DOE-2 Reference Manual Version 2.1 - DOE2.com

C. ALTERNATIVE LOAD CALCULATION METHODS1. Load Calculation MethodsThe purpose of the following discussion is to give the user enough of theDOE-2 load calculation theory to permit decision making. At the conclusion ofthis section the user will be asked to make a decision on which of twoavailable load calculation methods he wishes to use. It is not necessary forthe user to understand the theory in detail, only enough to make the decision.In Heating, Ventilating and Air Conditioning (HVAC) calculations, fourdistinct heat flows, each varying with time, take place. These heat flows are:1. Space Instantaneous Heat Gain (or Space Instantaneous Heat Loss)2. Space Cooling Load (or Space Heating Load)3. Space Heat Extraction Rate (or Space Heat Addition Rate)4. Cooling Coil Load (or Heating Coil Load).The following discussion speaks in terms of "heat gain and the associatednecessary cooling" but the same discussion could be made for the reversesituation, that is, "heat loss and the associated necessary heating."Space Instantaneous Heat Gain is the rate at which heat enters or is generatedwithin a space at a given instant. The space instantaneous heat gainresults from: (1) solar radiation through transparent surfaces; (2) heat conductionthrough exterior walls and roofs; (3) heat conduction through interiorpartitions, ceilings, and floors; (4) heat generated within the space byoccupants, lights, and appliances; (5) energy transfer as a result of enteringoutside air; or (6) miscellaneous heat gains. Space instantaneous heat gaincalculations may be done based on either transient (dynamic) or steady-stateformulas, depending on the form of the space instantaneous heat gain. Forexample, transient formulas may be used in the calculation of the heatconduction through an exterior wall. Using transient calculations makes itpossible to account for the storage of energy that is due to the thermal massbetween the high and low temperature sides of the energy transmitting media.Space Cooling Load is the rate at which heat must be removed from thespace to maintain space air temperature at a constant value. It should benoted that the summation of all space instantaneous heat gains, at any giventime, does not necessarily equal the cooling load for the space at that sametime. For example, the space instantaneous heat gain by solar radiation isfirst partially absorbed by the surfaces (walls, floor, and ceiling) and thecontents (furniture) of the space. Therefore, it does not affect the spaceair until these surfaces and contents become warmer than the space air; whenthis happens, some of the heat will be transferred to the space air byconvection.Space Heat Extraction Rate is the rate at which heat is removed from theconditioned space. It equals the space cooling load only when space airtemperature is kept constant, which rarely occurs. Usually, the controlsystem, in conjunction with intermittent operation of the HVAC equipment, willcause a "swing" in the space air temperature.II I.141 (Revised 5/81)