Cooling capacity balance in a building and selection of the HVAC system

The cooling capacity balance in a building is the starting point for selecting a suitable HVAC system. Without it, it is easy to fall into two classic pitfalls: either the system is too weak and fails to maintain the required conditions during peak demand periods, or it is oversized and operates erratically, with poor control and unnecessarily high capital costs. In practice, therefore, it is not simply a question of ‘how much air conditioning is needed’, but of how much heat actually accumulates in the building and how this excess energy is to be removed.

This is important not only at the design stage. An incorrectly calculated balance will later manifest itself during operation as excessively high temperatures, humid air, noise, short equipment operating cycles, user complaints, or bills that do not match the original estimates. Simply selecting the cooling capacity is not enough to choose the right system. One must also understand the load structure, the division of the building into zones, the role of ventilation, and whether the problem concerns mainly temperature or also humidity.

What is a cooling capacity balance, and what does it actually involve?

The cooling capacity balance determines how much heat needs to be removed from a building or a specific zone within it in order to maintain the desired indoor conditions. This usually refers to air temperature, but in many buildings, humidity is equally important. This is why two buildings of similar size may require completely different approaches to cooling. They differ not only in their envelope, but also in their glazing, orientation to the sun, usage, number of occupants, volume of fresh air, and the presence of heat-emitting equipment.

In design practice, the heat balance is not a single, universal figure. Rather, it is the sum of several categories of heat gains that occur simultaneously or overlap at specific times. It is therefore necessary to distinguish between average operating conditions and design conditions, i.e. those under which the system must still function correctly. This is precisely where the first problem often arises: someone looks at the average temperature or average load, whereas the equipment must, after all, cope with the most demanding moment of the day or season.

The second important distinction concerns the building as a whole and its individual zones. The total power requirement for the building does not, in itself, indicate how the load is distributed across the rooms. If one façade is heavily exposed to sunlight whilst the other remains in the shade, localised overheating of parts of the building can occur even if the total power requirement is correct. From an HVAC design perspective, this is not a minor detail, but one of the fundamental decisions to be made.

Where does the cooling load in a building come from?

The sources of heat gain are generally well known, but in practice the problem arises when they are treated too broadly. The simplest approach distinguishes between external and internal heat gains. External heat gains result from the climate, solar radiation and the properties of building envelopes. Internal gains come from people, lighting, equipment and technological processes. Added to this are ventilation and uncontrolled infiltration of outside air.

In buildings with extensive glazing and lightweight structures, solar radiation has a significant impact. The heat transfer coefficient alone does not fully capture the issue. In summer, solar gains from sunlight passing through the glazing and the heating of building elements are equally important, and often more so. This is why two offices of similar size can behave completely differently if one has a west-facing façade with extensive glazing, whilst the other has smaller, better-shaded windows.

In commercial, office and industrial buildings, internal heat gains are a significant factor. People emit heat, lighting fixtures raise the temperature, and computer equipment, servers, technological devices or production facilities can completely alter the load profile. In such a building, floor area alone is not very telling. What actually happens inside and at what times becomes more important.

Ventilation plays a particularly important role. Fresh air is necessary for hygiene, comfort and regulatory compliance, but at the same time it brings heat and moisture into the building. The higher the temperature and humidity of the outside air, the greater the load on the system. This is precisely why, in many buildings, the zonal cooling system alone does not solve the problem unless it works in conjunction with a properly designed air handling unit.

The main sources of heat gain in a building

One thing is clear from this table: errors in the balance sheet usually do not stem from the complete omission of a particular component, but from oversimplifying it. On paper, everything looks correct, but the assumptions made are too lenient when it comes to real-world use.

It is also clear that not every building has the same ‘cooling profile’. In some buildings, solar heat gain is the dominant factor; in others, it is heat generated by people and equipment; and in others still, it is heat from ventilation. It is this profile that should determine the choice of HVAC system, rather than the total kilowatt rating alone.

Explicit and implicit power – a distinction that shapes the choice of system

When it comes to cooling buildings, it is not enough simply to know the total amount of heat that needs to be removed. One must also understand which part of the load relates to lowering the temperature and which to removing moisture. Sensible heat refers to cooling that is perceived as a drop in temperature. Latent heat is associated with the condensation of water vapour, i.e. with dehumidifying the air.

This distinction is crucial, as different systems handle this task in different ways. A system may appear to have sufficient cooling capacity, yet still fail to provide comfort if it does not control humidity. In an office or residential building, this manifests as stuffy, heavy air. In a technical or warehouse facility, it can lead to durability issues, condensation on surfaces, or unsuitable conditions for processes and equipment.

The greatest risk arises where there is a high proportion of outside air. If warm, humid air is constantly entering the building, a comfort cooling system alone may not be sufficient. In such cases, appropriate air treatment within the air handling unit is required, and sometimes different control logic and operating parameters for the entire system are also needed.

In practice, selecting an HVAC system without distinguishing between apparent and latent loads is often one of the main reasons for disappointment once the system is up and running. The user is told that ‘the capacity is correct’, but the building still lacks comfort. The problem then lies not in the number of kilowatts itself, but in the poor match between the cooling method and the nature of the load.

What data is required to calculate the cooling capacity correctly?

An accurate energy balance starts with the input data. The more arbitrary the assumptions, the less reliable the result. This applies to both simple structures and complex buildings with multiple zones. The designer must know the building’s location, climatic parameters, orientation, geometry, the type of partitions, the area and type of glazing, and how it is shaded.

Equally important is information regarding usage. This includes the number of people, occupancy schedules, the nature of the work carried out in the premises, lighting operating times, the power ratings of equipment, and the expected variation in load throughout the day. In many cases, this is precisely where the weakest link lies, as the developer or user provides approximate figures, only for actual usage to turn out to be significantly more intensive.

Ventilation should not be treated as an afterthought either. The flow of fresh air, how the air handling unit operates, heat recovery, the building’s airtightness, and the opening of doors, gates or windows – all these factors influence cooling. In modern buildings, the problem is often not just the temperature itself, but also how the system responds to rapid changes in load and whether it remains stable in different operating modes.

Input data for the refrigeration balance

The most insidious errors are those that seem minor at first glance. A difference of just a few people, a slightly longer operating time for the equipment, or a higher proportion of fresh air does not seem particularly serious. The problem arises when several such oversimplifications occur simultaneously. At that point, the overall result no longer reflects the actual conditions.

The second point is even more important. The design should not be based on a wish-list scenario for the building’s use. If the building is to function in a specific way, the system must be selected to suit that function, rather than a simplified version that only looks good on paper.

The most common mistakes when estimating cooling requirements

The most common mistake is basing decisions solely on floor area. This approach can only be useful as a very rough guide for simple, repetitive spaces. In any more complex building, it quickly leads to poor decisions. Floor area tells us nothing about sunlight exposure, the number of people, heat generated by equipment, ventilation or fluctuations in load over time.

Another common problem is inadequate zoning. The building is treated as a single entity, even though different parts of it operate under different conditions. An office on the east side behaves differently in the morning, a room on the west side behaves differently in the afternoon, and a server room or conference room behaves differently still. When everything is subjected to a single operational logic, the system may be technically correct but functionally poor.

Another very common mistake is underestimating the importance of ventilation and humidity. In buildings with a high volume of fresh air, or where occupants frequently open doors and gates, temperature control alone is insufficient. Then comes the typical complaint: ‘The equipment is running, but there’s still no comfort.’ This is usually a sign that the load profile has been misjudged, rather than the capacity of the equipment itself.

Oversizing is no less problematic. Many people view excess capacity as a safety net, but in HVAC it is not quite that simple. Oversized units may operate in short cycles, regulate temperature less effectively, dehumidify the air less efficiently and cause operational problems more quickly. Sometimes the client pays more not to have a better system, but simply to have a less stable one.

Design fault and its impact during operation

It is worth noting that some faults do not become apparent straight away on first start-up. The system may operate correctly under partial load or in moderate conditions, with problems only arising on hot days, when the building is at full capacity, or when the way it is used changes.

That is precisely why a well-prepared assessment is more than just a formality. It is not intended to justify the purchase of equipment, but to predict how the building and its systems will perform under real-world operating conditions.

Refrigeration capacity balance and selection of the HVAC system

The cooling load calculation should guide the choice of system design, rather than merely its nominal capacity. If the load is fairly even, ventilation plays a minor role, and zoning is not complex, a simpler system without extensive infrastructure is often sufficient. This applies to some small offices, commercial premises, individual functional zones, or simple residential buildings.

The situation changes when a building has multiple zones with different load profiles. If some rooms are in constant use whilst others are only used periodically, and this is compounded by varying levels of sunlight, a zoned system or a solution allowing for more flexible power control makes more sense. In such a building, the central kilowatt rating alone is of no use. What matters is the ability to distribute the load across different parts of the building.

Another scenario involves buildings where a significant proportion of the load is caused by ventilation and humidity. In such cases, the room cooling system alone is not sufficient. Cooperation with the air handling unit is required, along with appropriate air treatment, sometimes energy recovery, and above all, proper coordination of the operation of both parts of the system. Without this, you may have formally selected the correct cooling capacity, but still be unable to control the indoor conditions.

In larger commercial, technical and industrial buildings, systems based on chilled water, fan coils or extensive air handling units make sense. Not because they are ‘better’ by definition, but because they are better suited to a large number of zones, the building’s greater scale, the need for integration and more complex automation. The choice of system must therefore be based on the nature of the building, rather than the contractor’s or client’s preferences.

How does the cooling load affect the zoning of a building?

Zoning is one of those decisions that may seem like a minor detail in the design phase, but which determines the quality of the entire system once it is in operation. A building is rarely subjected to uniform loads. Conditions vary in rooms with large glazed areas, in technical zones, in areas used only occasionally, and in spaces where large numbers of people gather.

If all these spaces share the same cooling logic, compromises soon arise. One part of the building will be under-cooled, another over-cooled, and the staff will start adjusting the settings manually. This usually means that the problem lies not in the quality of the equipment itself, but in the poor functional layout of the building for HVAC purposes.

A well-prepared load balance shows which zones have a similar load profile and which need to be treated separately. It is on this basis that the control method, the layout of the units, the air supply parameters, and sometimes even the architecture of the entire system are selected. Without this, the selection is only partially correct, as it does not reflect how the building actually operates.

Comfort, humidity and control – power alone isn’t enough

In practice, users do not judge a system by whether the power rating on the nameplate is correct. They judge it by how comfortable it is. If the temperature fluctuates, the air is too humid or too dry, or conditions vary drastically from one part of the building to another, the system is perceived as poorly designed, even if it technically ‘matches the calculations’.

That is why regulation is so important. The system should not only cope with peak loads, but also operate reliably under partial loads. It is precisely then that the quality of control, power modulation, coordination with ventilation and the appropriate selection of operating parameters become apparent. A building does not operate under extreme conditions for most of the time, so everyday comfort depends more on the quality of control than on the maximum power output itself.

Humidity is particularly important here. In many buildings, it is the key factor in determining comfort levels, yet it is also the easiest aspect to overlook when selecting a system based on simplified criteria. A system that cools the room quickly but fails to manage humidity effectively may provide a poorer user experience than one with better-optimised operating logic and more balanced control.

What does the refrigeration balance sheet mean for the investor, the designer and the user?

For the investor, the cooling capacity balance sheet should be a decision-making tool, not merely an appendix to the project. It is this document that determines whether the budget is allocated to a simple system with a small number of zones, or to a more complex system featuring an air handling unit, automation and more precise control. Incorrect assumptions made at the outset often result in costs that exceed the difference between the options.

For the designer, the load calculation forms the basis for selecting equipment, dividing zones, determining how cooling is generated and distributed, and establishing the control logic. It is here that one must distinguish between peak and average loads, sensible and latent heat, and the conditions specified by the user and the building’s actual operating conditions.

For the user and maintenance staff, the outcome is clear. A cooling load calculation that is accurately calculated and properly translated into the HVAC system design means more predictable comfort, fewer manual adjustments, more stable operation, and a lower risk of the system becoming a source of day-to-day problems. That is precisely why the cooling balance is not a secondary step. It is at the heart of the entire decision on how the building is to be cooled.

Summary

A cooling capacity assessment for a building is not intended for the mechanical selection of equipment from a catalogue, but rather to understand the actual heat load and translate it into a suitable HVAC system. A well-executed assessment helps to avoid both under- and over-sizing, and consequently results in improved comfort, more stable control and more predictable operation of the entire facility.

FAQ – Cooling capacity balance in a building