How to Size an HVAC System: A Guide to Load Calculations and Capacity

• Sizing has a direct cost impact: cooling and ventilation are major sources of electricity consumption, so getting capacity right matters.^[1]
• HVAC capacity is based on calculated loads: capacity is expressed in BTU/h, tons, or kilowatts, where one ton of refrigeration equals 12,000 BTU/h.^[2]
• Commercial capacity should come from an engineering load calculation, not a square-feet-per-ton rule of thumb. The calculation considers building characteristics, outdoor conditions, solar radiation, occupancy, internal gains, ventilation, and infiltration. For cooling loads, ASHRAE methods such as Heat Balance (HB) and Radiant Time Series (RTS) account for the time-dependent effects of heat gains and help determine the peak cooling load.^[3][4]
• Building type also determines what factors must be considered beyond floor area: for offices, ASHRAE 62.1 assumes a default density of approximately 5 people per 1,000 ft² (ASHRAE 62.1), an operating room needs a minimum of 20 air changes per hour (ASHRAE 170), and a data center is cooled to a server-inlet envelope of 18–27°C (ASHRAE TC 9.9).^[5][6][7]
• LATS Load can calculate building heating and cooling loads from entered site specifications, providing an input for capacity planning. Other LG LATS tools can assist with equipment selection, automated design, and system verification as a project develops.

[1] U.S. Energy Information Administration (EIA). https://www.eia.gov/tools/faqs/faq.php?id=1174&t=1
[2] ACHR NEWS. “Understanding What a Cooling Ton Means.” https://www.achrnews.com/articles/144557-understanding-what-a-cooling-ton-means
[3] Air Conditioning Contractors of America (ACCA). https://www.acca.org/standards/technical-manuals/manual-j
[4] ASHRAE. ASHRAE Handbook—Fundamentals, Chapter 18.https://handbook.ashrae.org/Handbooks/F25/SI/F25_Ch18/F25_Ch18_si.aspx
[5] ASHRAE. ASHRAE Terminology: Cooling. https://terminology.ashrae.org/?term=cooling
[6] MEP Academy. https://mepacademy.com/how-operating-room-hvac-systems-work/
[7] Uptime Institute Journal. “New ASHRAE Guidelines Challenge Efficiency Drive.” https://journal.uptimeinstitute.com/new-ashrae-guidelines-challenge-efficiency-drive/

"What size HVAC do I need?" is really a question about capacity: how much heating and cooling a system must deliver to hold indoor conditions on the hottest and coldest days a location sees. In commercial buildings, that number comes from a proper load calculation rather than floor area alone, and it shifts significantly with how a space is built, occupied, and used.

Every building type has different capacity requirements. Understanding these differences and choosing the right system type is key to maintaining both comfort and efficiency.

2-1. What Is a BTU in HVAC?
A British thermal unit (BTU) is a unit of heat energy. One BTU is the amount of heat required to raise the temperature of one pound of water by 1°F.^[1] In HVAC, cooling capacity is the rate at which equipment removes heat under specified operating conditions, typically expressed in BTU per hour (BTU/h), kilowatts (kW), or tons of refrigeration.^[2]

One ton of refrigeration equals 12,000 BTU/h, or approximately 3.517 kW.^[2] Total cooling capacity includes both sensible cooling, which removes sensible heat from the air, and latent cooling, which removes latent heat from the air under specified operating conditions.^[3]

[1] U.S. Energy Information Administration (EIA). British Thermal Units (Btu).  https://www.eia.gov/energyexplained/units-and-calculators/british-thermal-units.php
[2] ACHR NEWS. Tomczyk, John. “Understanding What a Cooling Ton Means.” Published March 4, 2021. https://www.achrnews.com/articles/144557-understanding-what-a-cooling-ton-means
[3] ASHRAE. ASHRAE Terminology: Cooling. https://terminology.ashrae.org/?term=cooling

2-2. How Do I Calculate HVAC Capacity?
HVAC systems and components are sized on the basis of calculated heating and cooling loads, together with relevant system loads and design requirements. These calculations estimate how much heat must be removed or added to maintain the desired indoor conditions.
The calculation considers not only building size, but also factors such as outdoor weather, windows, building materials, orientation, occupants, lighting, equipment, and ventilation. ASHRAE provides several approaches for these calculations, including the Heat Balance (HB) and Radiant Time Series (RTS) methods.^[1]
The RTS (Radiant Time Series) method is based on a simple idea: heat entering a building does not always become a cooling load immediately. For example, solar radiation can heat floors, walls, and furniture. Some of this radiant heat is stored in these surfaces and objects and released gradually, so the cooling load can continue even after the amount of sunlight changes.
RTS takes this time delay into account when calculating the cooling load. For heat conducted through walls, roofs, and other building components, the CTS (Conduction Time Series) accounts for the time delay between heat input and its contribution to the indoor cooling load. RTS then considers how radiant heat is gradually converted into a cooling load.^[1]

In general, the calculation considers:
• Heat through walls, roofs, floors, and windows
• Solar heat entering through windows and skylights
• Heat generated by people, lighting, and equipment
• Heat brought in by ventilation and infiltration

The basic process can be summarized as:
Heat gain → conduction/radiant time delay → hourly space cooling load → peak load → system load and equipment sizing

The resulting peak load provides a basis for HVAC system and component sizing.^[1] The resulting cooling load is commonly expressed in Btu/h. For air-conditioning equipment, cooling capacity can be expressed in Btu/h; one ton of refrigeration is defined by ASHRAE as 12,000 Btu/h (approximately 3.517 kW).^[2]

For commercial buildings, cooling capacity should not be estimated from floor area alone. ASHRAE provides nonresidential load-calculation procedures that account for building characteristics, internal heat gains, ventilation and infiltration, weather conditions, and system loads.^[1] Two buildings with the same floor area can have different cooling loads depending on their windows, orientation, insulation, occupancy, equipment, and ventilation.

In short, HVAC capacity is not determined by building size alone. It is based on the calculated peak load, which depends on heat gains, system loads, and their variation over time. The RTS method provides a practical way to account for this time-dependent behavior when estimating peak cooling loads.^[1]

[1] ASHRAE. ASHRAE Handbook—Fundamentals, Chapter 18 https://handbook.ashrae.org/Handbooks/F25/SI/F25_Ch18/F25_Ch18_si.aspx
[2] ASHRAE. ASHRAE Terminology: Cooling https://terminology.ashrae.org/?term=cooling

A load calculation turns a building's characteristics into a required capacity, and those characteristics vary widely by building type. Each type carries its own mix of occupant density, operating hours, equipment, and required indoor conditions, so two buildings of the same size can need very different systems. In the sections that follow, you’ll see what to pay attention to in offices, hotels, retail, healthcare, and data centers when sizing HVAC capacity for each use case.

3-1. Offices
Moderate, predictable loads. ASHRAE Standard 62.1 assigns office space a default occupant density of about 5 people per 1,000 ft² for ventilation design.^[1] The cooling load, separately, is driven by occupants, lighting, equipment, and outdoor air intake. VRF systems with zone-level individual control may offer efficiency and comfort benefits in suitable office applications.

3-2. Hotels
A hotel behaves like several buildings in one and runs around the clock with guest rooms, lobbies, kitchens, laundries, and pools each creating different load patterns. Because lodging facilities operate for far more hours than a typical office, heating and cooling demand can remain significant even during periods of lower occupancy.^[2] VRF systems are commonly used where individual room control and quiet operation are important, while heat-recovery-based heat pump or hot water solutions may be considered for energy-saving opportunities in suitable applications. 

3-3. Retail and malls
High, fluctuating occupancy in large open volumes. For typical spaces, ASHRAE 62.1 assigns a higher people outdoor air rate to retail sales floors than to offices (7.5 versus 5 cfm per person),^[3] and retail lighting is typically denser than office lighting according to ASHRAE 90.1. Long operating hours and frequent door openings can further increase operating loads.^[4] High-capacity, wide-coverage units such as Ceiling Convertible and 4-Way Cassette models may be suitable in some layouts, and occupancy-sensor-equipped models may offer additional energy-saving potential.

3-4. Healthcare
The most tightly regulated vertical, because here ventilation is an infection-control measure, not just comfort. ANSI/ASHRAE/ASHE Standard 170 sets minimum air-change rates by space, along with pressure relationships and temperature and humidity requirements. Filtration and airflow strategies are space-specific, and system selection may depend on regulatory requirements and the intended clinical use.^[5][6][7]

3-5. Data centers and industrial facilities
These facilities are dominated by continuous sensible heat, since most of the power drawn by IT equipment becomes heat that must be removed nonstop. The ASHRAE TC 9.9 guidelines target a server-inlet range of 18–27°C, and because downtime is costly, data center designs often incorporate cooling redundancy in addition to these thermal guidelines.^[8][9] Industrial facilities may also require solutions that address process heat, high ceilings, and large exhaust volumes.

[1] MEP Academy. “How to Calculate Ventilation Air.” Published October 11, 2024. https://mepacademy.com/how-to-calculate-ventilation-air/
[2] U.S. Energy Information Administration (EIA). https://www.eia.gov/todayinenergy/detail.php?id=46118
[3] ASHRAE. ANSI/ASHRAE Standard 62.1-2022, Addendum w: Ventilation and Acceptable Indoor Air Quality. https://www.ashrae.org/file%20library/technical%20resources/standards%20and%20guidelines/standards%20addenda/62_1_2022_w_20250930.pdf
[4] ASHRAE and IES. ANSI/ASHRAE/IES Standard 90.1-2022 https://www.ashrae.org/file%20library/technical%20resources/standards%20and%20guidelines/standards%20addenda/90_1_2022_b_20230914.pdf
[5] MEP Academy. “How Operating Room HVAC Systems Work.” https://mepacademy.com/how-operating-room-hvac-systems-work/
[6] Augmintech. “ASHRAE Standard 170-2017 Healthcare.” https://augmintech.com/ashrae-standard-170-2017-healthcare/
[7] American Society for Health Care Engineering (ASHE). “Air Filtration.” https://www.ashe.org/compliance/ec_02_05_01/01/airfiltration
[8] Uptime Institute Journal. “New ASHRAE Guidelines Challenge Efficiency Drive.” https://journal.uptimeinstitute.com/new-ashrae-guidelines-challenge-efficiency-drive/
[9] POWER Magazine. “Understanding Uptime Institute’s Tier III Standard: A Guide to Data Center Electrical System Design.” https://www.powermag.com/understanding-uptime-institutes-tier-iii-standard-a-guide-to-data-center-electrical-system-design/

HVAC load calculations consider more than floor area. Insulation, occupant density, internal heat gains, outdoor conditions, and operating schedules all affect the heating and cooling capacity a building requires. The table below summarizes seven key factors to evaluate during load calculations and explains how each influences building loads, providing a basis for determining the appropriate HVAC system capacity.

A load calculation determines the heating and cooling capacity a building or zone requires. The next step is to translate that calculated capacity into an HVAC system suited to the application and site conditions.

LG Air-conditioner Technical Solution (LATS) is a suite of HVAC design tools that helps translate building, site, and load information into system plans. Its workflow spans Model Selection, Automated Design, and Design Verification.

Liquid cooling has changed where a data center's heat is captured, but not the fact that the room still has to be cooled. While liquid cooling removes heat directly from targeted IT components, air-side cooling remains essential for managing the remaining heat within the data hall and maintaining stable operating conditions. The CRAH is the unit built for that job, using a chilled water coil and efficient fans to hold a high-density room within safe limits while keeping fan energy in check.

Treating the air side as a first-class part of the design, rather than an afterthought to the liquid loop, is what makes a high-density facility both reliable and efficient. A well-chosen CRAH, engineered for efficient airflow and coordinated with the rest of the system, can be an important factor in achieving reliable and efficient operation.