Your contractor says the restaurant needs 15 tons. The supplier quotes 12. Nobody has shown you a calculation, and the equipment order closes on Friday.
The bottom line first. You compute HVAC load by totalling every source of heat and moisture entering a space, then converting that total into tons of refrigeration. How to compute HVAC load correctly on a Philippine project turns on one variable that most online guides ignore, and that variable is moisture. In Metro Manila conditions, roughly two thirds of the fresh air load is moisture removal rather than temperature drop. Size the equipment on temperature alone and you will land near a third of the cooling capacity the building actually needs.
What follows is the method step by step, the numbers to expect by building type, why Manual J does not transfer here, and who has to seal the calculation before your mechanical permit moves. For the wider picture, our Mechanical and HVAC design page covers where load calculation sits in the full design sequence.
What Does a Cooling Load Computation Actually Cover?
A cooling load calculation totals the heat and moisture a space gains during a design day, then sizes equipment to remove it. It covers sensible heat, which raises air temperature, and latent heat, which raises air moisture content.
Both are measured in British thermal units. One British thermal unit, shortened to BTU, is the amount of heat required to raise one pound of water by one degree Fahrenheit, and one ton of refrigeration equals 12,000 BTU per hour. Philippine drawings and equipment schedules usually shorten tons of refrigeration to TR.
The heat sources split into two groups. Sensible heat arrives through glass as solar radiation, through walls and roof as conduction, and from lights, equipment, and the people inside. Latent heat comes from occupants, from cooking and dishwashing, and above all from the outdoor air you are required to introduce for ventilation.
Two points about the result. Your total cooling load is a peak value, not an average, because the system has to hold conditions on the worst hour of the worst month. And the peak for the whole building is almost always smaller than the sum of the room peaks, since not every space peaks at the same hour.
How to Compute HVAC Load, Step by Step
The sequence is fixed. Set your design conditions, take off the building fabric, count the internal gains, add the outdoor air, apply diversity, then convert the total to tonnage. Skipping straight to tonnage is where most projects go wrong.
Here is what each step involves in practice.
- Design conditions. Establish the outdoor design temperature and humidity for the site, and the indoor condition you are holding. Most Philippine commercial work targets around 24°C at 55 to 60 percent relative humidity, though hotel guest rooms and hospital areas have their own requirements.
- Building fabric. Take off glass area by orientation, then wall, roof, and slab areas with their construction types. Solar heat gain through west-facing glass on a Metro Manila afternoon is the heaviest fabric load on most buildings we work on, which is why the same room facing east and west calculates differently. Roof insulation is one of the few fabric decisions that moves the tonnage meaningfully here.
- Internal gains. Count people by activity level, lighting load in watts per square meter, and every piece of equipment that runs during occupied hours. Kitchen equipment schedules matter more than owners expect, and a coffee shop with three espresso machines and two undercounter chillers is not a small load.
- Outdoor air and infiltration. Determine the ventilation rate required for the occupancy, then calculate what it costs to cool and dry that air. Infiltration belongs here too, since air leaking around doors and poorly sealed glazing carries moisture straight into the space. This is the step that separates a real calculation from a guess in this climate.
- Diversity. Apply realistic occupancy and equipment usage patterns rather than assuming everything runs at once. On a school building this is significant, because classrooms, the canteen, and the library do not peak together.
- Conversion. Total the sensible and latent heat, add a defensible allowance for duct gain and fan heat, then divide by 12,000 to get your tonnage in TR.
The how to compute HVAC load question usually arrives at our office in the form of a spreadsheet somebody has already prepared. More often than not the fabric and internal gains are fine and the outdoor air line is either missing or badly understated.
Why the BTU Per Square Meter Rule of Thumb Misleads on Philippine Projects
A BTU per square meter figure is a sanity check, not a design. It works reasonably for a bedroom and fails badly for anything commercial.
Philippine residential practice commonly assumes roughly 500 to 600 BTU per hour for every square meter of floor area, which is higher than the figure used in temperate markets because our outdoor conditions are hotter and far more humid. That range is fine for choosing a 1.0 HP or 1.5 HP split-type air conditioner for a condo bedroom. It quietly assumes a modest ceiling height, ordinary window area, two or three occupants, and almost no fresh air requirement.
Now change one variable. Put twenty people in that same floor area and give the space the ventilation rate a dining room requires. The load can more than double while the square meters stay exactly the same. Floor area is a proxy for load, and proxies break when the thing they were standing in for changes.
We still run a per-square-meter check at the end of every cooling load calculation, because a number that lands far outside the expected range for that building type usually means an input error somewhere. Using the square footage method as the starting point, though, is how a project ends up with equipment that cannot hold humidity.
Is Manual J the Right Method for a Philippine Project?
No. Manual J is a residential load calculation developed by the Air Conditioning Contractors of America, or ACCA, to estimate the amount of heating and cooling a house needs and determine the heating and cooling capacity you need to install. It was built for North American homes.
Search this topic in English and almost every result points you toward Manual J load calculation software. For a house in Ohio that advice is sound. The method assumes a heating season, draws on US climate data, and works in square footage. None of those assumptions survive contact with a project in Marikina or Makati, where there is no heating season at all and the moisture load runs year round.
Philippine commercial design generally follows ASHRAE methods instead. The American Society of Heating, Refrigerating and Air-Conditioning Engineers publishes the heat balance and radiant time series procedures, both of which handle high latent heat properly, and its climatic design data covers Philippine stations. Those are the methods behind the numbers on our permit drawings.
There is a related trap with free tools. An online HVAC load calculator will accept your floor area and hand back a tidy number, but nearly every load calculator you will find is built on the same residential assumptions. The output looks like accurate results and is not one.
How Much Does Humidity Add to the Load in Manila?
Roughly two thirds of the fresh air load in Metro Manila is latent, meaning the energy goes into removing moisture rather than lowering temperature. This is the number that catches out designs copied from temperate-climate templates.
The climate data explains why. PAGASA’s climatological normals for 1991 to 2020 at the Port Area station in Manila put the mean maximum temperature at 33.8°C in April, with relative humidity peaking at 82 percent in August and averaging 74 percent across the year. Mean wet bulb temperature sits near 26°C through the hottest months. Wet bulb is the number that matters for moisture, and ours stays high all year.
Here is what that does to a real calculation. Take 1,000 litres per second of outdoor air, which is a typical fresh air quantity for a mid-sized restaurant, at 34°C dry bulb and 26°C wet bulb, cooled to an indoor condition of 24°C and 55 percent relative humidity. The sensible portion works out to about 12 kW. The latent portion is close to 24 kW. Total, roughly 36 kW, or about 10 tons of refrigeration, for the fresh air alone before you have cooled a single square meter of dining room.
A designer working from a template built for a dry climate will size that same air handler on the sensible load and land somewhere near a third of the required capacity. This is the part of how to compute HVAC load that imported templates get wrong most often. The unit will hit 24°C on the thermostat and the room will still feel damp, because the coil never gets cold enough for long enough to condense moisture out. That is the condition behind most complaints about a space that is cool but clammy.
Humidity control is also why oversizing hurts here rather than helping. An oversized unit satisfies the thermostat quickly, shuts off, and never runs long enough to dehumidify. We have been called to sites where the fix for visible mold on ceiling tiles was a smaller unit, not a bigger one.
What Load Densities Should You Expect by Building Type?
Expect wide variation, because occupancy drives load far more than floor area does. A heat load computation for buildings in the hospitality and food sectors will always land denser than an office of the same size.
Rough ordering, and please treat these as ranges to check against rather than design values:
- Open-plan offices in high-rise towers usually fall around one ton for every 18 to 25 square meters, depending on glass area and how densely the tenant fits out. Our work at The Linear in Makati sat in this territory.
- Restaurants and coffee shops run much denser, sometimes double an office, driven by occupant density, kitchen equipment, and the makeup air needed to balance kitchen exhaust. The restaurant MEPF and coffee shop MEPF pages go into how we handle that.
- Classrooms sit between the two, with high occupant density during class hours but low equipment load and long unoccupied periods that make diversity worth applying carefully.
- Hotel guest rooms compute low individually and add up quickly, and the corridors, function rooms, and back-of-house areas often carry more total tonnage than the rooms themselves. If the property runs heat pump water heaters, their heat rejection lands in the back-of-house load as well.
- Healthcare spaces are their own category, since air change rates, pressure relationships, and filtration requirements set the airflow before comfort does.
One more consideration that owners rarely hear early. If your project crosses the Philippine Green Building Code thresholds, which begin at 10,000 square meters of total gross floor area for hotels, schools, hospitals, offices, and mixed occupancy, your cooling equipment has to meet minimum efficiency requirements as well. The code itself notes that air conditioning typically accounts for more than half the total electricity cost in a centrally air conditioned building, which is a useful figure to keep in mind when someone suggests rounding the tonnage up to be safe.
What Goes Wrong When the Load Is Wrong
The cost of a bad load number is almost never the equipment. It is everything downstream that was sized from it.
Oversize the cooling and the electrical design inherits the mistake. Larger compressors mean larger breakers, heavier feeders, and sometimes a transformer or service capacity you now have to apply for and wait on. We have seen a fit-out stall for weeks because the utility application had to be revised after the mechanical scope grew late in the design. Our electrical design work is coordinated against the mechanical load for exactly this reason.
Undersize it and the problems surface after turnover, when they are expensive and public. Compressors run continuously, energy bills climb, and the space never reaches its setpoint on a hot afternoon. In a restaurant that means covers lost during peak hours.
Oversized and undersized HVAC units both result in wasted energy, just through different mechanisms. An oversized unit cuts its run time short and stops before it has dehumidified. An undersized one never stops at all, which causes increased wear and tear on the compressor and shortens equipment life. A system sized just right runs long enough to pull moisture out of the air without cycling, and proper system sizing is the only thing that gets you there.
There is a third failure that gets less attention. Get the load wrong and the ductwork gets wrong with it, and a duct sized for the wrong airflow will not fit the ceiling void the architect allowed. That clash gets discovered on site, usually by the crew installing it, and resolving it costs either ceiling height or a redesign. Running BIM clash detection against a load computation nobody verified simply produces a very precise model of the wrong system.
Who Has to Sign the Cooling Load Computation for a Mechanical Permit?
A Professional Mechanical Engineer has to prepare, sign, and seal the mechanical plans and computations. This is not optional, and it is not something the equipment supplier can do for you.
RA 8495, the Philippine Mechanical Engineering Act of 1998, requires that designs, plans, and specifications for mechanical works be prepared under the responsible charge of a PME and carry that engineer’s seal. Section 302.3 of the Revised IRR of PD 1096, the National Building Code, carries the same requirement into the building permit process. The Philippine Mechanical Engineering Code adds that plans, engineering calculations, and diagrams must be submitted with the permit application itself.
What this means in practice is that your HVAC load calculation formula and its output become a permit document. On a Philippine project, how to compute HVAC load is therefore not purely a technical question. The Office of the Building Official reviews the computation alongside the plans, and a load sheet that does not reconcile with the equipment schedule is a common reason for a comment sheet coming back.
Requirements and reviewer expectations do vary between LGUs, and the format one Office of the Building Official accepts without comment may draw questions in the next city. Confirm the current checklist with the OBO covering your site before submission rather than assuming last year’s requirements still apply.
Frequently Asked Questions
What is the formula for HVAC load calculation?
There is no single formula. A heat load calculation uses Q equals U times A times temperature difference for the building fabric, then adds internal gains. Latent load is worked out separately from moisture content difference. Total the two, then divide by 12,000 to get tons of refrigeration.
How many BTU per square meter do I need in the Philippines?
Philippine residential practice commonly uses 500 to 600 BTU per hour per square meter for ordinary bedrooms with two or three occupants. Commercial spaces run far higher because of occupant density, equipment, and mandatory fresh air. Treat any per-square-meter figure as a check, never as a design basis.
How many square meters can one ton of refrigeration cool?
One TR covers roughly 20 to 24 square meters of ordinary residential space in Philippine conditions. Air conditioning for offices lands near one ton per 18 to 25 square meters. Restaurants and kitchens need considerably more cooling capacity for the same floor area.
Is ACCA Manual J used in the Philippines?
Manual J is a United States residential standard and is rarely used here. Philippine commercial design generally follows ASHRAE methods, which handle tropical latent loads properly, alongside the Philippine Mechanical Engineering Code. Manual J assumes heating seasons and climate data that do not apply to Philippine projects.
Can I compute the cooling load myself?
You can estimate it for your own budgeting, and doing so is useful. For a building permit you cannot, since the mechanical computation must be prepared, signed, and sealed by a Professional Mechanical Engineer under RA 8495. An owner’s estimate and a permit document are two different things.
What is the difference between sensible load and latent load?
Sensible load is the heat that changes air temperature and registers on a thermometer. Latent load is the energy needed to remove moisture from the air, and it does not change temperature at all. In Philippine conditions latent load often makes up the larger share.
Send Us the Plans Before the Equipment Gets Ordered
Most of the load problems we fix would have cost nothing to prevent at design stage and a great deal to correct after turnover. If you have architectural plans and a rough scope, that is enough for us to review the cooling load basis and tell you whether the tonnage on the table makes sense.
The bottom line, restated. HVAC sizing in this climate is decided by moisture, not floor area, and knowing how to compute HVAC load properly is what separates a system that holds 24°C at 55 percent humidity on a wet August afternoon from one that runs constantly while people complain. Send us your drawings through the proposal request form and we will come back within 48 hours with scope, timeline, and fees. If you would rather talk it through first, call us at +63 960-420-9863.