HVAC Heat Load Calculator
Estimate the heat that flows through a building's envelope – walls, windows, doors, roof and floor – at your design temperatures. Use quick estimation from floor area, or enter actual surfaces for a detailed component-by-component calculation, in metric or imperial units.
- Quick and detailed modes
- Metric and imperial units
- Formulas and assumptions shown
- Printable PDF report
Results appear here
Enter the building and temperature details, then select Calculate. You will get the total transmission load in W, kW, BTU/hr and TR, a component-wise breakdown with a chart, the formulas applied, and a printable PDF report.
The basics
What is HVAC heat load?
Heat load is the rate at which heat must be removed from (cooling) or added to (heating) a space to hold the indoor design temperature. It is the starting point for sizing air-conditioning and heating systems.
A complete load has several parts: heat conducted through the building envelope, solar radiation through glazing and onto opaque surfaces, heat from people, lighting and equipment, outdoor air from infiltration and ventilation, and moisture (latent) load. This calculator works out the first of these – envelope transmission – which is often the largest single part in poorly insulated buildings and the one most directly controlled by the choice of walls, roof and windows.
How does heat load calculation work?
Heat always flows from warm to cold. When it is hotter outside than inside, heat conducts in through every exterior surface; when it is colder outside, heat leaks out. The rate depends on three things: how easily the construction lets heat through (its U-value), how large the surface is, and how big the temperature difference across it is.
Multiplying these for each surface and adding the results gives the steady-state transmission load. Openings are treated as their own surfaces – a window has a much higher U-value than the wall around it – so their area is taken out of the wall before the wall is calculated.
Understanding U-value
The U-value (thermal transmittance) is the heat flow through one square metre of a construction for each degree of temperature difference across it, in W/m²·K (BTU/hr·ft²·°F in imperial units). Lower is better: a lower U-value means less heat gained in summer and less lost in winter.
It is the reciprocal of the total thermal resistance: U = 1 / (Rsi + Σ thickness / conductivity + Rse), where Rsi and Rse are the inside and outside surface resistances. Adding insulation, which has a very low conductivity, raises the resistance sharply – 50 mm of XPS on a bare RCC roof slab can cut its U-value from about 3.5 to about 0.6 W/m²·K.
| 230 mm brick wall, plastered | ≈ 2.05 W/m²·K |
|---|---|
| 200 mm AAC block wall, plastered | ≈ 0.69 W/m²·K |
| 150 mm RCC roof slab, uninsulated | ≈ 3.5 W/m²·K |
| RCC slab + 50 mm XPS | ≈ 0.56 W/m²·K |
| Single clear glass, aluminium frame | ≈ 5.8 W/m²·K |
| Double glazed low-E, uPVC frame | ≈ 1.7 W/m²·K |
Indicative values for typical constructions; use your specification or manufacturer data for design.
Formulas
Calculation methodology
- Total transmission load
- Q = Σ (U × A × ΔT)
- Each component
- Q_wall = U_wall × A_wall,net × ΔT, Q_window = U_window × A_window × ΔT, Q_door = U_door × A_door × ΔT, Q_roof = U_roof × A_roof × ΔT, Q_floor = U_floor × A_floor × ΔT
- Net wall area
- A_wall,net = A_wall,gross − Σ A_openings in that wall
- Temperature difference
- ΔT = |T_indoor − T_outdoor|
- Effective average U-value
- U_eff = Q_total / (A_envelope × ΔT)
| Symbol | Meaning | Unit |
|---|---|---|
| Q | Heat transfer rate | W (BTU/hr) |
| U | Overall heat transfer coefficient of the construction | W/m²·K (BTU/hr·ft²·°F) |
| A | Surface area, net of any openings | m² (ft²) |
| ΔT | Indoor–outdoor design temperature difference | K (°F) |
Quick estimation assumptions
- Quick estimation builds the envelope from the floor area: a square plan of side √(A ÷ floors), walls of the ceiling height on the exposed sides, and the roof equal to the footprint where the configuration has one.
- Room: 1 exterior wall. Corner room: 2 walls. Top-floor corner room: 2 walls + roof. One-story building: 4 walls + roof. Two- and multi-story: 4 walls on every floor + roof, with the floor area split equally between floors.
- Glazing is entered as a percentage of floor area (15 % is a common reference value) and spread evenly over the whole perimeter; only the exposed walls' share is counted, and it is taken out of the wall area.
- The ground floor and walls, floors or ceilings next to other conditioned rooms are assumed to have no heat flow.
Detailed engineering mode
Detailed mode uses the areas and U-values you enter. Windows and doors are placed in a wall or roof and deducted from its gross area; an optional floor (for floors over open or unconditioned space) uses the same ΔT.
Conversions: 1 W = 3.412142 BTU/hr; 1 kW = 3,412 BTU/hr; 1 m² = 10.7639 ft²; 1 W/m²·K = 0.17611 BTU/hr·ft²·°F; 1 TR = 12,000 BTU/hr = 3.51685 kW. A temperature difference of 1 K equals 1.8 °F.
Worked example
Example calculation
A single-story office of 150 m² in Coimbatore, cooled to 24 °C against an outdoor design temperature of 38 °C (ΔT = 14 K). Walls are 230 mm plastered brick (U = 2.05), the RCC roof has 50 mm XPS insulation (U = 0.56), windows are single clear glass in aluminium frames (U = 5.8), and glazing is 15 % of the floor area. Ceiling height is 2.7 m.
1. Envelope areas
- Footprint side
- √150 = 12.247 m
- Gross wall area (4 sides)
- 4 × 12.247 × 2.7 = 132.27 m²
- Window area
- 15 % × 150 m² = 22.50 m² (window-to-wall ratio 17.0 %)
- Net (opaque) wall area
- 132.27 − 22.50 = 109.77 m²
- Roof area
- 150.00 m²
2. Heat gain per component (ΔT = 14 K)
- Q_wall = 2.05 × 109.77 × 14 = 3,150 W
- Q_window = 5.80 × 22.50 × 14 = 1,827 W
- Q_roof = 0.56 × 150.00 × 14 = 1,176 W
3. Total
Q_total = 6,153 W = 6.15 kW = 20,997 BTU/hr ≈ 1.75 TR (reference). Envelope area 282.27 m², effective U-value 1.557 W/m²·K.
Shares of the load: walls 51 %, windows 30 %, roof / ceiling 19 %. The largest share (walls) is usually where an envelope improvement saves the most. Enter the same values in the calculator above to reproduce this result.
Applications
Use it to compare envelope options and get an early, transparent view of transmission load for:
- Residential buildings
- Commercial buildings
- Offices
- Hospitals
- Factories
- Warehouses
- Industrial facilities
Limitations
This calculator estimates envelope transmission load only. It is not a complete HVAC design load and does not include:
- Solar radiation and solar heat gain through glazing and on opaque surfaces
- Occupancy heat gain (sensible and latent)
- Lighting and electrical equipment
- Air infiltration
- Mechanical ventilation (outdoor air)
- Duct heat gains and losses
- Latent (moisture) cooling load
- Detailed ground heat transfer
It does not replace ACCA Manual J, the ASHRAE Heat Balance or Radiant Time Series methods, or a load calculation to the code that applies to your project. Do not select air-conditioning equipment capacity from transmission load alone.
FAQ
Frequently asked questions
Can I size an air conditioner from this result?
No. The result is envelope transmission only. Cooling equipment must also handle solar gain, people, lighting, equipment, outdoor air and latent load, which together are often larger than transmission. Use the result to compare envelope options, then carry out a full load calculation before selecting equipment.
Why is the TR value marked as reference only?
The refrigeration-ton figure is a straight unit conversion of the transmission load (1 TR = 12,000 BTU/hr = 3.517 kW). It is shown to give a sense of scale, not as a recommended equipment capacity.
What outdoor temperature should I use?
Use a published design condition for the location – for example ISHRAE climatic data in India or ASHRAE design conditions elsewhere – rather than a monthly average or a record extreme. For cooling, the 0.4 % or 1 % dry-bulb value is typical; for heating, the 99 % or 99.6 % value.
Are the U-value presets Indian code values?
No. The 'typical construction' values are indicative figures for common Indian constructions worked out from typical material properties. The 2021 IECC values are maximums from a US residential code and are listed only for reference. For compliance, use ECBC 2017 (commercial) or Eco Niwas Samhita (residential) and your actual specification.
How are windows and doors handled so they are not counted twice?
In detailed mode you enter gross wall and roof areas and place each window or door in the surface it sits in; its area is deducted from that surface. In quick mode the glazing area is taken out of the exposed wall area before the wall load is calculated.
Does switching between metric and imperial change my inputs?
Your values are converted, not lost: each field keeps its value in SI internally and is shown in the selected units, so switching back returns the numbers you typed.
Why is the ground floor not included in quick estimation?
Heat flow to the ground depends on soil, slab edge insulation and perimeter length and needs a separate method (for example ISO 13370). Quick estimation leaves it out; in detailed mode you can add a floor over open or unconditioned space with its own U-value.
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