperature 99.6% or 99.0% of all annual hours. The peak heating load
design temperatures for Dayton, OH are shown below. To ensure that the heating
system is large enough to handle the coldest expected temperatures, use the 99.6%
design temperature.
For peak cooling load calculations, ASHRAE publishes the “0.4%” and “1.0%” design
conditions for temperature (Tdb) and humidity (Twb & Tdp), such that the actual hourly
temperatures were greater (warmer) than the design temperatures 0.4% or 1.0% of all
annual hours. In addition, ASHRAE publishes the mean coincident wet bulb temperature
(MCWB) for each design condition, which is the mean wet bulb temperature at the
specified dry bulb temperature. MCWB temperature is used for calculating peak latent
cooling loads. The peak cooling load design temperatures for Dayton, OH are shown
below. To ensure that the cooling system is large enough to handle the warmest
expected conditions, use the 0.4% design temperatures.
For Dayton
99.6%
99%
DB
-1 °F
5 °F
For Dayton DB
0.4%
90 °F
1.0%
88 °F
MCWB
74 °F
73 °F
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The outdoor air specific humidity can be determined from dry bulb and mean coincident
wet bulb temperature using a psychrometric chart.
Example
Determine the specific humidity of air when the dry bulb temperature is 88 F and the
wet bulb temperature is 73 F using a psychrometric chart for air at sea level.
w(Tdb = 88 F, Twb =73 F) = 0.014 lbw/lba
Solar Radiation
ASHRAE also publishes a method to calculate peak solar radiation on a surface. An
alternative is to use solar radiation data from Typical Meteorological Year files. To use
this method, use WeaTran to create a “12 24-hour Day Types” file. To create the file,
WeaTran selects a representative 24-hour period for each month, and calculates hourly
solar radiation on east, south, west and north exposures from the hourly solar radiation
on a horizontal surface published in the TMY3 data. Using this method, peak solar gains
can be estimated by selecting the hour with the greatest solar radiation on a horizontal
surface from a WeaTran output file of “12 24-hour Day Types”.
Example
For peak cooling load calculations, find solar radiation on horizontal, east, south, west
and north exposures for Sacramento California using the TMY/WeaTran method.
Using TMY3 data for Sacramento California, WeaTran selects a representative 24-hour
period for each month, and calculates hourly solar radiation on east, south, west and
north exposures from the solar radiation on a horizontal surface, and publishes the data
in “Sacramento_CA_dt_us.txt”.
The peak solar radiation on a horizontal surface in this file is 319 Btu/ft2-hr and occurred
on June 24 at 12:00. The column names and record is shown below.
Mo Dy Yr Hr Ta(F) Sol-H(Btu/ft2hr) Sol-E(Btu/ft2hr) Sol-S(Btu/ft2hr) Sol-W(Btu/ft2hr) Sol-N(Btu/ft2hr) w(lbw/lba)
06 24 1995 12 80.96
319
91
123
44
44 0.0086
Thus, peak solar radiation for design purposes can be estimated as:
Horizontal: 319 Btu/hr-ft2
East: 91 Btu/hr-ft2
South: 123 Btu/hr-ft2
West: 44 Btu/hr-ft2
North: 44 Btu/hr-ft2
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Ground Temperature
In many locations, annual minimum and maximum ambient temperatures occur about
1.5 months after the winter and summer solstices. Thus, February 1 and August 1 are
good dates on which to calculate effective ground temperatures for peak heating and
cooling load calculations. A good estimate of effective ground temperature on these
dates for load calculations can be derived from TMY3 data and WeaTran “12 24-hour
Day Types” data.
Example
For peak heating and cooling load calculations, find effective ground temperature for
Sacramento California using the TMY/WeaTran method.
Using TMY3 data for Sacramento California, WeaTran selects a representative 24-hour
period for each month and publishes the data in “Sacramento_CA_dt_us.txt”.
Effective ground temperature is found using the following formula:
Tg = (1.7 Toa,yr + 1.0 Toa,3mo) / 2.7
From “Sacramento_CA_dt_us.txt”, the average annual air temperature is:
Ta,avg = 59.94 F
From “Sacramento_CA_dt_us.txt”, the average air temperature during November,
December and January is 47.91 F
From “Sacramento_CA_dt_us.txt”, the average air temperature during May, June and
July is 69.96 F.
Thus, the effective ground temperatures are:
Winter: Tg = (1.7 Toa,yr + 1.0 Toa,3mo) / 2.7 = (1.7 x 59.94 F + 1.0 47.91) / 2.7
Winter: Tg = 55.48 F
Summer: Tg = (1.7 Toa,yr + 1.0 Toa,3mo) / 2.7 = (1.7 x 59.94 F + 1.0 69.96) / 2.7
Summer: Tg = 63.66F
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