Heating and Cooling Load Calculations Principles:

Heating load calculations estimate the conduction heat losses in the winter through exterior surfaces like roof, walls, windows, any adjoining spaces like ceiling, partition walls, and floor. Other losses include heat losses due to cold air infiltrating into the space from outdoor (or adjacent spaces) through doors, windows, and building’s cracks. Internal heat sources such as occupants or equipment/appliances are beneficial as they compensate for some of the heat losses. Heat load calculations are carried out assuming steady state conditions, no solar radiation, and neglecting the internal heat sources. Conduction heat loss equation is as follows:

Where:
Q = Heat gain by conduction, BTU/hr (W)
U = Overall heat transfer coefficient of the surface, BTU/hr x ft² x °F (W/m² x °K)
R = Thermal resistance, hr x ft² x °F/ BTU (m² x °K/W)
A = Area of the surface, ft² (m²)
ΔT = Dry-bulb temperature difference across the surface, °F (°C)

Cooling load calculations estimate the conduction heat losses in the summer through the same surfaces as for the heating load calculations. Internal and external cooling load calculations on a building (sensible and latent parts, meaning: the sensible cooling load involves lowering the temperature of air provided to a space without causing the moisture in the air to undergo a phase change and condensate, on the other hand, the latent cooling is responsible for dehumidifying the air by producing a phase change and causing condensation to occur without affecting the temperature) can be calculated using the Cooling Load Temperature Difference/Solar Cooling Load/Cooling Load Factor (CLTD/SCL/CLF) load estimation method developed by ASHRAE along with the conduction heat loss principle.

Conduction through a shaded wall: Q=U x A x ΔT
Conduction through sunlit surfaces: Q=U x A x CLTD
Solar radiation through glass: Q=A x SC x SCL

Internal heat gains:
Lights: Q=Power density (W/ft²) x A x CLF
People: Q=Sensible heat (BTU/hr)x No. of people x CLF
Equipment: Q= Sensible heat (BTU/hr)x CLF

Where:
Q = Heat gain by conduction, BTU/hr (W)
U = Overall heat transfer coefficient of the surface, BTU/hr x ft² x °F (W/m² x °K)
A = Area of the surface, ft² (m²)
ΔT = Dry-bulb temperature difference across the surface, °F (°C)
CLTD: Cooling load temperature difference factor, °F (°C)
SC: Shading coefficient of the window, dimensionless
SCL: Solar cooling factor, BTU/hr x ft² (W/m²)
CLF: Cooling load factor
People Sensible/Latent Heat: Rates at which heat and moisture are given off by human beings in different states of activity.
Equipment Sensible Heat: Radiant and convective heat gain from equipment and appliances.
Power Density: Lighting power density using space by space method.

2. Establish Design Conditions

• Outdoor temperature based on location of the building. Use the latest ASHRAE climatic design conditions at this website: http://ashrae-meteo.info/v2.0/
• Indoor temperature required for the building. Depending on the type of building, it could be as dictated by the local code (Ontario Building Code), as requested by the client or it could be assumed based on application (ex. electrical room heating temp during shutdown).
• Building geometry. Coordinate with Architectural/Structural/Civil and/or client for existing buildings.
• Building construction details like wall, roof, and floor assemblies. Coordinate with Architectural/Structural/Civil and client for existing buildings.
• Building occupancy loads and activity level. Occupancy and parameters usually provided by the client, or it could be assumed based on application.
• Any building internal sources of heat dissipation from process equipment, office equipment, etc. Coordinate with electrical and/or client.
• Any building sources of outside air infiltration.
• Lighting heat gain as per electrical design or assumed based on electrical discipline recommendation.

3.1 Conduction Load

a) Outdoor temperature:

• ✔ Summer: Dry Bulb (DB) °F, Wet Bulb (WB) °F, Grains of Moisture (W) grains/lb. Refer to a Psychrometric chart to obtain any missing parameters.
• ✔ Winter: Dry Bulb (DB) °F, Grains of Moisture (W) grains/lb. Refer to a Psychrometric chart to obtain any missing parameters.

b) Indoor Temperature:

• ✔ Summer: Dry Bulb (DB) °F, Wet Bulb (WB) °F, Grains of Moisture (W) grains/lb (50–60% Relative Humidity is typical). Refer to a Psychrometric chart to obtain any missing parameters.
• ✔ Winter: Dry Bulb (DB) °F, Grains of Moisture (W) grains/lb (30% Relative Humidity is typical). Refer to a Psychrometric chart to obtain any missing parameters.

3.2 Solar Loads

Solar loads are based on SCL and SC, which are the maximum solar cooling load and the shading coefficient respectively, these are obtained from ASHRAE Handbook Fundamentals section 18, or text book chapter 8 based on the zones type (determined based on the No. of exterior walls, floor covering, partition type, and inside shade), exposure of the window (North, South, East, West), solar time, and the type of glass and internal shading device.

3.3 Internal Loads

• Light density load or lighting power density (Watts/ft²). Coordination with electrical based on space application. From ASHRAE Handbook Fundamentals – Section 18, table 2.
• Occupancy loads as per client and space application. From ASHRAE Handbook Fundamentals – Section 18, table 1, or text book chapter 8. Determined based on type of activity.
• Equipment loads (BTU/hr or kW) as per application. From ASHRAE Handbook Fundamentals – Sections 17 and/or 18, or Manufacturer technical data sheets.

3.4 Infiltration/Exfiltration Loads

Air leaks into or out of a space through windows, doors, and small cracks in the building envelope. Heat gain from infiltration can be calculated using any of the following methods:

• Crack method: based upon the average quantity of air known to enter through cracks around windows and doors. Infiltration calculations use a reference rule of thumb of 1 CFM/ft of door/window perimeter cracks.
• ACH method: based on an estimation of the number of air changes per hour that can be expected in a volume of space of a certain construction quality. Infiltration calculations use 1 ACH for poor constructions, 0.5 ACH for average construction and 0.3 ACH for tight constructions.
• Area method: this method considers the wind speed, shielding, and stack effect per window and door areas. Infiltration calculations are based on ASHRAE/IES standard 90.1 which establishes an air leakage 0.2 CFM/ ft² for most gross window product area, 0.4 CFM/ ft² for non-swinging opaque doors. Refer to ASHRAE Handbook Fundamentals – section 15, point No. 7 Air Leakage, for more information.
• Pressurization requirements: this will be required to take into consideration when a building/room required to be under a pressure differential. Exfiltration is calculated based on negative pressure and corresponding velocities through door crack openings, refer to the Industrial Ventilation Handbook section 7, table 7-1 for negative pressure per air velocity. Engineering good practice recommend to use an air velocity of 380 fpm for an negative pressure of 0.025” w.g. along with the following crack areas to exfiltration calculations:
  • 3/8” x 1ft/12” x bottom crack (ft)
  • 1/8” x 1ft/12” x side crack (ft)
  • 1/8” x 1ft/12” x top crack (ft)
  • 1/4” x 1ft/12” x in between crack (double doors) (ft)
• Infiltration through roll up door when opening and closing frequently (Ex. Dealership car maintenance roll up doors): based on volume of infiltration and estimated amount of time for which the door is opened each time.

Where:

• Volume of infiltration in CFM to be taking into consideration for the ventilation system at the heating and cooling load calculations.
• Air Velocity: Engineering good practice recommend using air velocity of 50 ft/min.
• Door opening area in ft².
• Working period: the duration for which a door is opened each time in minutes, it will depend on the application.

3.5. Ventilation Loads

Ventilation load based on:

• Outside air requirement: calculation is based on the type of the building application, total or space area, and the total occupancy load. Refer to ASHRAE 62.1 Ventilation for Acceptable Indoor Air Quality, Table 6.1 for detail information.
• Outside air requirement based pressurization requirements will need to be considered when applicable.

3.6. Results

The following values will be used to determine the size of the HVAC system required to maintain the indoor design conditions expected for the application during summer and winter.

• Total Sensible and Latent Cooling
• Grand Total of Cooling
• Subtotal of Heating
• Safety/Pickup Factor of 15% is usually applied
• Grand Total of Heating

Important note: It is to consider that the equipment size should be able to meet both total sensible and latent cooling loads and total of heating including the 15% pick up factor.

1. Dry Bulb: temperature of air indicated by a regular thermometer.
2. Wet Bulb: temperature measure by a thermometer that has a bulb wrapped in wet cloth. The evaporation of water from the thermometer has a cooling effect, so the temperature indicated by the wet bulb thermometer is less than the temperature indicated by a dry bulb.
3. Sensible heat: is the heat energy that, when added to or removed from a substance, results in measurable change in dry-bulb temperature. Sensible heat load is total of:
   • Heat transmitted through floors, ceiling, walls;
   • Occupant’s body heat;
   • Appliance and light heat;
   • Solar heat gain through glass;
   • Infiltration of outside air;
   • Air introduced by ventilation.
4. Latent heat: changes in the latent heat content of a substance are associated with the addition or removal of moisture. Latent heat can also be defined as the “hidden” heat energy that is absorbed or released when the phase of a substance is changed. For example, when water is converted to steam, or the steam is converted to water. Latent heat load is total of:
   • Moisture-laden outside air from infiltration and ventilation.
   • Occupant respiration and activities.
   • Moisture from equipment and appliances.
5. Daily Range (DR): mean daily range of the dry bulb temperature, which is the mean of the temperature difference between daily maximum and minimum temperatures for the warmest month (highest average dry-bulb temperature). These are used to correct CLTD values.
6. Peak Load: the energy analysis calculations compare the total energy use in certain period with various alternatives in order to determine the optimum one to size and select the HVAC system.