Room Air-Conditioner Size
The peak cooling load of one room, or of several rooms on one ducted unit, in watts, Btu/h and tons, hour by hour on the 21 July design day in 27 Saudi cities: the unit to buy, the air each room needs, and the room or the whole plan in 3D.
An hour-by-hour cooling load on the ASHRAE clear-sky design day, 21 July, from 6 to 18 solar time, with the design conditions of 27 Saudi weather stations from the 2025 ASHRAE Handbook: Fundamentals. Every exterior wall and an exposed roof conducts heat at its sol-air temperature, windows add conduction and solar gain through their SHGC, and people (ASHRAE Table 3), lighting, equipment and outdoor air add the rest, sensible and latent; the load is checked again at the design wet bulb, the most humid hot hours. For one room, the hour with the highest total plus your design margin is the capacity to buy, turned into a wall split, a floor-standing unit, a cassette, a ducted unit or a window unit. For several rooms on one unit (a villa floor or an office) you list the rooms on each side of a corridor; the page draws the plan, finds which part of each wall faces outside, adds the rooms hour by hour and sizes the ducted unit on the largest sum, with the supply air each room needs and the wall split each room would need on its own.
Location and design conditions
The load is also computed at the design wet bulb with its mean coincident dry bulb (the most humid hot hours) and the larger peak sets the size. Leave both empty to skip the check.
The plan: spaces on both sides of a corridor
Enter the spaces on each side in their order along the corridor. The plan is drawn from them, and each side of each room is found to face outside, an unconditioned space or another air-conditioned room.
Corridor lighting, unconditioned spaces and supply air
Choosing a use fills the ordinary values for that room: people, activity, lighting and equipment. They are your own inputs: change them.
Room, walls and windows
Walls: tick the exterior ones and enter their windows
Wall and roof construction
People and internal gains
What to buy
The room and the chosen unit
Supply air to each room
Room by room
| Room | Area m² | Its own peak W | At the block hour W | Share % | Supply air L/s | A wall split for it alone Btu/h |
|---|
Each room at its own peak and at the block hour that sizes the unit; the supply air is at the room's own peak. The last column is what the room would need on a wall split of its own, for comparison.
The load hour by hour on 21 July
Steady-state load on the clear-sky design day, solar hours 6 to 18: outdoor air at the design dry bulb all day, sol-air on every sunlit surface, no credit for thermal storage.
Component by component at the peak hour
| Component | Area / quantity | How it is computed | Sensible W | Latent W |
|---|
Hour by hour
| Solar hour | Sun altitude β ° | Sun azimuth φ ° | Walls W | Roof W | Glass: solar W | Glass: conduction W | Sensible W | Latent W | Total W |
|---|
Azimuth φ is measured from south, positive to the west and negative to the east.
The surfaces at the peak hour
| Surface | Total irradiance E_t W/m² | Sol-air t_e °C | Heat entering W/m² |
|---|
The two air states
| State | Outdoor air | Room air |
|---|
Export
Questions engineers ask
How many tons do I need for my room?
Enter the room, its exterior walls and windows, the build-up of the walls and roof, and who uses it. The page computes the load at every solar hour of 21 July, takes the highest, adds your margin (10% by default) and converts it at 12,000 Btu/h per ton. A west-facing room gains most in the afternoon, an east-facing one in the morning, and a top-floor room carries its roof as well.
One unit for several rooms: why is it smaller than the rooms added up?
Because the rooms do not peak at the same hour: a west room peaks in the afternoon, an east room in the morning. The page adds the rooms hour by hour and sizes the unit on the largest hourly sum, the block load, and shows it beside the sum of each room's own peak. Each room still gets its own peak in the supply air it needs, and the table shows the wall split that room would need on its own, for comparison.
Why check the T3 capacity and not only the nominal size?
SASO 2663:2021 rates a unit at T1, 35 °C outdoor, and also tests it at T3, 46 °C outdoor. At 46 °C the same unit delivers less than its nominal rating, and the 0.4% design dry bulb is 45.1 °C in Riyadh and 47.5 °C in Al Ahsa. So the figure to hold against the load per unit is the T3 cooling capacity on the model's specification sheet; enter it and the page says whether it is enough.
Which unit type does the page suggest?
Up to 24,000 Btu/h a wall-mounted split; up to 36,000 a wall split or a floor-standing unit; above that a floor-standing, cassette or ducted unit, or more than one unit. You can pick any of the five types and the number of units yourself: the size of each is the smallest nominal step that covers the load divided by the number of units.
Which cities are listed?
The 27 Saudi weather stations, in all 13 regions, that have a cooling design table in the 2025 ASHRAE Handbook, Fundamentals, at 0.4%, 1% and 2%: the design dry bulb with its mean coincident wet bulb, and the design wet bulb with its mean coincident dry bulb. Hafr Al Batin is represented by Al Qaisumah. Al Kharj, Al Duwadimi, AlUla and Jubail have weather files but no ASHRAE table, so they are not listed: choose Custom and enter their values.
Why is the latent load from outdoor air zero in Riyadh?
At its design dry bulb Riyadh air holds less moisture than a room at 24 °C and 50%, so the air leaking in would dry the room, and the arithmetic gives a negative latent load. The page does not let that lower the size: a negative outdoor-air latent load is taken as zero. People still add their latent heat.
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