Quick cooling and heating load calculator
A quick estimate for early design, budgeting and HVAC exam practice. Space type and floor area give the sensible and latent loads, room SHF and supply airflow; Open in the psychrometric chart draws the room point, SHF line and outdoor point in the calculator.
Basis
Room loads are sums of per-item rates; the outdoor-air load is the same psychrometric calculation as the calculator's process card. Cooling counts heat to remove as positive, heating counts heat to supply as positive.
| Item | Equation | Notes · basis |
|---|---|---|
| People | qs = N·s, ql = N·l | s and l from the people table below (W per person). N = density × A / 100, or a head count |
| Lighting, equipment | q = L·A, q = E·A | All sensible, all at once (no diversity or storage). The rates are reference values |
| Envelope (simplified) | q = U·Ae·(t_o − t_r) + S·A | Conduction from the dry-bulb difference only; solar and other as one per-floor-area value. Heating uses U·Ae·(t_r − t_o) |
| Outdoor airflow | Q = (Rp·N + Ra·A)/Ez × 3.6 | ASHRAE 62.1 ventilation rate procedure. Ez from the 62.1 zone air distribution effectiveness table: 1.0 by default for cooling (ceiling supply of cool air), 0.8 for heating (ceiling supply of warm air, ceiling return). Or Q = n·A·H, or a direct value |
| Outdoor-air sensible | qs = ṁ·(1.006 + 1.86 W_o)·(t_o − t_r) | ṁ = Q / (3600·v_o) at the outdoor specific volume; the same definition as the process card (outdoor to room) |
| Outdoor-air latent | ql = ṁ·(h_o − h_r) − qs | Total minus sensible (as in the process card); moisture ṁ·(W_o − W_r) |
| Room SHF | SHF = Σqs / (Σqs + Σql) | Room loads only, without outdoor air; this draws the SHF line on the chart |
| Supply airflow (cooling) | ṁ_s = q_r / (h_r − h_s), Q_s = ṁ_s·v_s | t_s = t_r − ΔT; the supply state makes the supply-to-room SHF equal the room SHF (as in the calculator's cooling panel) |
| Supply airflow (heating) | ṁ_s = q_r / ((1.006 + 1.86 W_s)·ΔT) | t_s = t_r + ΔT, W_s = W_r (as in the calculator's heating panel) |
| Units | 1 RT = 3.517 kW, 1 kcal = 4.1868 kJ | US refrigeration ton, IT calorie |
Heat gain from occupants (ASHRAE Fundamentals chapter 18, Table 1)
Adjusted values for the normal mix of men, women and children at a 24 °C room (W per person, low air velocity). At a warmer room the same total shifts towards latent heat. The restaurant value includes heat from food. Editions differ slightly, so check against the edition you use.
| Activity | Typical application | Sensible W | Latent W |
|---|---|---|---|
| Seated at theater | Theater | 65 | 30 |
| Seated, very light work | Offices, hotels, apartments | 70 | 45 |
| Moderately active office work | Offices, hotels, apartments | 75 | 55 |
| Standing, light work; walking | Department store, retail store | 75 | 55 |
| Walking, standing | Drug store, bank | 75 | 70 |
| Sedentary work (restaurant) | Restaurant | 80 | 80 |
| Light bench work | Factory | 80 | 140 |
Outdoor air rates (ASHRAE 62.1 Table 6-1)
Per-person and per-area outdoor air and default occupant density from the ventilation rate procedure. Breathing-zone outdoor air Vbz = Rp·Pz + Ra·Az is divided by Ez from the 62.1 zone air distribution effectiveness table (1.0 for ceiling supply of cool air, 0.8 for ceiling supply of warm air with ceiling return; the calculator starts at 1.0 for cooling and 0.8 for heating, and either can be picked). Values change between editions, so check the one that applies. The Korean required ventilation rates for multi-use facilities are not included because the source text could not be checked here.
| Occupancy | Rp L/s·person | Ra L/s·m² | Density /100 m² | m³/h per person |
|---|---|---|---|---|
| Office space | 2.5 | 0.3 | 5 | 30.6 |
| Conference/meeting | 2.5 | 0.3 | 50 | 11.2 |
| Sales (retail) | 3.8 | 0.6 | 15 | 28.1 |
| Restaurant dining rooms | 3.8 | 0.9 | 70 | 18.3 |
| Classrooms (age 9 plus) | 5 | 0.6 | 35 | 24.2 |
| Lecture classroom | 3.8 | 0.3 | 65 | 15.3 |
Space-type starting values
Only values with a source in the basis column are sourced; values marked reference are generous starting values that could not be checked against a source. Enter your own.
| Space type | Occupants, heat | Outdoor air | Lighting, equipment W/m² |
|---|---|---|---|
| Office | 5 /100 m² (62.1), 75/55 W | Rp 2.5, Ra 0.3 (62.1) | 10 · 15 (reference) |
| Conference room | 50 /100 m² (62.1), 70/45 W | Rp 2.5, Ra 0.3 (62.1) | 10 · 5 (reference) |
| Retail sales | 15 /100 m² (62.1), 75/55 W | Rp 3.8, Ra 0.6 (62.1) | 15 · 5 (reference) |
| Restaurant dining | 70 /100 m² (62.1), 80/80 W | Rp 3.8, Ra 0.9 (62.1) | 12 · 10 (reference) |
| Classroom | 35 /100 m² (62.1), 70/45 W | Rp 5, Ra 0.6 (62.1) | 10 · 5 (reference) |
| Residential (apartment) | 4 people (reference), 70/45 W | 0.5 ACH (Korean rule, art. 11, 30+ units) | 6 · 5 (reference) |
| Server room | 2 people (reference), 70/45 W | 100 m³/h (reference) | 10 · 500 (reference) |
Limits of the quick method
This method adds up per-item rates, so it does not work out solar gain (orientation, hour, glazing shading), the time lag from wall and room storage, the peak hour, infiltration, duct and fan gains, reheat or safety factors. Use it for early sizing, budgeting and exam practice; before selecting equipment, run a detailed load calculation (HAP, TRACE and other programs using the ASHRAE RTS method).
Heating leaves out internal and solar gains and counts only envelope conduction and outdoor air, which errs on the safe side. Humidification covers outdoor air only; moisture released in the room (people and so on) is not counted.
Worked example
Office of 100 m², room 26 °C and 50 %, outdoor 32 °C and 60 %, density 5 per 100 m² gives 5 people, moderately active office work 75/55 W, lighting 10, equipment 15 and solar 20 W/m², U 1.0 W/m²K × 50 m², 62.1 outdoor air (2.5 × 5 + 0.3 × 100) × 3.6 = 153 m³/h, supply difference 10 K.
Room: people 0.375/0.275 kW, lighting 1.000, equipment 1.500, envelope 1.0 × 50 × 6 = 0.300, solar 2.000 kW, so sensible 5.175, latent 0.275, SHF = 5.175 / 5.450 = 0.950.
Outdoor air: W_o 18.033, W_r 10.496 g/kg, h_o 78.37, h_r 52.91 kJ/kg, v_o 0.8895 m³/kg, so ṁ = 153 / (3600 × 0.8895) = 0.04778 kg/s, sensible 0.04778 × 1.0395 × 6 = 0.298 kW, total 0.04778 × 25.45 = 1.216 kW, latent 0.918 kW, moisture 1.30 kg/h.
Total: sensible 5.473, latent 1.193, total 6.666 kW = 1.90 RT = 5,732 kcal/h. Supply 16 °C and 10.282 g/kg, ṁ = 5.450 / (52.91 − 42.12) × 3600 = 1,817 kg/h, so 1,513 m³/h (the ρ = 1.2, cp = 1.006 shortcut 5.175 kW / (1.2 × 1.006 × 10) gives 1,543 m³/h, 2 % apart).
The 2 % gap comes from the shortcut fixing ρ·cp at 1.2 kg/m³ and 1.006 kJ/kgK; this calculation uses the moist-air specific heat 1.006 + 1.86·W (1.025) and the supply specific volume (0.833 m³/kg per kg of dry air).
Sources
- ASHRAE Handbook, Fundamentals (SI), chapter 18, Nonresidential Cooling and Heating Load Calculations, Table 1: heat gain from occupants by activity
- ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality: Table 6-1 minimum ventilation rates, zone air distribution effectiveness (Ez) table
- Korean Rules on Building Equipment Standards, article 11: at least 0.5 air changes per hour in new apartment buildings of 30 or more units
- ASHRAE Handbook, Fundamentals (SI), chapter 1, Psychrometrics: moist-air equations (calculator engine)
Last updated 2026-10-07.