Commercial HVAC Load Calculator
Estimate cooling and heating load, ASHRAE 62.1 ventilation, and supply airflow for offices, retail, restaurants, classrooms, clinics, warehouses, and light industrial spaces.
Inputs
Open or private offices with desk-based work.
Results
Climate zone: Zone 4 — Mixed (DC, Nashville, Seattle). Recommended unit: 14-ton at 168,000 BTU/hr cooling capacity.
Visualization
Where the cooling load comes from
In a commercial building, envelope conduction is rarely the biggest load. Internal gains (lighting, plug loads, occupants) and outdoor air ventilation often dominate. Switch building types below to see the share each component contributes to total cooling load.
Live commercial load breakdown
Where the cooling load comes from
Commercial loads differ from residential because internal gains (people + equipment + lighting) and outdoor air for ventilation dominate. Switch building types above to see how the share of each component shifts. Defaults from office: occupancy 5 people / 1,000 sq ft, lighting 0.8 W/sq ft, equipment 1.5 W/sq ft.
Methodology
The commercial load formula
Commercial cooling load is the sum of envelope conduction, internal gains, and outdoor air conditioning. ASHRAE 62.1 ventilation airflow is computed first, then its cooling and heating contribution is added to the envelope and internal load.
Rp = CFM/person, Pz = occupants, Ra = CFM/sq ft, Az = floor area.
Apply the lighting + plug load W/sq ft from ASHRAE 90.1 tables.
Office activity. Restaurants and gyms use higher rates.
Plus a latent component (~3–4.5 × OA CFM in humid zones).
Whichever is larger — cooling-driven or ventilation minimum.
Reference
Default occupancy and ventilation by building type
Per ASHRAE 62.1 occupant categories and minimum ventilation rates. Real designs should pull values from the current edition of the standard.
| Building type | Occupants / 1,000 sq ft | Rp (CFM/person) | Ra (CFM/sq ft) |
|---|---|---|---|
| Office | 5 | 5 | 0.06 |
| Retail / sales floor | 15 | 7.5 | 0.12 |
| Restaurant (dining) | 70 | 7.5 | 0.18 |
| Classroom / school | 35 | 10 | 0.12 |
| Medical exam / clinic | 20 | 10 | 0.18 |
| Warehouse / storage | 1 | 10 | 0.06 |
| Light industrial | 5 | 10 | 0.18 |
Reference
Typical cooling load and tonnage by building type
| Building type | Sq ft | Cooling BTU/hr | Tons | Supply CFM |
|---|---|---|---|---|
| Office | 5,000 | 150,000–185,000 | 12.5–15 | 5,000–7,500 |
| Retail | 5,000 | 175,000–225,000 | 15–20 | 5,500–8,000 |
| Restaurant (dining) | 3,000 | 275,000–375,000 | 23–32 | 9,000–13,000 |
| Classroom (1) | 900 | 30,000–40,000 | 2.5–3.5 | 1,200–1,800 |
| Medical clinic | 4,000 | 140,000–180,000 | 12–15 | 5,000–7,000 |
| Warehouse | 10,000 | 120,000–160,000 | 10–13 | 4,000–6,000 |
| Light industrial | 8,000 | 300,000–400,000 | 25–33 | 10,000–14,000 |
Ranges reflect climate-zone variation (zones 2 hot, 5 mixed). Real values shift with envelope, glazing, occupancy, and equipment density.
Workflow
The commercial sizing sequence
- 1. Identify ASHRAE 62.1 occupant category and ventilation rate (Rp + Ra)
- 2. Calculate ventilation airflow (Vbz) for each zone
- 3. Calculate envelope load using climate-zone design temperatures
- 4. Add internal gains: lighting (per ASHRAE 90.1 LPD) + plug load + occupants
- 5. Add outdoor air load (sensible + latent) for the ventilation airflow
- 6. Compute total cooling and heating load per zone
- 7. Select equipment with appropriate diversity for multi-zone systems
- 8. Size supply air at the greater of cooling CFM and ventilation minimum
Pitfalls
Common commercial sizing mistakes
- Skipping ASHRAE 62.1 ventilation — leads to IAQ complaints and code violations
- Underestimating equipment / plug load — modern offices are 1.5–3 W/sq ft, labs much higher
- Sizing on cooling alone in cold climates — heating ventilation load can be the binding constraint
- Not separating kitchen / lab exhaust — needs Type 1 hood calc, not block load
- Ignoring zone diversity — sum-of-peaks oversizes the central plant for multi-zone systems
- Missing latent load in humid climates — restaurants can have 30%+ latent load
- Selecting one big RTU when several smaller ones with VAV serve diverse occupancy better
Commercial HVAC FAQ
Quick answers to common HVAC sizing questions.