TAHA Engineering Toolsby TAHA Knowledge Lab

HVAC & Building Services

AC Cooling Load Estimator

Preliminary room cooling load from its separate heat gains — sensible and latent — in kW, Btu/h and tons, with a suggested AC size.

Inputs

Section through a room. The sun at the top left shines on the roof and through a window in the left wall. Inside are a person, a ceiling light and a computer. Outside air enters through the left wall. An air conditioner on the right wall removes the sensible and latent heat.sunroof: U × A × ΔTglass:U and SHGCoutside airlightspeopleequipmentAC unitremovesQs + QL
The cooling load is the sum of separate heat gains: conduction through walls, roof and glass, sun through the glass, people, lights, equipment and outside air. The air conditioner removes the sensible part (temperature) and the latent part (moisture). Diagram not to scale.

Use the summer design dry-bulb temperature for your location from climatic design data (for example ASHRAE or local data), not a record maximum.

For example 24 °C at 50% RH ≈ 9.3 g/kg. Use the Psychrometric calculator to convert from relative humidity.

At the same design condition, from climatic design data or the Psychrometric calculator.

Outside walls only, after subtracting windows and doors. Leave the three wall boxes empty to skip walls.

From the U-Value calculator or data for your construction.

Leave empty to use outdoor − indoor. For sunlit walls enter an equivalent temperature difference (CLTD or ETD from a load-calculation table), which includes solar heating of the wall.

Only a roof or ceiling exposed to outdoors or to a hot attic. Leave the three roof boxes empty to skip it.

Leave empty to use outdoor − indoor. A sunlit roof gets much hotter than the air: enter an equivalent temperature difference (CLTD or ETD) for a realistic result.

Leave the four window boxes empty to skip windows.

For conduction through the glass and frame, e.g. from the window data sheet.

Fraction of the solar radiation on the window that enters as heat, 0 to 1, from the window data sheet. Reduce it for internal blinds or external shading.

Peak solar irradiance falling on the window for its orientation and the design month, from solar or climatic data. Use 0 for a window that never receives sun.

The usual number of occupants at the time of peak load. Leave empty for none.

About 75 W for moderately active office work (ASHRAE table of heat gain from occupants). Use the table value for your activity.

About 55 W for moderately active office work. Higher for heavy activity.

Total electrical power of the lights that are on at the time of peak load, including drivers or ballasts.

Heat actually given off by computers, appliances and machines in use. Name-plate ratings are usually much higher than the real heat; use measured or typical values.

Only for equipment that releases moisture, such as cooking, kettles or open water. Leave empty otherwise.

All outdoor air entering the room and cooled by this unit: fresh-air supply plus leakage. Enter it once here only. Leave empty if outdoor air is cooled by a separate system.

Added to the total load for uncertainty and future changes. Many designers use about 10%; a large margin gives an oversized unit that removes moisture poorly.

Result

Sensible load
2.98621
Latent load
0.784226
Total cooling load
3.77043
Total cooling load in Btu/h
12,865.3BTU/h
Total cooling load in tons
1.0721TR
Sensible heat ratio (SHR)
0.792
Design load with safety margin
4.14748
Suggested nominal size (rounded up to the next 0.5 TR)
1.5
Breakdown
Sensible gains: Walls (U × A × ΔT = 0.6 × 12 × 16) = 0.1152 kW Roof (U × A × ΔT = 0.45 × 20 × 16) = 0.144 kW Windows, conduction (U × A × ΔT = 2.8 × 3 × 16) = 0.1344 kW Windows, solar (A × SHGC × I = 3 × 0.6 × 400) = 0.72 kW People, sensible (4 × 75 W) = 0.3 kW Lighting = 0.2 kW Equipment, sensible = 0.6 kW Outside air, sensible (ρ × V̇ × cp × ΔT = 1.2 kg/m³ × 0.04 m³/s × 1006 J/(kg·K) × 16 K) = 0.7726 kW Sensible total = 2.986 kW Latent gains: People, latent (4 × 55 W) = 0.22 kW Outside air, latent (ρ × V̇ × h_fg × ΔW = 1.2 kg/m³ × 0.04 m³/s × 2 501 000 J/kg × 0.0047 kg/kg) = 0.5642 kW Latent total = 0.7842 kW Total = 2.986 kW + 0.7842 kW = 3.77 kW = 1.072 TR With 10% margin = 4.147 kW = 1.179 TR → next 0.5 TR step: 1.5 TR

The estimated peak cooling load is 3.77043 kW (1.0721 TR). With the safety margin, a unit of about 1.5 TR nominal capacity is a starting point — check its actual capacity at your design conditions.

  • Preliminary estimate only. It is not a professional HVAC design or a substitute for a detailed room-by-room load calculation.
  • Walls: outdoor − indoor temperature difference used. It does not include solar heating; for sunlit surfaces enter an equivalent temperature difference.
  • Roof: outdoor − indoor temperature difference used. It does not include solar heating; for sunlit surfaces enter an equivalent temperature difference.

Educational estimate — check the assumptions below.

About this calculator

This estimator adds up the separate heat gains of a room at the time of peak load — heat conducted through walls, roof and windows, sun through the glass, people, lights, equipment and outside air — and splits them into sensible heat (which raises the temperature) and latent heat (moisture, which the coil must condense). The total, in kW, Btu/h and tons of refrigeration, gives a reasoned starting point for choosing an air conditioner.

It does not use a “watts per square metre” or “tons per square foot” rule. Such rules ignore orientation, insulation, glass, occupancy and ventilation, and can be wrong by a factor of two. Here you enter the actual areas, U-values, temperatures and loads of your room.

This is a preliminary estimate, not a professional HVAC design. It does not include heat storage in the building (the time delay of solar heat), hour-by-hour sun positions, duct gains or fan heat. For a final design, use a detailed room-by-room method such as the ASHRAE heat-balance or radiant time series method, or software based on it.

Formula

Walls, roof: Q = U × A × ΔT    Windows: Q = U × A × (to − ti) + A × SHGC × I

People: Qs = n × qs, QL = n × qL    Lights, equipment: Q = power in use

Outside air: Qs = ρ V̇ cp (to − ti)    QL = ρ V̇ hfg (Wo − Wi)

Total = ΣQs + ΣQL    Design load = Total × (1 + margin)    1 TR = 12 000 Btu/h = 3.517 kW

A = area, U = U-value, ΔT = temperature difference (outdoor − indoor, or an equivalent ΔT for sunlit walls and roofs), SHGC = solar heat gain coefficient of the window, I = solar radiation on the glass (W/m²), n = number of people, q = heat per person, V̇ = outside airflow, ρ = 1.2 kg/m³, cp = 1.006 kJ/(kg·K), hfg = 2501 kJ/kg, W = humidity ratio. The suggested nominal size is the design load rounded up to the next 0.5 TR.

Worked example

The default values describe an example office (replace them with your own data): indoor 24 °C and 9.3 g/kg, outdoor 40 °C and 14 g/kg, so to − ti = 16 K.

  • Walls 12 m² × 0.6 × 16 K = 0.115 kW; roof 20 m² × 0.45 × 16 K = 0.144 kW; window conduction 3 × 2.8 × 16 = 0.134 kW.
  • Window solar 3 m² × 0.6 × 400 W/m² = 0.720 kW — the largest single gain from the envelope.
  • 4 people: 0.300 kW sensible, 0.220 kW latent; lighting 0.200 kW; equipment 0.600 kW.
  • Outside air 40 L/s: sensible 1.2 × 0.040 × 1006 × 16 = 0.773 kW; latent 1.2 × 0.040 × 2 501 000 × 0.0047 = 0.564 kW.
  • Sensible 2.986 kW + latent 0.784 kW = 3.770 kW (12 865 Btu/h, 1.07 TR); SHR 0.79.
  • With 10% margin: 4.147 kW = 1.18 TR → suggested nominal size 1.5 TR.

Note that the walls and roof here use the air temperature difference only. If the roof is sunlit, its equivalent temperature difference may be two or three times larger, and the roof gain rises in proportion.

Assumptions and limitations

  • Preliminary estimate of the peak cooling load at one design condition. Not a professional HVAC design or a substitute for a detailed room-by-room load calculation.
  • All gains are added as if they peak at the same time, and heat storage in walls, floors and furniture is ignored. This usually gives a cautious (higher) peak, but very sunny or heavy-construction rooms can behave differently.
  • No double counting: window conduction (U-value) and window solar gain (SHGC) are separate effects and are each counted once. Outside air is entered once, as the total of ventilation and infiltration. Do not enter lighting or equipment both here and in another item.
  • Walls and roof use the temperature difference you enter. With the box empty, outdoor − indoor is used, which ignores solar heating of the surfaces; for sunlit surfaces enter an equivalent temperature difference from a recognized load-calculation table.
  • Air properties are fixed at ρ = 1.2 kg/m³, cp = 1.006 kJ/(kg·K) and h_fg = 2501 kJ/kg (standard air near sea level). At high altitude the outside-air load is smaller; use the Air Sensible & Latent Heat calculator for other values.
  • Duct heat gains, fan and pump heat, and heat from adjacent unconditioned rooms are not included.
  • Suggested size: the design load (with margin) rounded up to the next 0.5 TR. Nominal ratings are at standard test conditions; the real capacity at your outdoor temperature can be lower, so check the manufacturer’s data at your conditions.

References

  • ASHRAE Handbook — Fundamentals, chapters “Nonresidential Cooling and Heating Load Calculations” and “Residential Cooling and Heating Load Calculations” (heat gain from occupants, lighting and equipment; fenestration solar heat gain; heat balance and RTS methods).
  • ASHRAE Handbook — Fundamentals, chapter “Climatic Design Information” (outdoor design conditions).
  • ACCA Manual J — Residential load calculation (for homes in North America).