designhcloads.docx

7/29/2019 DesignHCLoads.docx

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Energy Efficient Buildings

Design Heating & Cooling Loads

Design Heating and Cooling Loads

Heating and cooling equipment must be sized to maintain interior conditions atcomfortable 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.

solQ

wallQ

peopleQ

elecQ

winQ

infQ

grndQ

ceilQ

Tia

Toa

Using the sign convention defined in the figure above, the net heating load out is:

peopelecsolinfgrndwinwallceiloutnet, QQQQQQQQQ

For most extreme case:

oasapeopelecsolTTand0QQQ

Therefore,

desoa,iainfgrndwinwallceilinfgrndwinwallceildesheat,

TTUAUAUAUAUA

QQQQQQ

And choose Toa = Toa,min = Toa,heating des

Use desheat,Q

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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max

,ma x,

,max,

II

TTT

descoolingoaoaoa

descoolingoaoaoa

maxoa,oalatseninf

maxoasa

maxelec,

maxpeople,

maxmaxsol,

QQQ

IIh

ITTUse

Q

Q

IIQ

standard sizes, round up. For example, if

hr

Btu58,000Q desheat, , then specify a furnace

with an output capacity of

hr

Btu60,000

Design Cooling Load

Major sensible heat flows into a building during summer are shown below.

solQ

wallQ

peopleQ

elecQ

winQ

infQ

grndQ

ceilQ

Tia

Toa

Using the sign convention defined in the figure above, the net cooling load in is:

grndpeopelecsolinfwinwallceilinnet, QQQQQQQQQ

To create the design heat load, consider the most extreme cases when:

Then descool,innet, QQ

And choose:


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Most cooling equipment is rated in tons of cooling capacity, where:

hr

Btu12,000ton1 .

Use descool,Q

to specify the size of cooling equipment. For non-standard sizes, round up.

For example, if

tons2.8Btu

hton

12,000

1

hr

Btu34,000

hr

Btu34,000Q descool,

then specify a 3.0-ton air conditioner.

Design Weather ConditionsAmbient 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 DB

99.6% -1 F

99% 5 F

For Dayton DB MCWB

0.4% 90 F 74 F

1.0% 88 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 thewet 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 radiationon 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-hourDay 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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