Cooling Loads

Cooling load represents the rate at which heat must be removed to maintain indoor design conditions during peak cooling conditions.

Calculation Method

HVAKR uses the Radiant Time Series (RTS) method for cooling load calculations, as recommended by ASHRAE. This method:

• Accounts for thermal storage in building mass
• Calculates a 24-hour load profile for every month of the year, not one design day — 288 month-hour points per scope
• Identifies the peak cooling month and hour from those 288 points
• Separates sensible and latent components

Internal gains are the exception to the monthly profile. People, lighting, equipment, and miscellaneous loads follow the space type’s usage schedule. HVAKR calculates one 24-hour profile for them and reuses it in every month. Envelope, ventilation, and infiltration loads change month by month with the weather profile.

Zero Clamping

HVAKR clamps each conduction and air-exchange term at zero, not each surface as a whole:

• A conduction term gives nothing when the outdoor or sol-air temperature is below the space setpoint
• A sensible air-exchange term gives nothing when the outdoor air is cooler than the setpoint
• A latent air-exchange term gives nothing when the outdoor air is drier than the space. It follows humidity ratio, not temperature

Solar gain through windows and skylights is a separate term. That clamp does not reach it. A window can carry solar gain in the same hour its conduction term is zero.

Internal gains are not clamped. A negative Misc. Cooling Sensible Load or Misc. Cooling Latent Load on the space reduces the load.

Load Components

Transmission Loads

Heat transfer through the building envelope:

Walls

• Sol-air temperature against the space cooling setpoint, so the outdoor-to-indoor difference already carries the solar absorption
• Wall U-value and net area, with window and door area deducted
• Solar absorption comes from the wall type’s Surface Absorptance, not from its Color — Color is the display color used to draw the wall
• Thermal mass delay comes from the ASHRAE wall type’s conduction time series
• Turn on Separate wall load between ceiling and space when assigning a wall and part of the load reports as Wall to Plenum instead of Wall

Roof

• Similar to walls but with higher solar exposure, using the roof’s tilt and azimuth
• Solar absorption comes from the roof type’s Surface Absorptance. As on wall types, Color is display only
• Skylight area is deducted from the roof area, so it is not counted twice
• Significant for single-story buildings
• Turn on Apply roof load to ceiling only on the roof and the load reports as Roof to Plenum instead of Roof

Windows

• Conductive heat transfer (U-value)
• Separate from solar heat gain

Doors

• Sol-air temperature against the space cooling setpoint, the same treatment walls get
• Door U-value, area, and the door type’s Surface Absorptance
• Reported on its own Door line, separate from door infiltration

Skylights

• Windows in the roof — the same conduction and solar treatment, at the roof’s tilt
• Reported on their own Skylight line

Floors

• Conductive gain through the slab (perimeter F-factor, or Overall U-Value when the slab type carries a Summer External Temperature Override)
• Never a credit — floor load is clamped to zero, not negative

Solar Loads

Heat gain from solar radiation:

Window Solar Gain

• Direct solar radiation through glass
• Diffuse solar radiation
• Based on SHGC and window area
• Varies by orientation and time of day

Opaque Surface Solar

• Solar absorbed by walls and roof
• Delayed by thermal mass
• Included in sol-air temperature

Internal Loads

Heat generated within the space:

People

• Sensible heat from body temperature
• Latent heat from respiration and perspiration
• Based on activity level and occupancy

Lighting

• All electrical energy becomes heat
• Based on lighting power density
• Modified by schedule
• Lighting Load to Ceiling Space on the space type sends that percentage to the ceiling plenum, where it reports as Lighting to Plenum instead of Lighting

Equipment

• Heat from appliances and machinery
• Based on equipment power density
• Sensible only — for a latent load (for example, kitchen equipment), add it to the space’s Misc. Cooling Latent Load field instead

Miscellaneous

• Extra sensible or latent load not covered by people, lighting, or equipment
• Entered directly on the space as Misc. Cooling Sensible Load and Misc. Cooling Latent Load
• The one cooling term HVAKR does not clamp at zero. A negative value reduces the load, so enter one only when you intend that credit

Ventilation Load

Heat to condition outdoor air. Both streams use the space outdoor air requirement for the cooling condition, and both are clamped at zero:

Sensible

• Outdoor dry-bulb temperature against the space type’s Summer (Cooling) setpoint, not against a supply temperature
• Based on outdoor air volume

Latent

• Outdoor humidity ratio against the indoor humidity ratio, which HVAKR derives from the Summer (Cooling) setpoint and the space type’s Relative Humidity
• Significant in humid climates

Both report with the system totals as Ventilation Total, not inside the space sensible subtotal.

Infiltration Load

Uncontrolled air leakage. Like the heating side, HVAKR reports it as three streams:

• General — a rate you set on the space type, or on the space when it overrides the method, not a wind calculation
• Window — the rate set on the window type
• Door — the only wind-driven stream: stack effect combined with a fixed wind on door types that have no seals, then scaled by the door type’s open fraction and the space type’s usage schedule. The cooling path uses 7.5 mph, half of the 15 mph the heating path uses

Each stream carries a sensible and a latent component, so cooling reports six infiltration lines where heating reports three. See Heating Loads for the fields behind each stream.

Peak Load Determination

Monthly Hourly Analysis

HVAKR calculates loads for every hour of every month, then takes the largest of those 288 points as the peak:

• Solar gains shift through the day and through the year
• Internal loads follow schedules
• Peak may not occur in the hottest month, or at the hottest hour of that month, because solar gain and outdoor temperature do not peak together

The search runs separately for each scope, so a space, its zone, its system, and the project can each peak at a different month and hour.

Peak Month and Hour

HVAKR reports the peak as a month and an hour together — for example, July at 4pm. It appears as Cooling Peak Time on the scope information card and as Space Sensible Peak Time on the project and space cooling data cards.

Each scope carries two peaks. The with plenum peak drives the reported loads. The without plenum peak ignores the ceiling-plenum share. It drives door infiltration sampling and the airflow calculation.

Results Display

Open Reports > Loads and pick a scope in the sidebar tree. The Cooling Sums at Peak card holds the results for that scope. See Loads Summary Report for the whole report.

Space Sensible Components

A Total Space Sensible figure, then every space-level line grouped into three totals:

• Internal Total — People Sensible, Lighting, Equipment, Miscellaneous Sensible
• External Total — Wall, Window, Door, Roof, Skylight, Slab
• Infiltration Sensible Total — Door Infiltration Sensible, Window Infiltration Sensible, General Infiltration Sensible

Alongside it, a details table adds Occupancy, Ave. Lighting, and Ave. Equipment, then the scope’s envelope areas and its infiltration airflows. At space scope it also shows the Setpoint.

System Sensible and Latent Components

The lines the space sensible subtotal leaves out:

• Total Load — everything at the peak, in both minor and major power units
• Plenum Total — Lighting to Plenum, Wall to Plenum, Roof to Plenum
• People Latent and Miscellaneous Latent
• Infiltration Latent Total — Door, Window, and General Infiltration Latent
• Ventilation Total — Ventilation Sensible and Ventilation Latent. Sum of Spaces Ventilation appears alongside them as a separate airflow row

Checksums

One card per cooling equipment mode: Sum of Space Peaks Total Load, Sum of Space Peaks Sensible Load, Sum of Space Peaks Supply Airflow, Load Distribution, Load Density, Airflow Density, Airflow Load Ratio, Space SHR, OA Airflow Density, OA Airflow Per Person, and OA Fraction.

The Lighting, Equipment, and Occupancy Diversity factors on a system scale internal gains, so all three change cooling results. Zone scope carries no diversity.

Charts

Switch the pane to the chart view with the two-icon toggle at the top:

• Cooling Loads Profile — the hourly load curve for the peak month, stacked by component, with a 2D / 3D toggle. The 3D view shows all twelve monthly profiles at once
• Peak Coincidence — at zone and system scope only, a month-by-hour plot of where each child space or zone peaks, so you can see how far the peaks spread

Verification Tips

1. Check solar gains - Largest component for perimeter spaces
2. Verify internal loads - Match expected density
3. Review ventilation - Significant in humid climates
4. Compare to benchmarks - Typical 25-40 BTU/h per sf for offices