Load Calculations Overview

HVAKR calculates heating and cooling loads using industry-standard methods based on ANSI/ASHRAE/ACCA Standard 183. These calculations determine the required capacity of your HVAC systems.

Calculation Methods

Cooling Load Calculation

HVAKR uses the Radiant Time Series (RTS) method for cooling loads:

• Accounts for thermal mass effects
• Calculates hourly loads through the design day of every month, not one design day
• Identifies the peak cooling month and hour
• Separates sensible and latent components

Heating Load Calculation

Heating loads use a steady-state calculation:

• Based on design heating temperature
• No solar credit (conservative)
• Includes transmission, infiltration, and ventilation

Ventilation Calculation

Outdoor air requirements per ASHRAE 62.1:

• People-based ventilation
• Area-based ventilation
• Zone air distribution effectiveness (Ez) — applied on the space’s VRP path and inside IAQP and CO2 Floor. A space set to Custom uses the airflow you type, so Ez never reaches it
• System ventilation efficiency (Ev) — applied only when a central unit uses the Multi-Zone outside-air method

See ASHRAE 62.1 for the equations behind each outside-air method.

Load Components

Transmission Loads

Heat transfer through the building envelope:

• Walls, roof, and floor
• Windows and doors
• Skylights

Solar Loads

Heat gain from solar radiation:

• Through windows (direct and diffuse)
• Absorbed by walls and roof
• Time-varying throughout the day

Internal Loads

Heat generated inside the building:

• People (sensible and latent)
• Equipment and appliances
• Lighting

Ventilation Loads

Conditioning of outdoor air:

• Sensible load (temperature difference)
• Latent load (humidity difference)
• Required for indoor air quality

Infiltration Loads

Uncontrolled air leakage. HVAKR reports it as three streams:

• General — a rate you set, not a wind calculation. It comes from the space type unless the space overrides the method itself: per wall area, per perimeter length, air changes, or a manual flow
• Window — the rate set on the window type
• Door — the only wind-driven stream: stack effect combined with a fixed wind speed on door types that have no seals

See Heating Loads for the fields behind each stream.

Design Conditions

Outdoor Conditions

Based on the ASHRAE or CIBSE weather data set you select, using the design percentile you choose on the Weather page:

• Cooling: 0.4%, 2%, 5%, or 10% design temperature — 0.4% by default
• Heating: 99% or 99.6% design temperature — 99.6% by default
• Humidity conditions

Indoor Conditions

Indoor setpoints come from the space type, not from the space or the zone. Set Summer (Cooling), Winter (Heating), and Relative Humidity under Indoor Design Conditions on the Space Types page. Typical values are 75°F and 50% RH for cooling and 70°F for heating.

HVAKR ships no default for these three fields. In the load calculation a missing setpoint zeros the envelope, ventilation, and infiltration loads for that condition — see Missing or Zero Loads. The Energy report is the exception: it falls back to 75°F and 70°F.

Calculation Hierarchy

Loads are calculated at multiple levels:

1. Space Level - Individual room loads
2. Zone Level - Sum of spaces in the zone
3. System Level - Sum of zones. Lighting and equipment diversity scale internal gains, so they reach cooling only. Occupancy diversity also scales the system’s outdoor airflow, so it reaches heating as well
4. Project Level - Total building load

When Loads Update

Calculations update automatically when you change:

• Weather or location
• Building envelope properties
• Space types or assignments
• System configurations
• Space geometry

Viewing Results

View load calculation results in the Reports section:

• Loads Summary report
• Scope drill-down — project, system, zone, or space, one at a time
• Load component charts
• Peak load identification