Heating and Cooling Load Calculations
Heat loss, solar and internal gains, ventilation load and the sensible and latent split, worked for a 100 square meter office.
Why calculate loads
The load is the heat that must be added or removed to hold the room at its set conditions. Equipment is chosen from it: an oversized chiller or furnace costs more, runs in short cycles, controls humidity poorly and wears out sooner; an undersized one fails on the hottest or coldest day. The calculation has two halves with different worst cases: heating (the coldest night, no sun, no internal gains) and cooling (a hot afternoon with the sun on the glass and the room full).
The heating load
It is the sum of the heat lost by conduction through each part of the envelope and by air leaving the building:
Q = Σ U A ΔT + 1.2 × V̇ × ΔT (V̇ in L/s gives W)
Ventilation and infiltration usually make up a third to a half of the load in a modern building. Add a pick-up allowance for warming the structure after a night setback. Ignore internal gains and sun (they may not be there when it is coldest). Better windows lower the load: replacing the 12 m² of U = 2.8 glazing with U = 1.4 cuts the glass loss at 26 K from 874 W to 437 W.
The cooling load: sources of heat
| Source | How to calculate it | Sensible or latent |
|---|---|---|
| Conduction through walls and roof | U A ΔTeq; the equivalent temperature difference includes sun and heat storage (the wall delays and smooths the peak) | sensible |
| Conduction through glass | U A (Tout − Tin) | sensible |
| Solar through glass | A × SHGC × solar irradiance (W/m², from the orientation and hour); shading and film lower SHGC | sensible |
| People | about 75 W sensible and 55 W latent each in an office; more with activity | both |
| Lights | installed W/m² × area (LED: 5 to 10 W/m²) | sensible |
| Equipment | actual measured use, not nameplate (a computer draws 30 to 100 W, not its 300 W rating) | sensible (latent for kitchens) |
| Ventilation air | 1.2 × flow × ΔT; 2 501 × ṁ × ΔW for the latent part | both |
| Infiltration | air changes per hour × volume, then as ventilation | both |
Heat does not all become cooling load at once: solar and lighting heat warms the surfaces first (radiant part), which then release it to the air later. Detailed methods (the ASHRAE heat balance and radiant time series methods, CIBSE admittance) account for this delay; the simple steady-state estimate used here gives a safe upper figure for a light building.
Worked example: the 100 m² office
Worked example: peak cooling load at 33 °C outdoors
Office of 10 m × 10 m × 3 m: 40 m² of sunlit wall (U = 0.321, ΔTeq = 12 K), 12 m² of glass (U = 2.8, SHGC 0.4, peak solar 550 W/m²), ten people, 10 W/m² of lighting, 8 W/m² of equipment. Outdoor design 33 °C and 40 percent; indoor 24 °C and 50 percent; outdoor air 10 × 2.5 + 0.3 × 100 = 55 L/s.
| Source | Calculation | Sensible (W) | Latent (W) |
|---|---|---|---|
| Wall conduction | 0.321 × 40 × 12 | 154 | — |
| Glass conduction | 2.8 × 12 × (33 − 24) | 302 | — |
| Solar through glass | 12 × 0.4 × 550 | 2 640 | — |
| People | 10 × 75 and 10 × 55 | 750 | 550 |
| Lighting | 10 W/m² × 100 m² | 1 000 | — |
| Equipment | 8 W/m² × 100 m² | 800 | — |
| Room load | 5 646 | 550 | |
| Ventilation air | 1.2 × 55 × 9 and ṁ ΔW hfg | 563 | 515 |
| Coil load | 6 209 | 1 065 |
- Total 7.27 kW (2.07 tons), which is 73 W/m². Solar gain through the glass alone is 36 percent of it.
- Room sensible heat ratio = 5 646 ÷ (5 646 + 550) = 0.91; with the ventilation air the coil SHR is 0.85.
- Supply air at 13 °C (11 K below the room): flow = 5 646 ÷ (1.2 × 1.006 × 11) = 425 L/s (901 cfm), about 4.3 L/s per m² of floor.
- External shading that drops the SHGC from 0.4 to 0.25 saves 990 W (14 percent of the total).
- Infiltration at 0.3 air changes an hour adds another 25 L/s and about 272 W sensible: keep the building pressurized and the envelope tight.
Sensible and latent: the heat ratio
Cooling equipment is chosen for both parts. The room SHR sets the supply air condition: a room SHR of 0.91 can be handled by a coil that dehumidifies only a little; a lecture hall or restaurant with an SHR of 0.7 needs a much colder coil, or a separate dehumidifier. If the coil only handles the sensible load the humidity creeps up; this is the commonest cause of a clammy, cold room in humid climates (see Psychrometric Processes).
Sizing and safety factors
The peak of every load never happens at once (the sun is on the west glass in the afternoon but the east glass in the morning), so a building’s block load is less than the sum of the room peaks: a diversity of 0.8 to 0.9 for people and equipment. A 5 to 15 percent margin is then added. For the office: 7.27 kW × 0.85 × 1.10 = 6.80 kW. Piling on a 30 or 50 percent “just in case” margin on top of the loads is the common way equipment ends up oversized (and poor at part load).
Rules of thumb and their limits
| Building | Cooling (W/m²) | Heating (W/m²) |
|---|---|---|
| Office (modern, good envelope) | 70 to 100 | 30 to 60 |
| Office (older, glazed) | 100 to 150 | 60 to 100 |
| Retail | 100 to 150 | 40 to 80 |
| School classroom | 90 to 130 | 50 to 90 |
| Dwelling (cold climate) | 30 to 70 | 50 to 100 |
Rules of thumb are good for a first estimate and for checking a calculation (the example office is 73 W/m²), but they hide glazing, orientation, occupancy and climate. Always do the calculation before selecting equipment.
Related
- Previous: Psychrometric Processes. Next: Ventilation and Indoor Air Quality.
- Heat Transfer for U-values and radiation.
- Psychrometric Calculator.
- HVAC and Psychrometrics: the whole guide.
FAQ
What is SHGC?
Solar heat gain coefficient: the fraction of the incident solar energy that enters through the glass as heat. Clear double glazing is about 0.7, low-e glass 0.25 to 0.4.
Do I include the sun in the heating load?
No. The design condition is a cold night; the sun may be gone.
Why is the cooling load not just U A ΔT?
Because the sun and the stored heat in the structure add much more than the air-to-air difference.
What is a block load?
The peak of the whole building at once, which is smaller than the sum of the room peaks because they peak at different hours.
References
- ASHRAE, Handbook – Fundamentals, Chapters 17 and 18 (nonresidential and residential heating and cooling load calculations); CIBSE Guide A and Guide J (weather and solar data).
- EN 12831 (heating load) and ISO 52016 (energy needs for heating and cooling).
- Spitler, J. D., Load Calculation Applications Manual, ASHRAE.
Last reviewed: October 2026. Design values are examples; use local weather data and the applicable code.