Internal Heat Gain: A Practical Guide for HVAC & Building Services Engineers

Internal heat sources are any heat-generating activities or equipment located inside the building envelope. Unlike solar or conductive gains that come from outside, internal heat gain is produced within the conditioned space itself.

The key rule: All energy (measured in BTU/hr or W) consumed inside a building ultimately becomes heat, including a computer running calculations, a person sitting at a desk, a light, or a server processing data.

What Is Internal Heat Gain

Internal heat gain is the heat added to a conditioned space by sources located within the building envelope itself. It includes heat from occupants, artificial lighting, and electrical equipment — all of which convert the energy they consume into heat that the cooling system must remove.

In modern commercial buildings, internal heat gain frequently exceeds solar and fabric gains combined, making it one of the most important factors in the cooling load calculation.

Sensible Heat vs. Latent Heat: A Quick Explainer

Not all heat affects a building the same way. In fact, internal heat gains are split into two types:

• Sensible heat warms the air — you can measure it with a thermometer. It raises the dry-bulb temperature, and the air is cooled to remove it.
• Latent heat adds moisture to the air — it comes from people breathing and sweating, cooking, and wet processes. It doesn’t raise the temperature but it raises humidity, affecting comfort, mold risk, and equipment performance. Removing it requires dehumidification in addition to cooling.

Understanding this split is critical for selecting the right cooling equipment.

Why Internal Heat Gain Matters in Cooling Load Calculations

For instance, in many modern office buildings, internal gains could account for 50% of the total cooling load.

Three Main Sources of Internal Heat Gain

Internal heat gains come from three primary sources: occupants, lighting, and equipment. However, each source contributes differently to the total load, in terms of magnitude, predictability, and the split between sensible and latent heat, and each requires a different approach to estimate accurately. Understanding all three is essential before selecting cooling plant or sizing ductwork.

1. Occupants (People)

People generate heat through metabolic processes, and the rate depends on the activity level:

Heat Output per Person by Activity (source: ASHRAE Fundamentals Handbook):

Key points:

• Latent heat (moisture from breathing and perspiration) becomes critical in dense spaces
• Gyms, auditoriums, and classrooms have high latent loads, which drives dehumidification requirements
• Therefore, always validate occupancy density with the client. For example, a law firm and a call center will have different occupant density and activity, and have different cooling and dehumidification needs.

2. Lighting

All electric lighting releases a portion of its energy as heat. The amount depends on luminaire type and installation method.

Typical installed lighting loads:

Key points:

• Recessed fittings discharge some heat to the ceiling void — factor in plenum return air if relevant
• Lighting controls (presence detection, daylight dimming) can significantly reduce the design load
• LED retrofits reduce heat gain as well as energy bills

3. Equipment and Appliances

Often, the most variable and hardest to estimate category. This can include:

• Office: Computers, monitors, printers, photocopiers, AV equipment
• Catering: Ovens, grills, fryers, dishwashers, refrigeration (heat rejection side)
• IT / data: Servers, UPS systems, networking hardware, storage arrays
• Medical / lab: Autoclaves, centrifuges, imaging equipment, incubators
• Industrial: Motors, compressors, process equipment

 
Typical equipment heat gain benchmarks:

Key points:

• Equipment gains are almost entirely sensible (dry heat)
• Exception: Catering and lab equipment can introduce latent gains via steam and moisture
• Benchmark data from CIBSE or ASHRAE is useful when measured data isn’t available

Sensible Heat vs. Latent Heat — Full Breakdown

Sensible Heat Ratio (SHR)

SHR = Sensible Heat Gain ÷ Total Heat Gain

• SHR of 1.0 = pure sensible load (e.g. server room)
• SHR of 0.8 = typical office
• SHR of 0.5–0.6 = high moisture space (kitchen, gym, pool)

 
Why SHR matters for design:

• Cooling coils must be selected to match the space SHR
• A coil with the wrong SHR will either over-cool (too cold, under-dehumidified) or leave the space at the right temperature, but damp and muggy
• Direct Expansion (DX) systems are particularly sensitive to SHR mismatch

Internal Heat Gain Examples

Example 1: Open-Plan Office

Space: 2,153 ft² (200 m²) | 30 occupants | LED lighting | Standard equipment

SHR: 0.79

This space has a moderate latent load from occupants. Therefore, the cooling system needs sensible cooling and dehumidification capacity.

Example 2: Commercial Kitchen

Space: 538 ft² (50 m²) | 5 staff | Heavy cooking equipment with hood

SHR: 0.66

A low SHR indicates dehumidification is critical. Consequently, a standard comfort cooling coil may not cope. Consider a low-SHR coil or dedicated energy-recovery ventilation.

Example 3: Server Room

Space: 20 m² (215 ft²) | No regular occupancy | High-density IT equipment

SHR ≈ 1.0

Almost entirely sensible load. Use precision cooling (CRAC/CRAH units) with high SHR performance. Standard comfort coils would be inefficient here.

SHR Comparison Across Space Types

Key Takeaways

• Every watt (BTU/hr) consumed inside a building becomes heat; there are no exceptions.
• Internal gains often dominate modern commercial buildings at 50% of the total cooling load
• Always validate inputs with the client as occupancy density, equipment schedules, and operating hours vary enormously between building types
• SHR drives coil and plant selection. Get it wrong, and the space will be either too cold, too humid, or both.
• Apply diversity factors to equipment loads — not everything runs at full capacity simultaneously.
• High internal gain spaces may need cooling year-round and this affects zoning, controls, and plant strategy.
• Use dynamic simulation for complex projects. Peak load timing depends on occupancy and equipment schedules, not just peak external conditions.

Internal Heat Gain Frequently Asked Questions

What does Internal Heat Gain mean and what causes it?

Internal Heat Gain is a measurement of all of the heat-generating activities or equipment located inside the building envelope. It is caused by any energy-consuming activity or equipment operating within the building envelope, including occupants, artificial lighting, and electrical equipment.

What is the difference between sensible heat and latent heat?

Sensible heat changes the air temperature, so you can measure it with a thermometer. Latent heat changes the air moisture content, so it affects humidity rather than dry-bulb temperature. In HVAC design, sensible heat usually comes from lighting and equipment, while latent heat often comes from occupants, cooking, steam, and other wet processes.

How does Internal Heat Gain affect building construction and costs?

Internal heat gain can affect energy consumption, cooling load sizing, and cooling coil selection. In many modern office buildings, internal gains could account for 50% of the total cooling load.

How do you control Internal Heat Gain when designing an HVAC system?

Calculating the Sensible Heat Ratio (SHR) will help you determine how to adequately cool the space while keeping energy costs reasonable.

How to reduce Internal Heat Gain​

The most effective strategies include switching to LED lighting, specifying energy-efficient equipment, and applying diversity factors to account for equipment not running at the same time. Occupancy-based controls and scheduled shutdowns can also reduce peak internal gains significantly.

Internal Heat Gain Calculations Are Coming to h2x

We’re currently building Internal Heat Gain calculations into h2x design software. Join the list below to get product updates and be first to know when the feature goes live.