Understanding Heat Gain: Where Your Room Gets Hot From
An air conditioner does not cool empty space. It removes heat that is actively entering the room from multiple sources, every hour the unit runs. Understanding where that heat comes from explains why two rooms of the same size can have very different cooling loads, and why a sizing approach that only counts floor area often gets the result wrong. This guide covers every significant source of heat gain in an Indian room.
Quick answer: Heat enters a room through five main pathways: conduction through walls, roof, and floor; solar radiation through windows; heat from people inside the room; heat from appliances and lighting; and warm outdoor air infiltrating through gaps. In Indian summer conditions, conduction from extreme outdoor temperatures and solar gain through windows are typically the two largest sources.
Source 1: Conduction Through the Building Envelope
Heat moves by conduction from any hot surface to any cooler surface in contact with it. In summer, the outdoor walls, roof, and sometimes floor are all significantly hotter than the cooled room air inside. Heat conducts inward continuously as long as the outside is hotter than the inside.
The rate of conduction depends on the temperature difference and the thermal resistance of the material. In Indian cities at 44 degrees Celsius outdoors with a room set to 24 degrees, the temperature difference is 20 degrees. Through a thin 115 mm brick wall with no insulation, this conducts a substantial amount of heat per square metre per hour. A 230 mm double-brick wall conducts roughly half as much per the same area.
The roof is particularly problematic. A flat concrete roof in direct sun can reach surface temperatures of 60 to 70 degrees Celsius by mid-afternoon. Even after sundown, a roof that has absorbed heat all day continues radiating it into the room through the night. Top-floor rooms therefore have among the highest conductive heat gains of any room in a building.
Source 2: Solar Radiation Through Windows
Solar radiation passing through glass is the most concentrated source of heat gain in most rooms. Unlike conductive heat, which is spread across a whole wall, solar gain can arrive through a single window at intensities of 500 to 800 watts per square metre. A 1.5 square metre west-facing window can deliver as much heat in an afternoon as a one-bar electric heater running continuously.
The window direction, glass type, and shading all determine how much of this enters the room. West-facing windows in India are the highest-risk orientation because they receive direct sun at peak outdoor temperatures. Unshaded single-pane glass passes the majority of incident solar radiation into the room. Reflective or double-glazed glass and external shading reduce this substantially. This is covered in more depth in how sunlight and window direction affect cooling load.
Source 3: Internal Gains from People
Every person in a room generates heat as a by-product of metabolism. At rest or doing light work, each person adds roughly 250 to 350 BTU per hour (about 75 to 100 watts) of sensible heat and a similar amount of latent heat (moisture). Four people in a room contribute more than 1,200 BTU per hour of heat that the AC must remove in addition to all the external gains. For offices, classrooms, or family rooms with regular multi-person occupancy, this is a significant fraction of total load.
Source 4: Internal Gains from Appliances and Lighting
Every electrical appliance in a room ultimately converts the electricity it draws into heat. A few common examples:
| Appliance | Approximate heat output |
|---|---|
| LED lighting (per 10 W bulb) | 10 watts (very low) |
| Television (50-inch LED) | 100 to 180 watts |
| Desktop computer and monitor | 200 to 400 watts |
| Gaming PC with dedicated GPU | 400 to 700 watts |
| Refrigerator (in kitchen) | 150 to 250 watts |
| Cooking range (active use) | 1,000 to 3,000 watts |
The practical implication: a home office with a desktop computer, monitor, and printer can add 500 watts or more of continuous heat load during working hours. For sizing an AC in a room with significant appliance load, this should be added to the area-based calculation. See how to size an AC for home offices with computers for a worked example.
Source 5: Air Infiltration
Any gap between the room and the hot outdoor environment allows warm air to flow in, directly adding heat and moisture to the cooled space. Common infiltration points are gaps under doors, poorly fitted window frames, and the wall penetrations where AC pipes pass through. In an older building with settled walls and ill-fitting doors, air infiltration can add a surprisingly large heat load.
Sealing door gaps with door sweeps and ensuring AC pipe penetrations are properly filled with foam or sealant reduces infiltration at low cost. This is one of the few heat gain sources that can be almost eliminated with simple DIY measures.
Thermal Mass: The Delayed Release
Thick concrete walls and floors act as heat stores. They absorb solar and conductive heat during the day and release it slowly over the following hours. This explains the common experience of a room that cools down reasonably during the afternoon but still feels warm at 10 pm: the walls, which absorbed heat all day, are still radiating it back into the room. High thermal mass means the AC must work longer after sundown than a simple outdoor temperature reading would suggest.
Calculate the combined cooling load for your specific room conditions.
Cooling Load CalculatorKey takeaways
- Heat enters a room through five pathways: conduction, solar radiation, people, appliances, and air infiltration.
- In Indian summer conditions, conduction from extreme outdoor temperatures and solar gain through windows are usually the two largest sources.
- Top-floor rooms with uninsulated flat roofs have very high conductive heat gain from the roof surface.
- Appliances contribute meaningfully to load, especially in home offices with computers and screens.
- Air infiltration through gaps can be reduced at low cost with door sweeps and pipe sealant.
Frequently Asked Questions
What is heat gain in a room?
Heat gain is the total rate at which heat enters a room from all sources. It is what the AC must remove to maintain a comfortable temperature. The main sources are conduction through walls and roof, solar radiation through windows, heat from people and appliances, and warm air leaking in through gaps.
Which heat source is biggest in Indian rooms?
In most Indian rooms, the dominant sources are conduction through the roof and walls due to extreme outdoor temperatures, and solar radiation through windows, especially west-facing windows in the afternoon. Top-floor rooms with uninsulated roofs can see the roof become the single largest source.
Do appliances add a lot of heat to a room?
It depends on the appliance. A desktop computer setup can add 200 to 400 watts continuously. A gaming PC can add 400 to 700 watts. A television adds 100 to 200 watts. LED lighting adds very little. In a home office with multiple screens and computers, appliance heat load is a significant fraction of total cooling load.
How does thermal mass affect cooling at night?
Thick concrete walls absorb heat during the day and release it slowly through the evening. This is why a room can still feel warm at midnight even after hours of AC operation: the walls are still radiating heat absorbed in the afternoon. High thermal mass extends the period the AC must run after outdoor temperatures drop.
Sources and Further Reading
- Bureau of Energy Efficiency, India, building energy and cooling load standards (beeindia.gov.in)
- ENERGY STAR, heat gain and room air conditioner sizing (energystar.gov)
- U.S. Department of Energy, residential cooling load and heat gain (energy.gov)
This article provides general educational guidance on heat gain in residential rooms. Heat load figures are illustrative. Use the Cooling Load Calculator or consult an HVAC professional for a precise assessment.