The 90.1 User’s Manual offers straightforward HVAC system requirements and multiple examples.
Standard 90.1-2001 is not an easy read. Energy efficiency is not a simple topic. Thankfully, however, the90.1 User’s Manualcuts to the heart of compliance for commercial buildings, spelling out simple solutions, what is required, what isn’t, how to make trade-offs (for instance, so that IAQ isn’t sacrificed to efficiency). It also details how to calculate the efficiency of more complex designs.

The manual includes a CD containing Envstd 4.0 energy calculation software and compliance forms. The entire package was prepared by Eley Associates of San Francisco and is available through ASHRAE (

The updated energy efficiency standard covers HVAC systems, service hot water, building envelopes, and lighting for commercial, industrial, institutional, and residential high-rise buildings. It is the result of cooperative work from the American National Standards Institute (ANSI), the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), and the Illuminating Engineering Society of North America (IESNA), and is therefore named ANSI/ASHRAE/ IESNA Standard 90.1-2001.

According to Charles Eley, “This manual illuminates the standard through the use of abundant sample calculations and examples, streamlines the process of showing compliance, and provides standard forms to demonstrate compliance.”

This edition of the User’s Manual “is consistent with Standard 90.1-2001, which includes addenda approved through October 2001,” according to Eley, who noted that the standard has reached a developmental stage called “continuous maintenance.” “Addenda, errata, and interpretations will be issued throughout its life,” Eley said.


Standard 90.1 does not claim to offer the ultimate solutions in energy-efficient building design. It sets forth design requirements to achieve baseline efficiency savings — and something to build on if still greater savings are desired.

The standard, which is written in mandatory language and intended for code use, must be referenced by every U.S. state. The states must certify that their building codes meet the requirements in the standard, under a ruling issued last year by the U.S. Department of Energy (DOE).

Energy standards from ASHRAE are also being included in the first full set of safety codes being developed through a consensus-based process accredited by ANSI.

The National Fire Protection Association (NFPA), overseeing the safety codes’ development, recently approved publication of a key element, the NFPA 5000™, Building Construction and Safety Code™.

What Is Covered, What Is Not

Standard 90.1 applies to all buildings except “low-rise” residential buildings (three habitable floors or less). Low-rise commercial buildings are covered by 90.1. Low-rise residential structures are covered by Standard 90.2.

Standard 90.1 also does not apply to “manufactured houses (modular or mobile homes); buildings that do not use either electricity or fossil fuel; or equipment and portions of building systems that use energy primarily to provide for industrial, manufacturing, or commercial processes,” stated Eley.

“The main focus of the standard,” he said, “is on new buildings. Every new building has its own site that presents unique opportunities and challenges; each building owner or user has different requirements; and climate and microclimate conditions can vary significantly among projects.

“The standard provides flexibility in a number of ways. Each of the technical sections provides multiple compliance paths.”

Existing buildings are also covered in the User’s Manual. Considerations include whether an existing building is getting new construction, such as an addition, or if unconditioned space is being converted to conditioned space (heating and/or cooling is added for the first time).

Addition: The standard applies to the addition alone, not to the rest of the existing building. In other words, “treat the addition as if it were its own separate building,” states the manual.

Alterations: “In general, the standard only applies to new building systems and equipment (e.g., building envelope, HVAC, service water, heating, powering, lighting, and electric motors),” states the manual. “The standard does not apply to building systems or equipment that are not being altered or repaired, unless there is a change in space conditioning.”

Historic buildings — those that have been designated as “historically significant” by the authority having jurisdiction or listed (or eligible for listing) in the National Register of Historic Places — are exempt from the requirements of the standard for building alterations.

HVAC Overview

This article cannot do justice to all the information contained in the manual regarding HVAC equipment. Contractors need to examine the manual themselves to appreciate its depth.

HVAC equipment “that is a direct replacement of existing equipment must meet the standard’s efficiency requirements. This applies, but is not limited to, air conditioners and condensing units, heat pumps, water chilling packages, PTACs and PTHPs, furnaces, duct furnaces, unit heaters, boilers, and cooling towers.”

According to the manual, “There are a number of important instances when the standard does not apply to replacement HVAC equipment.” For example:

  • When equipment is repaired but not replaced;

  • When the replacement of existing equipment with complying equipment requires extensive revisions to other systems, equipment, or building elements, and the replacement equipment is a like-for-like replacement;

  • When refrigerant in existing equipment is changed; and

  • When existing equipment is relocated.

    Compliance Methods

    The manual describes three approaches to HVAC compliance:

    1. Simplified Approach — Applicable to relatively simple systems in small buildings.

    2. Prescriptive Path — May be used for any HVAC system, but it is primarily used for the complex systems in larger buildings, where the simplified approach is not applicable, such as VAV systems and central hydronic heating and cooling plants.

    3. Energy Cost Budget (ECB) method — Designed for building systems that are unable to meet prescriptive requirements. It allows for trade-offs between various building systems and components.

    The Simplified Approach (the only approach this article will touch upon) will suit contractors whose products and services go into “small” buildings (those less than 25,000 square feet). According to ASHRAE, buildings in this category represent more than 80% of new construction building starts in the U.S. “They are generally served by simple, single-zone HVAC systems,” states the manual.

    “Many HVAC 90.1 requirements do not apply to these simple systems; rather than require designers to search through the entire section for requirements that do apply, these requirements are grouped into one section, under the heading of ‘Simplified Approach.’”

    This approach can be used with:

  • Buildings served by the system that are two stories or less in height;

  • Buildings less than 25,000 square feet in gross floor area;

  • Buildings in which the HVAC system must serve a single zone. “Systems with any level of subzoning (i.e., systems with more than one thermostatic control) cannot use this approach for showing compliance,” points out the manual; and

  • Systems that provide no mechanical cooling or that use a unitary packaged or split-system air conditioner that is air cooled or evaporatively cooled.

    The section also covers heating systems, outdoor air heat recovery, thermostats, heat pump auxiliary heat control, reheat for humidity control, off-hour shutoff and setback controls, piping and ductwork insulation, air balancing, simultaneous heating and cooling, shutoff dampers, and optimum start controls.

    Figure 1. Perimeter system zoning served by two independent HVAC systems is an exception to the zone control requirement.

    Mandatory Provisions

    Some things are mandatory whenever the contractor or designer uses the Prescriptive Path or ECB method. Among them are:

  • Part-load as well as full-load operation considerations — According to ASHRAE, an earlier version of the standard addressed full-load efficiency at standard rating conditions, “representative of typical peak design conditions.” In the 1989 version, part-load efficiency limits were added for most equipment types. Standard 90.1-2001 “continues to recognize full- and part-load efficiency, but the level of stringency has increased.”

  • Load calculations — “The designer must make heating and cooling load calculations before selecting or sizing HVAC equipment.” This helps ensure “that equipment is neither oversized nor undersized for the intended application.”

    It doesn’t mandate which load calculation procedure to use, only that it be in accordance with “engineering standards and handbooks acceptable to the adopting authority (for example, ASHRAE Fundamentals Handbook 2001).” That’s the most recent edition of the Handbook, not the 1989 edition.

  • Zone thermostatic controls — “The supply of heating and cooling to each zone whose load characteristics are sufficiently similar, must be individually controlled by a thermostatic controller that senses temperature within the zone.” The exceptions are perimeter zones served by two independent HVAC systems. (See Figure 1.)

  • Off-hour controls — “The standard requires that the system be equipped with automatic controls to provide automatic shutdown, setback controls, optimum start controls, and zone isolation.” Exceptions include HVAC systems serving hotel/motel guestrooms and systems intended to operate continuously (such as hospital systems).

  • Humidifier preheat.

  • Humidification and dehumidification — These cannot run simultaneously, except desiccant cooling systems with direct evaporative cooling in series, and systems serving zones where specific humidity levels are required, such as computer rooms and museums.

    The User’s Manual includes several examples of all the methods cited. It should prove to be a valuable tool for HVAC contractors and designers, as well as to the owners of the systems they create.

    Publication date: 01/20/2003