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HVAC Design Criteria and Guidelines COMMERCIAL/INSTITUTIONAL LOAD CALCULATIONS Prior to 1967, heating/cooling load calculations were performed manually, using data developed from experience. The evolution of the current heating/cooling load calculation methodology started with Total Equivalent Temperature Difference/Time Average (TETD/TA) methods, published by ASHRAE in 1967. Like earlier methods, the TETD/TA computations proceeded in logical steps and were easy to understand, and the results were validated by field tests. However, there were disadvantages: In the days before common use computers, the equations were too repetitive and time-consuming for many designers and the academic members of ASHRAE continued to point out that the method, though valid, remained an “approximation” based on the Transfer Function Method (TFM). The more rigorous TFM method factored together heat gain by conduction through exterior walls and roofs; conversion of the cooling load from heat gain; and use of room transfer functions. The method had the benefit of being flexible for load variations, but the calculations were formidable and the computers available at the time were too costly for most engineering firms to purchase and maintain. TETD/TA, essentially, was a manual simplification of TFM calculations. In the 1980s, the Cooling Load Temperature Difference/ Cooling Load Factor (CLTD/CLF) method was devised, again based on TFM calculations, that offered better accuracy and greater simplicity than TETD/TA due to its single-step calculation that could easily be done manually. But, CLTD/CLF had a limited range of application and, in 1984, caveats were announced that stated limitations to the manual method and guidance for its use. Then, in the 1990s, as personal computers came along and quickly improved in performance, the Radiant Time Series (RTS) and Heat Balance (HB) methods, which more accurately models real-world conditions, was developed. The RTS method is rigorous and accurate due to the degree of detail it includes, but this method of load computation (now called “modeling”) requires the computer Manual methods of heating/cooling load computation for commercial or institutional buidlings, except perhaps for preliminary estimating, has gone by the wayside. All new and renovated commercial or institutional buildings must have heating/cooling load calculations performed and an energy model created. Load calculations and energy modeling must be performed via Carrier “HAP,” DOE “Equest,” Elite “CHVAC,” Energy Soft “EnergyPro,” or Trane “Trace 700,” all of which use the RTS/HB load computation methods, using the following design criteria: 1. Determine design outdoor weather conditions (temperature and humidity) for the building location, summer and winter from Chapter 14, 2013 ASHRAE Handbook-Fundamentals. Unless the project requires otherwise, utilize the 99% for heating/1% for cooling design criteria. 2. For normally occupied spaces, select the indoor conditions to be maintained (temperature, humidity, etc.) Unless required otherwise by project program, utilize 75F/50% RH for cooling and 70F for heating. (For more detailed evaluation of indoor design conditions, see ASHRAE Standard 55.) 3. For other levels of space utilization, select indoor conditions based on the following: Minimally Occupied Space (Space that is used for purposes that are not intended to be fully occupied or used for assembly. These include but are not limited to stairwells and storage rooms.): Heating: 65°F and 50%RH HVAC Design Guidelines LOAD CALCULATIONS 1 HVAC Design Criteria and Guidelines Cooling: 85°F and 50% RH Unoccupied Mechanical Space (Space that houses the equipment and systems that give the building its functionality. These include but are not limited to mechanical rooms, electrical rooms and fire pump rooms.): Heating: 55°F Cooling: 85°F Unoccupied Space Requiring Freeze Protection (Any unoccupied space that has piping (plumbing, HVAC, sprinkler) in which is at risk for freeze burst if the room space temperature falls below freezing.): Minimum 45°F No additional safety factors are required when load estimates are based on accurate information pertaining to the building envelope construction, internal heat gains, etc. However, large errors are possible if there is uncertainty about insulation levels, fenestration performance, envelope tightness, etc. and the designer attempts to compensate for the lack of information with safety factors. Loads must be computed using accurate building data…apply safety factors only if absolutely necessary. While every project is unique, there are typical similarities and it is to important to understand the calculations to ensure their validity. Therefore, unless more specific information is available, the "defaults" defined below should be utilized. Envelope Heat Gain/Loss: Obtain window cut sheets from the architect that provide specific U-value and Shade Coefficient data. Use the data from Chapter 26, 2013 ASHRAE Handbook - Fundamentals and information supplied by the architect to determine the thermal properties of opaque envelope assemblies. Remember to de-rate wall insulation that is located in a wood stud wall. A wall with 16-inch center-to-center wood studs has an average of 25% wood /75% insulation (with an allowance for headers and plates) and a wall with 24-inch center-to-center studs has an average of 22% wood / 78% insulation. For metal stud walls, use the following table to determine wall assembly U-value: Nominal Stud Depth, Inches 4 4 4 6 6 8 Nominal Insulation R-Value R-11 R-13 R-15 R-19 R-21 R-25 Overall Assembly U-Factors 16" O.C. 24" O.C. 0.14 0.13 0.13 0.12 0.12 0.11 0.11 0.10 0.11 0.09 0.10 0.09 Window loads may be calculated based on actual window sizes or on generic 1’x1’ units. If there is an unusually large amount of glass or the calculations are being used for a detailed energy analysis, then actual window sizes must be used. All assumptions and calculations for wall, roof, window, etc. heat transfer factors must be recorded in the design file. Internal Heat Gain: Use equipment cut sheets whenever possible to determine heat gain. When adequate information is not available, use values found in the 2013 ASHRAE Handbook-Fundamentals. The following is a list of typical internal heat gains: HVAC Design Guidelines LOAD CALCULATIONS 2 HVAC Design Criteria and Guidelines People: The reduced numbers for children should only be used in pre-school and elementary schools; use adult numbers for middle and high schools. Adults, stationary1 Adults, office work2 Adults, strenuous work3 Children, classrooms Children, activities Other Notes BTUH sensible BTUH latent 230 120 245 205 525 925 200 160 400 700 See Table 1, Chapter 18, 2013 ASHRAE Handbook Fundamentals 1. Examples include performance theatres and auditoriums 2. Examples include offices, conference rooms, classrooms, and lobby areas 3. Examples include gyms, exercise areas and shops Lighting: In new buildings, calculate general lighting load based on the applicable energy conservation code. In specialty applications, such as retail or performance spaces, use 120% of the designed fixtures. In existing buildings, use the sum of existing fixtures wattages plus applicable ballast factors. Equipment: Heat release (internal heat gain) data from common equipment can be obtained from Tables 5-12 of Chapter 18, 2013 ASHRAE Handbook - Fundamentals. All university, college and high school classrooms should be designed to carry the load of one laptop per student unless otherwise instructed by the client. Obtain heat loss (inefficiency) information from the project electrical engineer for transformers, variable frequency drives, etc. If not available, use the following: Transformers, dry-type Variable Frequency Drives 3% of kW (kVA) rating 2% of kW (kVA) rating Confirm equipment quantities, locations, and heat release data with the owner and include this information in the design file. Ventilation Heat Gain/Loss: See the Dilution Ventilation section of these guidelines for discussion of ventilation air requirements. Note that ventilation heat gains/losses do not apply to space heat gains/losses, but do impact heating/cooling coil loads and subsequent air system(s) and central plant capacity requirements. Heating/Cooling Load Computation Input: For a load analysis, the software depends on your input data. The software has the capability to perform energy analysis, but if you are not using it for this level of modeling then avoid entering information that is unnecessary and that might somehow alter the result. (For example, do not add an economizer section to an air system, even if it will have one. The economizer controls should have no effect on the peak loads you are trying to calculate.) Unless there is something unique about your project, use the following defaults for your load calculations: HVAC Design Guidelines LOAD CALCULATIONS 3 HVAC Design Criteria and Guidelines Occupancy and thermostat schedules Thermostat throttling range Coil bypass factor Diversity factors Infiltration, walls and windows1 Safety factors (sensible/latent/heating) Supply ductwork heat gain Supply ductwork leakage Plenum heat gains (wall/roof/lighting) Zone and space sizing method 1Determine 100% “on” 24 hrs/day 7 days/week 1F 0.1 100% 0 cfm (design for positive pressure) 5%/5%/20% 0% 0% 0%/70%/30% peak zone at coincident space loads infiltration for renovation projects if the building envelope warrants it. Computed Loads Validation: Computer-based heating/cooling load calculations can easily fall victim to the "garbage in, garbage out" syndrome. Review load calculation results carefully and evaluate the "reasonableness" of the heat losses and gains in each space and zone using experienced-based "check figures". If results appear outside the range of typical loads computed previously, find out why…a non-obvious data entry mistake could easily skew results. To help ensure that computed heat gains and losses are “reasonable,” the following “check figures” have been developed for comparison against computed loads: Check Figures No. 1: Room/Zone Internal Het Gains Check Figures No. 2: Room/Zone Perimeter Heat Gains and Losses Check Figures No. 3: Typical Block Heating/Cooling Loads While, obviously, computed loads will almost never exactly match these check figures, any computed load that varies significantly from check figures should be carefully investigated to verify its validity. Minimum Air Change Requirements: For hospitals, laboratories, cleanrooms, etc., the amount of air circulated in a given space may be dictated on the basis of minimum air changes per hour (AC/Hr). The following table can be used to determine airflow requirements in terms of CFM/sf to meet AC/Hr requirements: Required AC/hr 2 3 4 5 6 7 8 10 12 14 16 8 0.25 0.40 0.53 0.67 0.80 0.93 1.07 1.33 1.60 1.87 2.13 Air Flow Required (CFM/sf) Ceiling Height (ft) 9 10 0.40 0.35 0.45 0.50 0.60 0.67 0.75 0.83 0.90 1.00 1.05 1.17 1.20 1.33 1.50 1.67 1.90 2.00 2.10 2.33 2.40 2.67 12 0.40 0.60 0.80 1.0 1.2 1.4 1.6 2.0 2.4 2.8 3.2 HVAC Design Guidelines LOAD CALCULATIONS 4 HVAC Design Criteria and Guidelines Required AC/hr 18 20 8 2.40 2.67 Air Flow Required (CFM/sf) Ceiling Height (ft) 9 10 2.70 3.00 3.00 3.33 12 3.6 4.0 HVAC Design Guidelines LOAD CALCULATIONS 5