- Area-based estimates ignore orientation, glazing, occupancy and equipment.
- An oversized system costs more and controls humidity poorly.
- HAP calculates the load hour by hour and zone by zone across the year.
- The accuracy of the inputs determines the accuracy of the results.
"So many square metres per ton of cooling" is a common market rule. It may do for a quick early estimate, but not for designing an HVAC system. An accurate heat-load calculation is the basis for selecting equipment and sizing ducts and pipes, and any error in it carries into everything that follows.
Why quick estimates aren't enough
Two rooms of the same area can have very different loads: one with a west-facing glass façade, the other internal with no windows. Area-based estimates ignore:
- Façade orientation and solar gain through glazing.
- Wall and roof construction and insulation.
- Number of occupants and their activity.
- Lighting, appliances and equipment.
- Fresh air required for ventilation.
- Operating hours and the timing of peak load.
What happens when the size is wrong?
Oversized system
Higher equipment and electricity costs, frequent cycling that shortens compressor life, and poor dehumidification because the system reaches the set temperature quickly and switches off — leaving a space that feels cold but damp.
Undersized system
The set temperature isn't reached at peak hours, the system runs flat out, and occupants complain repeatedly.
How HAP works
Carrier's HAP (Hourly Analysis Program) is one of the most widely used tools for load calculation and energy analysis. Using climate data for the location, it calculates the load for each zone and each hour, identifying the true peak for each zone and for the whole building — which usually don't occur at the same hour.
The inputs that determine accuracy
- Climate data: for the project's actual city.
- Envelope: wall, roof and glazing build-ups and their heat-transfer values.
- Zoning: by orientation, use and operating hours.
- Internal loads: people, lighting and equipment with realistic schedules.
- Fresh air: by type of use and code requirements.
The software is precise, but its results are only as accurate as the data fed into it.
From loads to design
The results are the starting point: equipment capacities are selected, airflows are calculated for each zone, and ducts and pipes are designed. Annual energy analysis also helps compare alternatives, such as better insulation or different glazing, before deciding.
FAQ
Is a load calculation needed for villas and small buildings?
Very useful even on small projects — it prevents buying units that are too big or too small and distributes capacity correctly across rooms.
What's the difference between sensible and latent load?
Sensible load raises air temperature; latent load relates to moisture. Good design deals with both.
Can an old load calculation be reused after design changes?
No. Any change to façades, glazing, use or layout requires an updated calculation.
Engineers and specialists in government tenders, technical office, MEP and central HVAC, with more than 15 years in the Saudi market.