Heating Loads

Heating load represents the rate at which heat must be added to maintain indoor design conditions during peak heating conditions.

Calculation Method

HVAKR uses steady-state calculation for heating loads:

Based on the outdoor heating design temperature — 99% or 99.6%, 99.6% by default
No solar credit (conservative, assumes nighttime peak)
No credit for people, lighting, or equipment gains
Sensible only — HVAKR calculates no latent heating load
Every envelope and air-exchange component is clamped at zero. A surface whose outdoor side is warmer than the space setpoint contributes nothing; it is never a credit
Results in maximum required heating capacity

Load Components

Transmission Loads

Heat loss through the building envelope. Each surface except the slab uses U-value × area × (space setpoint − outdoor design temperature).

Walls

Indoor-to-outdoor temperature difference
Wall U-value and net area over the slab-to-slab height, with window and door area deducted
No solar effect (nighttime condition)

Roof

Similar to walls
Higher U-value increases loss
Skylight area is deducted from the roof area, so it is not counted twice

Windows and Skylights

Conductive heat loss
Higher U-value than walls
No solar credit
A skylight uses its window type’s U-value with the roof type’s temperature

Floors and Slabs

Slabs use the F-Factor perimeter method by default, for slab-on-grade and basement slabs alike
Usually smaller than other components

Doors

Conductive heat loss
Higher U-value than walls
Include all exterior doors

Winter External Temperature Override

Wall Types, Roof Types, and Slab Types each carry a Winter External Temperature Override — an advanced field in Basis of Design > Building Data. It replaces the outdoor design temperature for that construction, so you can model a wall against soil or a roof under an unconditioned attic:

Wall — the override also applies to the windows in that wall
Roof — the override also applies to the skylights in that roof
Slab — the override replaces the perimeter method with Overall U-Value × space area × temperature difference, and needs an Overall U-Value to calculate

See Building Data for the fields and Slab for the slab methods.

Infiltration Load

Heat to warm cold outdoor air leaking in. HVAKR reports it as three streams — General Infiltration, Window Infiltration, and Door Infiltration:

General infiltration comes from the rate you set, not from a wind calculation. A space type carries Wall Infiltration (per unit area of exterior wall), Perimeter Infiltration (per unit length of exterior perimeter), and Infiltration Air Changes (ACH). HVAKR uses whichever of them produces the largest airflow, unless the space overrides the method itself
Window infiltration comes from the window type, not the space type: Summer Area Infiltration (per unit area of window) and Summer Perimeter Infiltration (per unit length of window perimeter), again the larger of the two
Door infiltration is the wind-driven stream. HVAKR combines stack effect with a fixed 15 mph wind on door types that have no seals, then scales the result by the door type’s open fraction

The general and window streams each have their own winter switch, and both default to the summer rates:

General — turn on Use Separate Winter Reqs on the space type or the space to apply the winter rates to heating
Window — turn on the window type’s own Use Separate Winter Reqs to apply its Winter Area Infiltration and Winter Perimeter Infiltration. The space-level switch does not reach it

Space rates are on the Infiltration Requirements section of Space Types; window rates are on the window type in Basis of Design > Building Data. Both winter switches and their rates are advanced fields, hidden until you turn on advanced settings.

Ventilation Load

Heat to warm outdoor ventilation air:

Based on the space outdoor air requirement for the heating condition
Outdoor-to-indoor temperature difference
No latent component — HVAKR calculates no humidification load
Reported with the system totals as Ventilation Total, not inside the space sensible subtotal

Miscellaneous Load

The only internal term in a heating load. Misc. Heating Load on a space adds a fixed heat loss the envelope model does not cover. See Spaces.

This is the one heating term HVAKR does not clamp at zero. A negative value reduces the space heating load, so enter a negative number only when you intend that credit.

No Credits Applied

For conservative sizing, heating loads exclude:

No Solar Credit

Peak heating often occurs before sunrise
Cloudy conditions are possible
Results in reliable capacity

No Internal Gains Credit

People, lighting, and equipment gains are never subtracted
Building may be unoccupied
The only internal term is the space’s Misc. Heating Load, which you enter yourself

Design Temperature

Outdoor Design Temperature

Choose the Winter Design Percent on the Weather page — 99% or 99.6% (99.6% by default):

99.6% — temperature exceeded 99.6% of hours, about 35 hours per year below this value
99% — temperature exceeded 99% of hours, about 88 hours per year below this value
A higher percentile is more conservative for equipment sizing

Winter Design DB below it shows the temperature looked up for that percentile. Type over it to size for your own design temperature, then reset the field to return to the lookup value.

Indoor Design Temperature

The heating setpoint comes from the space type, not the space. Set Winter (Heating) under Indoor Design Conditions on the Space Types page. Typical values:

68-72°F (20-22°C) occupied
May vary by space type

A space type with no Winter (Heating) value produces a load error for every space assigned to it. HVAKR has no setback input — it sizes for the value you enter.

Peak Load Identification

When Heating Peaks

Heating loads peak when:

Outdoor temperature is at design minimum
No solar gain (night or cloudy)
Building is unoccupied (no internal gains)

There is one heating design point rather than an hourly profile, so the Loads report always shows the heating peak time as Winter Design.

Morning Warm-Up

HVAKR adds no warm-up allowance and no safety factor. A building that cooled down overnight needs extra capacity to reach setpoint before occupancy — add that margin yourself when you select equipment.

Results Display

Open Reports > Loads and pick a scope in the sidebar tree. The Heating 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 — Miscellaneous
External Total — Wall, Window, Door, Roof, Skylight, Slab
Infiltration Total — Door Infiltration, Window Infiltration, General Infiltration

System Sensible Components

Total Load — the space sensible total plus the ventilation load
Ventilation Total — the outdoor air heating load for the scope
Sum of Spaces Ventilation — the outdoor airflow the spaces in the scope require

The Lighting and Equipment diversity factors on a system scale internal gains, which heating loads do not include, so they change cooling results only. Occupancy Diversity also scales the outdoor airflow the system’s spaces require, so it moves both rows above at system and project scope. Zone scope carries no diversity. See Occupancy Diversity in the Rollup.

Checksums

Heating Load Density and Heating Airflow Density for the scope, one card per heating equipment mode.

Verification Tips

Check envelope areas - Verify wall and window areas
Review U-values - Compare to specifications
Consider infiltration - May be significant
Compare to benchmarks - Typical 15-25 BTU/h per sf for offices
Verify temperature difference - Design delta-T is correct