Internal Heat Gains
Heat generated inside a building from occupants, lighting, and equipment that reduces the heating load in winter but increases cooling demand in summer.
What are internal heat gains?
Internal heat gains are all sources of thermal energy generated inside the building envelope by occupants, lighting systems, and electrical equipment. Every watt of electrical energy ultimately converts to heat; energy from appliances, computers, and lights diffuses into the indoor air. Even metabolic heat from people breathing and perspiring contributes. These gains reduce the heating load in winter but must be removed by cooling systems in summer, making them critical to both heating and cooling calculations.
Where do internal heat gains come from?
The three main sources are occupants, lighting, and equipment. A seated adult produces approximately 70-100 W of sensible heat, depending on activity level. Sleeping generates about 50 W, while moderate physical activity can reach 200 W. Lighting efficiency varies sharply: older incandescent systems released 95% of energy as heat (5% as light), while modern LED lighting releases only 20–40% as heat and converts 60–80% to light. Electrical equipment includes computers, monitors, printers, cooking appliances, washing machines, and refrigerators. In residential buildings, internal gains typically average 4 W/m2 under standardized occupancy assumptions. Office buildings with more occupants and equipment generate 10–50 W/m2 depending on use type; data centers and commercial kitchens can exceed 500 W/m2.
| Building Type | Typical Internal Gains (W/m2) | Primary Sources |
|---|---|---|
| Residential (detached house) | 4 | Occupants, kitchen, occasional lighting |
| Residential (apartment block) | 5–7 | Occupants, common area lighting, shared equipment |
| General office | 10–15 | Occupants (30% of total), computers/monitors (40%), lighting (20%) |
| Dense trading floor | 30–50 | High occupant density, continuous computing, bright lighting |
| School classroom | 8–12 | Occupants, minimal equipment, moderate lighting |
| Laboratory | 100–500 | Specialized equipment, fume hoods, high equipment load |
Why are internal heat gains more significant in a passive house?
A passive house has an extremely well-insulated envelope (low thermal losses) and minimal air leakage. Because heating demand is so low, internal gains that might seem negligible in a conventional building become proportionally large. In a typical passive house, internal gains can offset 30–50% of the theoretical heating load during occupied periods, especially in shoulder seasons. This means the building may need little or no active heating during spring, autumn, or mild winter days. The risk is overheating: if internal gains exceed the design assumption, the house cannot shed excess heat passively. The mechanical ventilation system is designed to distribute fresh air and recover heat; it does not actively cool. Poor summer performance in passive houses often results from underestimated occupancy, equipment, or lighting loads combined with inadequate shading or night cooling strategies.
How do internal gains differ between heating and cooling seasons?
In winter, internal gains are beneficial. They contribute to reducing the specific heat demand, lowering the annual energy requirement for heating. Occupants, lighting, and appliances provide free warmth that passive design exploits. In summer, internal gains become a burden. All heat generated indoors must be removed via mechanical ventilation, opening windows to cooler night air, or by accepting higher indoor temperatures. A building with high summer internal gains (for example, an office with many people, computers, and lights running continuously) faces a larger cooling load than one with identical envelope properties but lower occupancy or better equipment efficiency. This is why passive house design often emphasizes occupancy schedules, lighting control (dimming, occupancy sensors), and energy-efficient equipment to keep summer gains manageable.
How are internal gains calculated and verified?
Slovak thermal regulations (standards for energy certification) apply default values based on building category: 4 W/m2 for residential buildings under average occupancy assumptions. This simplified approach assumes roughly 1–2 people per bedroom, standard household appliances, and basic electric lighting. For non-residential buildings (offices, schools, shops), specific values are mandated by use type. Detailed calculations use PHPP (Passive House Planning Package) software, which allows occupancy schedules, appliance inventories by wattage and daily runtime, and lighting power density broken down by zone. Verification of actual internal gains is difficult without energy monitoring. Prediction relies on design assumptions; post-occupancy audits comparing calculated heat demand to measured energy use often reveal assumptions were too conservative or too optimistic. High-performance buildings frequently find that actual occupancy, equipment use, or occupant behavior differs from design intent, shifting the balance between heating and cooling loads.
| Gain Component | Input Method (Standard) | Input Method (PHPP Detailed) |
|---|---|---|
| Occupancy | Fixed value per m2 (e.g., 0.05 people/m2) | Hour-by-hour schedule; weekday/weekend variation |
| Lighting | Fixed W/m2 by building type | Lighting power density (W/m2) + daily operating hours + dimming/occupancy sensor profile |
| Equipment | Implicit in occupancy assumptions | Itemized appliances: refrigerator, washing machine, computers (hours/day, standby/active power) |
What role do internal gains play in cooling design?
Cooling load calculations account for internal gains alongside solar gains and transmission losses. In hot climates or buildings with high occupancy, internal gains can dominate the cooling load. A typical office building might see 50% of its summer cooling demand driven by internal gains (people, equipment, lights) and 40% from solar heat through windows. Reducing internal gains through LED lighting, occupancy sensors, heat-efficient equipment, and occupancy management directly lowers cooling equipment size and energy cost. In passive house summer cooling strategy, controlling internal gains (via appliance scheduling, ventilation dampers, or thermal zoning) becomes as important as managing solar gains through shading or stack effect ventilation.
What misconceptions exist about internal gains?
A common error is assuming internal gains are always helpful. In winter, they reduce heating need; in summer, they hurt. Another misconception is that energy-efficient equipment has no impact on calculated heat demand in residential buildings. Under Slovak default standards, appliance efficiency does not directly change the fixed 4 W/m2 value. However, in detailed calculations or in passive house design, improved efficiency (especially of always-on loads like refrigerators) genuinely lowers design demand. A third confusion: treating internal gains as independent of climate. Seasonal patterns shift; a building designed for continental Central European climate (cold winters, warm summers, high solar gain potential) gains differently from one in a marine climate. Internal gains matter most in shoulder seasons when heating or cooling demand is borderline; the timing and magnitude of gains relative to outside temperature determine whether the building overheats or undercools.
Frequently asked questions
- Why do internal heat gains matter more in a passive house than a conventional building?
- In a passive house, the building envelope is so well insulated that internal gains (occupants, appliances, lighting) become a substantial portion of the heating load, especially during shoulder seasons and mild winters. This means careful management of when and where heat is used is critical. Oversizing equipment or generating excess heat in one zone can cause overheating throughout the whole house, since internal gains spread evenly via the ventilation system.
- How much heat does a single person contribute to a room?
- A seated adult typically generates 70-100 W of sensible heat (skin and breath), depending on activity level. Sleeping requires about 50 W, light office work 70 W, and moderate physical activity can reach 200 W or more. In an occupied office space with computers and lighting, occupants may account for only 20-30% of total internal gains.
- Do LEDs really make a significant difference to building cooling load?
- Yes. LED lighting is typically 60-80% efficient (converting electricity to light), meaning 20-40% becomes heat. Older incandescent bulbs were only 5% efficient for light and 95% heat. In a modern office, reducing lighting power density from older systems (25 W/m2) to LED (10-12 W/m2) alone can drop internal gains by 4-6 W/m2, directly reducing summer cooling demand.
- What happens if internal gains exceed passive house design assumptions?
- If more people, equipment, or lighting operate than the PHPP calculation assumed, indoor temperature rises. The mechanical ventilation system alone cannot cool the building (it is not air conditioning). The building must rely on night cooling, window shading, or ventilation that brings in cooler outside air when available. Poorly designed passive houses can overheat in summer if occupancy or appliance load was underestimated.
- How are internal gains factored into Slovak building energy calculations?
- Slovak thermal calculation standards apply standardized default values: typically 4 W/m2 for residential buildings. This covers average occupancy (1-2 people per bedroom), basic lighting, and household appliances. The standard does not account for server rooms, commercial kitchens, or unusually high equipment loads. For non-residential or specialized buildings, detailed inventories of lighting power, equipment, and occupancy schedules must be specified.
- Can you reduce a building's heat demand by using more efficient appliances?
- In a conventional building, improved appliance efficiency mainly lowers your electricity bill, not the calculated heat demand (which assumes standard loads). In a passive house or high-performance home using PHPP, detailed appliance inventory reduction can lower the calculated heating demand, but only if the heating system design was sized to match the lower gains. Real-world benefit depends on your actual usage pattern.