HVAC Heat Load Calculation: Method, Formulas and a Worked Example for an Indian Office

An HVAC heat load calculation estimates how much heat enters a room, so you can size the cooling equipment in tonnes of refrigeration (TR) and the supply air in CFM or m³/s. It adds up heat from walls, roof, glass, people, lights, equipment and fresh air. 

One TR equals 12,000 BTU/h, or 3.517 kW. This guide walks through the inputs, the method and one full worked example for an Indian office. You'll end up with a TR number and an airflow number you can actually defend.

What is a heat load calculation?

A heat load calculation (also called a cooling load calculation) is the tally of every heat source that pushes a space above its comfort setpoint on a design day. The total tells you how much heat the air conditioning must remove.

It's simpler than it sounds. Imagine a room at 24°C on a 42°C afternoon. Heat leaks through the west wall, pours through the window, rises off every laptop, and walks in with every person and every litre of fresh air. You list each one, give it a number, and add them up.

Why MEP engineers calculate load before choosing equipment and ducts

Because the load drives everything after it: the AC tonnage, the supply airflow, the duct sizes, the coil selection and the plant size.

Get the load wrong and every later decision inherits the mistake. Oversize and the unit short-cycles. It hits the setpoint, switches off, and never runs long enough to pull moisture out, so the room feels cold and clammy. Undersize and it runs flat out on the hottest afternoon and still loses. Neither is a good first month at a new job.

Also read: How a centralised AC system works | ASHRAE standards MEP engineers use

Types of heat load: sensible and latent, external and internal

Heat load splits two ways. Sensible heat changes the air temperature. Latent heat is moisture, and removing it takes extra cooling. Separately, the heat is either external (it comes through the building) or internal (it's made inside).

In India, don't skip the latent column. Humid cities like Mumbai and Chennai live and die on it.

Want to run this inside a Revit model, not a spreadsheet? Kaarwan's Professional BIM Certification for MEP Engineers covers HVAC modelling and load analysis.

Inputs you need

Collect these before you touch a formula.

One thing people get wrong: use a statistically based design condition for the city, not the all-time record high. Don't reuse numbers from another city either. They differ more than you'd think.

Heat load calculation: step by step

1. Fix the indoor condition (24°C is common for offices) and the outdoor design condition for your city.
2. Work out areas: each wall, the roof, and the glass, separately by orientation.
3. Calculate conduction through the walls and roof.
4. Calculate glass heat gain, both conduction and solar.
5. Add the people load, sensible and latent.
6. Add lighting and equipment.
7. Calculate the fresh air load, sensible and latent.
8. Sum the sensible loads and the latent loads separately.
9. Add one documented safety margin (10% is common). One only.
10. Convert the total to TR, and the sensible part to airflow.

Formulas for each load

These are the standard forms, in SI with the imperial version alongside.

• Wall or roof conduction: Q = U × A × CLTD (W). U in W/m²K, A in m², CLTD in K.
• Glass conduction: Q = U × A × ΔT.
• Glass solar gain: Q = A × SHGC × solar intensity (W/m²).
• Lighting and equipment: watts are watts. In imperial, BTU/h = W × 3.412.
• Fresh air, sensible: Q = 1.23 × L/s × ΔT (W). Imperial: 1.08 × CFM × ΔT(°F).
• Fresh air, latent: Q ≈ 3.0 × L/s × Δg (W), with Δg in g/kg of moisture difference. Imperial: 0.68 × CFM × Δgrains.
• Fresh air quantity (ASHRAE 62.1, office): 2.5 L/s per person plus 0.3 L/s per m². In imperial that's 5 cfm per person plus 0.06 cfm per sq ft.
• Airflow from sensible load: CFM = Qs (BTU/h) ÷ (1.08 × ΔT in °F). In SI, m³/s ≈ Qs ÷ (1,200 × ΔT in K).
• Tonnage: TR = total kW ÷ 3.517, or total BTU/h ÷ 12,000.

The tabulated CLTD method works for hand calculation. The Radiant Time Series method and the full Heat Balance method are what software like HAP runs behind the scenes. For India, NBC 2016 Part 8 (Section 3) covers air conditioning design and minimum outdoor air, and designers often use it alongside ASHRAE 62.1. Check which values your project or green rating actually requires.

Worked example: a 100 m² top-floor office

Let's do the sums for a plain open-plan office. These are illustrative round numbers, not a published design condition for any one city. Swap in your own city's ISHRAE or ASHRAE values for a real project.

Inputs

Load build-up

Grand total: 10,383 + 2,648 = 13,031 W, or about 13.0 kW.

Notice that fresh air alone is about a quarter of the total (3,317 W of 13,031 W). That's the line item students most often forget. The roof is the second surprise. Move this same office to a middle floor and the load drops noticeably.

Converting the load to TR and CFM

TR. 13.03 kW ÷ 3.517 = 3.7 TR. Add a 10% margin and the design load becomes 14.33 kW, about 4.1 TR. Then round up once, to the next size your supplier actually sells. Skip two sizes and you've brought short-cycling back.

Quick sanity check: 1,076 sq ft ÷ 4.1 TR ≈ 264 sq ft per TR. Rule-of-thumb figures for Indian offices vary widely by source, and this lands at the roomier end because only one wall is glazed. A glass-heavy west-facing floor would push the number down. That's exactly why the rule of thumb is a sanity check, never the answer.

CFM. Take the design sensible load, 10,383 W × 1.1 ≈ 11,421 W. Say the supply air leaves the coil at 13°C into a 24°C room, so ΔT = 11 K (19.8°F). In imperial: 11,421 W × 3.412 = 38,970 BTU/h, then 38,970 ÷ (1.08 × 19.8) ≈ 1,820 CFM. In SI, 11,421 ÷ (1,200 × 11) ≈ 0.87 m³/s. Same answer, two unit systems.

That airflow is what sizes your diffusers and ducts next.

Also read: MEP drawings in construction | HVAC workflow in Revit

Tools: Excel, HAP, Revit heating and cooling load analysis

Excel is fine for learning. Build the table above once and you'll understand every row. For real projects, most consultants run software built on ASHRAE methods, such as Carrier HAP or Trane TRACE, which calculate hour by hour across design days.

Revit adds a different angle. Its heating and cooling load analysis reads loads from your spaces and zones in the BIM model, so a change in glazing or layout updates the load without redoing it by hand. Garbage in still means garbage out, though. The model has to carry correct spaces, envelope data and schedules.

If you want to learn this workflow properly, Kaarwan's Professional BIM Certification for MEP Engineers covers it in the HVAC Modelling and Analysis and Auditing modules. (State the Revit version used when you publish any screenshot.)

Stuck between theory and site work? Kaarwan's MEP BIM certification takes you from heat load basics to a modelled HVAC system, week by week.

Common mistakes

• Oversizing. Stacking a safety factor in the load, again in equipment selection, and again in the ducts. Use one.
• Missing fresh air. It's roughly a quarter of the load in our example, and far more in humid cities.
• Sizing on sensible heat only. The room reads 24°C and still feels sticky.
• Wrong design temperature. Using a record high, an average, or another city's data.
• Treating the calculation as one-and-done. Glazing, headcount and equipment change between concept and construction drawings. Re-run it.

FAQs

What is the formula for heat load calculation?

There isn't one formula, but a set of them added together. Walls, roof and glass use Q = U × A × CLTD. People, lights and equipment use their watts. Fresh air uses 1.23 × L/s × ΔT for sensible heat. Sum everything, then divide the total kW by 3.517 to get TR.

How do you convert heat load to TR?

Divide the total in kW by 3.517, or the total in BTU/h by 12,000. For example, 13.03 kW ÷ 3.517 ≈ 3.7 TR, before margin.

What is the difference between sensible and latent heat?

Sensible heat changes the air temperature. Latent heat is moisture, from people breathing and from humid outdoor air. AC equipment must be sized for both, or the room feels cold and clammy even when the thermostat is satisfied.

How is fresh air load calculated?

First find the airflow, which for offices under ASHRAE 62.1 is 2.5 L/s per person plus 0.3 L/s per m². Then multiply by the temperature difference for the sensible part (1.23 × L/s × ΔT) and by the moisture difference for the latent part (3.0 × L/s × Δg).