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PDHonline Course M196 (4 PDH)HVAC Made Easy: A Guide to Heating& Cooling Load EstimationInstructor: A. Bhatia, B.E.2012PDH Online PDH Center5272 Meadow Estates DriveFairfax, VA 22030-6658Phone & Fax: 703-988-0088www.PDHonline.orgwww.PDHcenter.comAn Approved Continuing Education Provider

www.PDHcenter.comPART 1PDH Course M196www.PDHonline.orgSUMMER COOLING LOADPrefaceThe term summer cooling load means much more than merely cooling the air in a building. In addition to cooling the air,it also implies controlling:1) The relative humidity2) Providing proper ventilation3) Filtering out contaminants (air cleaning) and4) Distributing the conditioned air to the lived-in spaces in proper amounts, without appreciable drafts or objectionablenoiseThis section deals with the design aspects and the equations used for summer cooling load calculations.Design ConditionsThe amount of cooling that has to be accomplished to keep buildings comfortable in hot summer depends on thedesired condition indoors and on the outdoor conditions on a given day. These conditions are, respectively, termed the“indoor design condition” and the “outdoor design condition”.Indoor Design ConditionsThe indoor design conditions are directly related to human comfort. Current comfort standards, ASHRAE Standard 551992 [4] and ISO Standard 7730 [5], specify a “comfort zone,” representing the optimal range and combinations ofthermal factors (air temperature, radiant temperature, air velocity, humidity) and personal factors (clothing and activitylevel) with which at least 80% of the building occupants are expected to express satisfaction. As a general guideline forsummer air-conditioning design, the thermal comfort chapter of the ASHRAE fundamentals handbook (Chapter 8, 2001)provides a snapshot of the psychrometric chart for the summer and winter comfort zones.For most of the comfort systems, the recommended indoor temperature and relative humidity are:1) Summer: 73 to 79 F; The load calculations are usually based at 75ºF dry bulb temperatures & 50% relativehumidity2) Winter: 70 to 72 F dry bulb temperatures, 20 - 30 % relative humidityThe standards were developed for mechanically conditioned buildings typically having overhead air distribution systemsdesigned to maintain uniform temperature and ventilation conditions throughout the occupied space. The Psychrometricchapter of the Fundamentals Handbook (Chapter 6, 2001) provides more details on this aspect.Outdoor Design ConditionsOutdoor design conditions are determined from published data for the specific location, based on weather bureau orairport records. Basic climatic and HVAC “design condition” data can be obtained from ASHRAE handbook, whichprovides climatic conditions for 1459 locations in the United States, Canada and around the world. The informationincludes values of dry-bulb, wet-bulb and dew-point temperature and wind speed with direction on percentageoccurrence basis.Design conditions for the United States appear in Table 1a and 1b, for Canada in Tables 2a and 2b, and theinternational locations in Tables 3a and 3b of 1997, ASHRAE fundamentals handbook chapter 26.The information provided in table 1a, 2a and 3a are for heating design conditions that include:1) Dry bulb temperatures corresponding to 99.6% and 99% annual cumulative frequency of occurrence,2) Wind speeds corresponding to 1%, 2.5% and 5% annual cumulative frequency of occurrence,3) Wind direction most frequently occurring with 99.6% and 0.4% dry-bulb temperatures and4) Average of annual extreme maximum and minimum dry-bulb temperatures and standard deviations.Page 3 of 79

www.PDHcenter.comPDH Course M196www.PDHonline.orgThe information provided in table 1b, 2b and 3b are for cooling and humidity control conditions that include:1) Dry bulb temperature corresponding to 0.4%, 1.0% and 2.0% annual cumulative frequency of occurrence and themean coincident wet-bulb temperature (warm). These conditions appear in sets of dry bulb (DB) temperature andthe mean coincident wet bulb (MWB) temperature since both values are needed to determine the sensible andlatent (dehumidification) loads in the cooling mode.2) Wet-bulb temperature corresponding to 0.4%, 1.0% and 2.0% annual cumulative frequency of occurrence and themean coincident dry-bulb temperature3) Dew-point temperature corresponding to 0.4%, 1.0% and 2.0% annual cumulative frequency of occurrence and themean coincident dry-bulb temperature and humidity ratio (calculated for the dew-point temperature at the standardatmospheric pressure at the elevation of the station).4) Mean daily range (DR) of the dry bulb temperature, which is the mean of the temperature difference between dailymaximum and minimum temperatures for the warmest month (highest average dry-bulb temperature). These areused to correct CLTD values.In choosing the HVAC outdoor design conditions, it is neither economical nor practical to design equipment either forthe annual hottest temperature or annual minimum temperature, since the peak or the lowest temperatures might occuronly for a few hours over the span of several years. Economically speaking short duration peaks above the systemcapacity might be tolerated at significant reductions in first cost; this is a simple risk - benefit decision for each buildingdesign. Therefore, as a practice, the ‘design temperature and humidity’ conditions are based on frequency ofoccurrence. The summer design conditions have been presented for annual percentile values of 0.4, 1 and 2% andwinter month conditions are based on annual percentiles of 99.6 and 99%.The term “design condition” refers to the %age of time in a year (8760 hours), the values of dry-bulb, dew-point andwet-bulb temperature exceed by the indicated percentage. The 0.4%, 1.0%, 2.0% and 5.0% values are exceeded onaverage by 35, 88, 175 and 438 hours.The 99% and 99.6% cold values are defined in the same way but are viewed as the values for which the correspondingweather element are less than the design condition 88 and 35 hours, respectively. 99.6% value suggests that theoutdoor temperature is equal to or lower than design data 0.4% of the time.Design condition is used to calculate maximum heat gain and maximum heat loss of the building. For comfort cooling,use of the 2.5% occurrence and for heating use of 99% values is recommended.The 2.5% design condition means that the outside summer temperature and coincident air moisture content will beexceeded only 2.5% of hours from June to September or 73 out of 2928 hours (of these summer months) or 2.5% ofthe time in a year, the outdoor air temperature will be above the design condition.Cooling Loads Classified by SourceCooling loads fall into the following categories, based on their sources:1) Heat tran