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 75F/50% RH for cooling and
70F 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
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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:
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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:
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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
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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
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