Properly-Sized HVAC Equipment

What is Properly-Sized HVAC Equipment?

Heating, ventilating, and air-conditioning (HVAC) systems consume 48% of the energy used in a typical US home.[1] Design engineers often oversize HVAC systems to comply with code or to meet the most extreme loads such as the coldest and hottest days of the year, and design systems with a built-in capacity for overloads which occur only 1% to 2.5% of the time.[2] Properly sized HVAC equipment considers the interaction among building systems, and the potential synergies and contradictions among these systems, taking into account the energy savings achieved through building envelope strategies, smart controls, and occupant behaviors when selecting equipment and designing air distribution systems (see Integrated Design Process).

Oversized equipment often results in higher installation and capital costs, reduced operating efficiency, and potential negative impacts on comfort. For example, short-cycling or excessive cycling of air conditioning equipment can lead to poor humidity control and moisture or mold issues and contributes to the wear and tear and longevity of equipment. Oversized systems use more fan power for blowers and can increase peak demand during the cooling season, leading to higher energy costs.[3] Oversizing can occur for a variety of reasons. Contractors may use standard rules of thumb based on experience rather than industry accepted practices and customers often request oversized units without understanding the calculations that go into sizing equipment or to plan for potential building additions.[4]

How to Incorporate Properly-Sized HVAC Equipment

First, consider all aspects of the building as part of an integrated design process and incorporate energy-efficient strategies that reduce heating and cooling loads. Use energy modeling to help select appropriately sized HVAC equipment that takes into account the impact of different energy-efficient design measures and building operations on peak heating and cooling loads and overall system loads as well as how often the system runs on part load (see Energy Modeling). Consider part-load performance when selecting equipment as most heating and cooling equipment fail to operate at peak efficiency unless fully loaded (see Part Load Efficiency).[5] One approach to downsizing HVAC equipment uses a small, efficient system to manage normal loads, and designates a backup system or relies on energy storage for extreme conditions that demand additional load capacity and occur less frequently (see Energy Storage and Back-up Power Generation, Thermal Energy Storage).[6] In general, properly sizing HVAC equipment avoids providing excess capacity and allocates extra physical space for additional equipment and designs modular distribution systems that can accept additional equipment for planned future expansion.[7]

According to the US DOE, there are several key elements to consider when sizing a residential heating and cooling system:[8]

• Local climate
• Size, shape, and building orientation
• Insulation levels
• Window area, location, and type
• Air infiltration rates
• Number and ages of occupants
• Occupant comfort preferences
• Selection of lights and major home appliances (which give off heat)

A home’s HVAC system design involves load calculations, system sizing, ductwork selection, and diffuser selection.[9] Hire certified contractors that use the Air Conditioning Contractors of America (ACCA) Manual J, “Residential Load Calculation” to calculate loads, the ACCA’s Manual D, “Residential Duct Design” for duct design, and the ACCA’s Manual S, “Residential Equipment Selection,” for guidance on selecting home heating and cooling systems[10] and the ACCA’s Manual T, “Air Distribution Basics,” for information on diffusers.[11]

Examples

HVAC Equipment Sizing Example, Chicago, IL.

This strategy guide prepared for the US Department of Energy’s Building America Building Technologies Program details the equipment selection of a split system air conditioner and furnace for a one-story, 2,223 square foot case study house with a full conditioned basement in Chicago, Illinois that meets the 2009 IECC prescriptive path.[12]

Benefits

Properly sized HVAC systems reduce energy consumption, water use and emissions, provide cost savings, manage occupant comfort and maintain indoor air quality.

Costs

 Incorporating a whole-building energy-saving approach including envelope strategies, high-performance lighting, and energy-efficient windows can reduce peak heating and cooling loads and result in smaller HVAC systems that cost less upfront than larger capacity models and save 30% on average on annual utility costs.[13] Additional upfront costs for energy modeling software and analysis often pay for themselves through realized energy and cost savings.

For information on available HVAC equipment incentives, see the New Jersey Clean Energy Program.

Resiliency

Properly sized HVAC equipment increases resiliency by decreasing energy and water consumption, and peak loads which decreases reliance and stress on the electricity grid natural gas, water and wastewater infrastructure.

[1] US DOE: Heating and Cooling. https://www.energy.gov/heating-cooling (accessed May 23, 2018).

[2] Whole Building Design Guide. 2016. High-Performance HVAC. National Institute of Building Science. https://www.wbdg.org/resources/high-performance-hvac (accessed Nov 28, 2018).

[3] NREL. Right-Size Heating and Cooling Equipment. https://www.nrel.gov/docs/fy02osti/31318.pdf  (accessed May 23, 2018).

[4] NREL. Right-Size Heating and Cooling Equipment. https://www.nrel.gov/docs/fy02osti/31318.pdf  (accessed May 23, 2018).

[5] Whole Building Design Guide. 2016. High-Performance HVAC. National Institute of Building Science. https://www.wbdg.org/resources/high-performance-hvac (accessed Nov 28, 2018).

[5] NREL. Right-Size Heating and Cooling Equipment. https://www.nrel.gov/docs/fy02osti/31318.pdf

[6] Roth, Amir. 2017. “Building Energy Modeling 101: HVAC Design and Operation Use Case.” US DOE Building Technologies Program. https://www.energy.gov/eere/buildings/articles/building-energy-modeling-101-hvac-design-and-operation-use-case (accessed Nov 26, 2018).

[7] Whole Building Design Guide. 2016. High-Performance HVAC. National Institute of Building Science. https://www.wbdg.org/resources/high-performance-hvac (accessed Nov 28, 2018).

[8] DOE. “HVAC Right Sizing Part 1: Calculating Loads” https://www.energy.gov/sites/prod/files/2013/12/f5/webinar_hvac_calculatingloads_20110428.pdf (accessed May 23, 2018).

[9] DOE Building Technologies Program Advanced Strategy Guideline: Air Distribution Basics and Duct Design https://www.nrel.gov/docs/fy12osti/53352.pdf (accessed May 23, 2018).

[10] DOE. “HVAC Right Sizing Part 1: Calculating Loads” https://www.energy.gov/sites/prod/files/2013/12/f5/webinar_hvac_calculatingloads_20110428.pdf (accessed May 23, 2018).

[11] DOE Building Technologies Program Advanced Strategy Guideline: Air Distribution Basics and Duct Design https://www.nrel.gov/docs/fy12osti/53352.pdf (accessed May 23, 2018).

[12] Burdick, Alan. 2012. “Strategy Guideline: HVAC Equipment Sizing” Prepared for the US DOE Building America Building Technologies Program. https://www.nrel.gov/docs/fy12osti/52991.pdf (accessed November 27, 2018).

[13] Whole Building Design Guide. 2016. High-Performance HVAC. National Institute of Building Science. https://www.wbdg.org/resources/high-performance-hvac (accessed Nov 28, 2018).