How Peak Load Can Impact the Cost of Your HVAC System

Air Conditioning Units on rooftop

One of the biggest challenges for the HVAC industry pertains to designing and installing equipment to meet end-user expectations for comfort while maintaining the balance of cost and efficiency. The U.S. Environmental Protection Agency (EPA) estimates that improperly sized HVAC systems can lose up to 30% of their overall efficiency, leading to unnecessary energy consumption and significant environmental damage. Identifying cost-optimization strategies helps architects, engineers, and contractors proactively mitigate the costs attributed to these changes and create sustainable buildings for the future. Buildings contribute significantly to the world's energy-related sustainability challenges; the adoption of right sized ventilation and air conditioning systems help curtail carbon emissions and offset the damaging effects of climate change.

What is Load?

When designing a building's HVAC system, engineers must assess a building's capability to gain or lose heat. A building's capability to gain or lose heat helps engineers identify HVAC equipment size, with the building's heat gains and losses characterized as either external or internal loads. Cooling load calculations are typically the first to be assessed by the mechanical engineer since these calculations help determine the adequate equipment sizes at the project's outset. They are a summation of the building's external and internal loads, which are impacted by its geometry, location, and use, in addition to other factors. 

What are External Loads?

External loads come from a variety of external factors, including solar radiation through windows and skylights and conduction through roofs and walls. Outside air brought into a building through the ventilation also contributes to a system's total load. 

What are Internal Loads?

Conversely, internal loads are produced from heat generated inside the building or zone from people, lighting, and equipment such as computers, monitors, etc. Lighting and equipment loads are sensible heat, whereas the metabolic heat from a building's occupants includes both sensible and latent portions of load.

What is Peak Load?

Peak Load is the absolute highest load for a room, system, or building, ignoring time when individual gains and losses occur. Peak is often used to size room-level equipment like diffusers and fan coil units. It uses the "Sum-of-Peaks" sizing methodology and is readily used for constant volume systems, where the system must supply maximum value without variation. The fan and coil sizes for the equipment are based on the sum of individual space loads or airflows and are referred to as non-coincident loads. Despite the ease of using Peak Load for system sizing, it is not recommended for sizing systems like AHU or Chillers because it will result in oversizing.

What is Oversizing?

Engineers will typically oversize a building's HVAC systems as a safety net when managing extreme periods beyond those assigned to the design conditions. This conventional practice allows engineers to offset professional risk; however, the sizing methodology transfers risk to the owner, who must address the increased cost of equipment, maintenance, and electricity. Often, the decision to oversize results from conservatively estimating the Peak Load or using an older "rule of thumb" which has not been checked against an actual load.

What is Block Sizing?

A "block" sizing methodology is commonly employed in mechanical engineering and for variable volume systems. Engineers use this method to assess the design load profiles and airflows specific to individual spaces within the zone or system to identify the collective maximum at any given time. 

The most significant difference between "block" or "peak" sizing is determined by the building type and orientation. Engineers should consider peak sizing factors such as the sum of peak Cubic Feet Per Minute (CFM) within each space. Conversely, they can base the "block" sizing on the peak time for the entire building which will almost always be lower than the sum of room peaks. Depending on whether a building's system is sized based on "peak" or "block" load, engineers can expect to see its size reduce  by ~10% to 25%.

How are Engineers Choosing System Sizes?

In the project's design phase, the engineer evaluates and recommends components needed for the HVAC project. Sizes and loads are calculated to define the equipment used, and a layout is developed for the entirety of the proposed system. Schematics are also created to illustrate the equipment and system in greater detail. 

The HVAC mechanical engineer oversees equipment installation to ensure the system is placed and connected according to the design plan. In addition, the engineer calls for performance tests to assess whether the new system is operating correctly and in accordance with regulations. 

Sizing building HVAC systems requires a high level of skill and technical expertise and requires engineers to navigate a host of variables to map out a building's expected heat profile and airflow. To evaluate proper system sizing, engineers must establish the precise design criteria used to determine the building's loads. By mirroring HVAC equipment to actual loads, engineers reduce initial costs and operating costs associated with a building's system. Moreover, engineers can achieve increased energy savings by using right sizing to avoid the problems that arise from oversizing a system.

Load Modeling and HVAC Design Software

The greatest challenges associated with the HVAC design process come from understanding airflow, maintaining comfort level despite temperature changes, and identifying the energy needed to operate an efficient system. 

Load modeling is a critical element in the design of HVAC systems and serves as a foundation for calculating a rate of energy in accordance with specified conditions for indoor environments. Specialized software lends itself to determining the optimal design of piping, ductwork, and heating/cooling components for a higher degree of confidence in the final product. The results from these tools offer better precision for sizing and improved HVAC cost-optimization. Using these tools also streamlines workflow to fulfill tasks in an innovative and organized manner. 

How Can Engineers Use Software to Communicate their Predictions with Owners? 

The design process for a building can be dynamic, and client expectations and communication are fundamental to the way engineers manage their project. Beyond wanting to ensure a building's comfort and efficiency, owners look to the skill and knowledge of the engineer to make recommendations. Once a building's manager is made aware of ways to reduce HVAC demands, and the advantages of doing so, they can make informed decisions, improving the project outcomes.. Strategies to reduce HVAC systems costs, as well as minimizing energy and maintenance expenses, might include:

  • Increasing energy efficiency

  • Installing space-saving systems.

  • Utilizing a low-maintenance system

  • Using HVAC systems in conjunction with renewable energy sources. 

Automated Reports 

loadmodeling.tool allows mechanical engineers to calculate the daily space, system, and plant loads; it considers the evolving conditions throughout different months and seasons and allows users to generate automated reports using this data. Automated reports serve as an essential communication tool to help bridge the communication gap between engineers and owners. They enable collaborators to see the breakdown of room-by-room Cooling Peak Load and Heating Peak Load and show the envelope gains and losses in Btu/h or % for your external loads such as Exterior Walls, Glazing, Roofs, Floors, etc. 

loadmodeling.tool also incorporates techniques recommended by the American Society of Heating, Refrigeration, and Air-Condition Engineers (ASHRAE), in accordance with ASHRAE Standard 183, "Peak Cooling and Heating Load Calculations in Buildings Except Low-Rise Residential Buildings," under industry best practices. 


Engineers and contractors employing the right software tools from early schematic design to construction for HVAC systems see cost-optimization through reduced energy usage and construction costs. These tools can help offer a big picture view that allows engineers to consider where changes need to be made to benefit the system's long-term performance and goals. 

Accurate load calculations help determine the amount of heat that must be continually removed or added to maintain the desired indoor temperature and ensure the right sized system is chosen for maximum efficiency and comfort. It helps engineers identify the optimal system for better results, reduces monthly utility bills, and allows HVAC professionals to diagnose large-scale issues that require repair or revision ahead of new unit installation. These calculations can ultimately save end-users money by reducing service calls or other expenses associated with maintenance.

Click here to discover loadmodeling.tool


cove.tool highlights