Cooling load temperature difference calculation method

The CLTD/CLF/SCL (cooling load temperature difference/cooling load factor/solar cooling load factor) cooling load calculation method was first introduced in the 1979 ASHRAE Cooling and Heating Load Manual (GRP-158) The CLTD/CLF/SCL Method is regarded as a reasonably accurate approximation of the total heat gains through a building envelope for the purposes of sizing HVAC equipment. This method was developed as a simpler calculation alternative to difficult and unwieldy calculation methods such as the transfer function method and the Sol-air temperature method. Error when using the CLTD/CLF/SCL method tends to be less than twenty percent over and less than ten percent under.

After its introduction in the 1979 ASHRAE handbook, research continued on increasing the accuracy of the CLTD/CLF method. Research completed in 1984 revealed some factors which were not accounted for in the original publication of the method; these findings were a result of the ASHRAE research project 359. In 1988 ASHRAE Research Project 472 worked to correct these oversights with the introduction of a classification system for walls, roofs, and zones. Additionally, a weighting factor database was generated to help correct for previous inaccuracies. Additional research in Thermal radiation and appliance heat gain with respect to CLTD data was also completed shortly after the original publication of the method. The advancements in each of these areas inspired a revision/compilation effort, and in 1993 the CLTD/CLF/SCL method was succinctly compiled by Spitler, McQuiston, and Lindsey.

The CLTD/CLF/SCL method uses predetermined set of data to expedite and simplify the process of cooling/heating load approximation. The data is divided into many different sections based on many different variables. These variables include, building material of the envelope, thicknesses of the building materials, day of the year, time of day, orientation of the surface (e.g. wall or roof, 90 degrees or 180), and wall face orientation (cardinal directions, i.e. N, NW, S, SE, etc.), to name a few. In order to determine which set of CLTD/CLF/SCL data to look at, all the requisite variables must be defined.

The respective tables of data were generally developed by using the more complex transfer function method to determine the various cooling loads for different types of heating. The results gained by doing so are then normalized for each type of heat gain used for the tables, CLTD, CLF, and SCL.

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