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Evgenii Konkin
Evgenii Konkin

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Quick Tip: The Four Heat Sources in a Cooling Load Calculation

#hvac #engineering #quicktip #cooling

Cooling load is not just heat through the walls. It comes from four sources:

Total Cooling Load = Envelope + Occupants + Equipment + Lighting
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Source 1: Envelope

Heat transfer through walls, roof, windows, floor.

Envelope load = Area × ΔT × thermal factor × ceiling adjustment
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For a 40 m² office, 9 ft ceilings, 20°F indoor-outdoor difference:

Envelope = ~2,640 W
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This is what most people calculate. It is also the only one that changes with weather.

Source 2: Occupants 

Sensible heat per person:
  Seated office work:    120 W
  Light physical work:   150–200 W
  Exercise / heavy work: 300+ W
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For 4 office occupants:

Occupant load = 4 × 120 = 480 W
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Small in an office. Dominant in a lecture hall (200 people = 24,000 W).

Source 3: Equipment

Every watt of electricity becomes a watt of heat.

Typical office workstation:  150–250 W
Laser printer (printing):    300–600 W
Commercial kitchen:          5,000–15,000 W
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Common mistake: using nameplate ratings instead of actual draw.

Computer PSU nameplate:  500 W
Computer actual draw:    150 W
Overestimate:            3.3×
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Source 4: Lighting 

All lighting power becomes heat.
LED office: 10–15 W/m² (1.0–1.5 W/ft²)

40 m² office: ~200 W
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Total 

Source        Load (W)    Share
───────────────────────────────
Envelope      2,640       69%
Occupants       480       13%
Equipment       500       13%
Lighting        200        5%
───────────────────────────────
Total         3,820      100%
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Equipment + lighting = 18% of total. Skip them and the system is 18% undersized.

When internal gains dominate 

Office:           envelope 60–70%, internals 30–40%
Server room:      envelope 10–20%, equipment 80–90%
Commercial kitchen: envelope 20–30%, equipment 50–70%
Lecture hall:     envelope 40–50%, occupants 40–50%
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The calculation method is the same. The dominant source changes with the space type.

For quick cooling load estimates with all four components, use the cooling load calculator on CalcEngineer.

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