How Standard 90.1-2010 will affect HVAC designs

ASHRAE 90.1-2010 is a substantial step forward in reducing building energy consumption. Designers should become familiar with the requirements of this energy standard.


ANSI/ASHRAE/IES Standard 90.1-2010 – Energy Standard for Buildings Except Low-Rise Residential Buildings includes many changes from the 2007 edition. These changes will affect the design, construction, and operation of all facilities. This article covers HVAC changes that have the greatest impact on most buildings.

Extensive energy simulations were performed by Pacific Northwest National Laboratory and included 16 building models in 17 climate zones. These simulations indicate that, on average, buildings conforming to 90.1-2010 will consume between 25.6% (including plug loads) and 32.7% (excluding plug loads) less energy than the same buildings conforming to 90.1-2004.

A major change adds to the scope of 90.1 “new equipment or building systems specifically identified in the Standard that are part of industrial or manufacturing processes.” The one process that is specifically identified in 90.1-2010 is computer rooms. Prior to the 2010 standard, computer rooms were exempted from the prescriptive economizer requirements, but they now must comply with the economizer requirements in 90.1-2010.

Supply-air temperature reset controls are required for multiple-zone HVAC systems that include reheat and that do not provide at least 75% of the reheat energy from site recovered or site solar energy. They shall reset at least 25% of the way between the supply-air temperature and the design room temperature based on building loads or outdoor air temperature or zone humidity.

Conforming to the fan power limits has been difficult in some building types. The fan power requirements of 90.1-2010 and 2007 are similar, but there are a few differences. The basic requirement for variable air volume (VAV) systems is that they should not require more than 1.3 bhp per 1,000 cfm of supply air, plus allowances for features such as fully ducted return and exhaust systems, filters with a MERV rating of more than 8, and energy recovery devices. These allowances tend to allow inpatient healthcare facilities approximately 2.2 bhp per 1,000 cfm of supply air. The fan power allowance for energy recovery devices was changed to be “(2.2 x energy recovery effectiveness) – 0.5 in. w.c. for each airstream,” which is intended to encourage designers to size energy recovery devices with low enough face velocities to provide high energy recovery effectiveness and low air pressure drop. The exception for systems exhausting fume hoods was replaced with an allowance of 0.35 in. w.c.

The 2010 edition includes a table that bases exhaust air energy recovery requirements on flow rate, climate zone, and percentage of outside air, which will increase the range of applications where energy recovery is required (see Table 1).

Table 1: Table shows ASHRAE’s energy recovery requirement. Courtesy: ASHRAE

Reheat can be a large percentage of building energy use, especially in buildings with high ventilation rate requirements. Many of the exceptions that permit reheat were eliminated or reduced. The remaining exceptions are:

  1. 30% of the zone design peak supply rate; or 50% during heating if the flow rate is reduced to 20% in deadband.
  2. The outdoor airflow rate required to meet the ventilation requirements of Section 6.2 of ASHRAE Standard 62.1 for the zone (this applies exclusively to 100% outside air systems).
  3. Any higher rate that can be demonstrated, to the satisfaction of the authority having jurisdiction, to reduce overall system annual energy usage by offsetting reheat/recool energy losses through a reduction in outdoor air intake for the system.
  4. The airflow rate required to comply with applicable codes or accreditation standards (e.g., pressure relationships or minimum air change rates).
  5. Laboratory exhaust systems complying with (which requires buildings with lab exhaust systems totaling more than 5,000 cfm to comply with one of three criteria relating to VAV, energy recovery, and non- or semi-treated makeup air).
  6. Zones where at least 75% of the energy for reheating or for providing warm air in mixing systems is provided from a site-recovered (including condenser heat as described below) or site-solar energy source.

The fourth exception replaces the 2007 exception b, which permitted constant volume reheat systems if any zone required pressure control. This essentially bans constant air volume central HVAC systems for most healthcare and laboratory facilities.

Another effect of the reheat changes to Standard 90.1 is that airflow rates to unoccupied pressure-controlled spaces without minimum unoccupied airflow rate requirements, such as operating rooms that are regulated by ASHRAE 170 and/or the 2010 Facility Guidelines Institute (FGI) guidelines, must be reduced to the amount necessary to maintain the required pressure relationship or 30% of the peak cooling flow rate, whichever is larger.

A change that is intended to further minimize reheat energy limits the supply air temperature of reheated air to 20 F above the space temperature setpoint if both the supply and the return/exhaust air openings are more than 6 feet above the floor. This will minimize short-circuiting of reheated air to the return or exhaust system. Standard 90.1 does not use the more restrictive 15 F limit that triggers higher outside air requirements in Standard 62.1. The temperature of 20 F was chosen as a compromise between the requirement of Standard 62.1 and concerns about added costs for perimeter heating systems. Occupancies that cannot meet their peak heating loads with their maximum allowed reheat flow rates will need to either add perimeter heat sources or use envelope construction with low enough heat loss to be heated by the maximum flow rate that is permitted to be reheated, if they are using the prescriptive compliance path.

ASHRAE 90.1-2007 included an exception to the economizer requirements for systems that provided humidification above 35 F dew-point temperature. This exempted most healthcare facilities from the economizer requirements. In the 2010 standard, this exception has been revised. For many healthcare, laboratory, and computer room air handling systems, this change will require economizers. Healthcare occupancies with some, but not more than 75%, of the air required to be humidified above 35 F dew-point will require water economizers. When the 2010 FGI guidelines and/or Standard 170 are adopted by the local AHJ, most humidification requirements will drop to 20%, which does not produce a dew point above 35 F at normal temperatures, and would therefore allow using air economizers. A comparison of economizer options on energy use can be found in “The AEDG for Small Healthcare Facilities.

Facilities will still be exempt from the economizer requirement if they comply with section, which contains requirements for the use of condenser heat for service water heating.

ASHRAE 90.1-2010 adds economizer requirements for HVAC systems in climate zones 2a, 3a, and 4a, and lowers the minimum system capacity threshold to 54,000 Btuh.

All ducts are now required to be constructed to seal class A, which requires sealing of all transverse joints, longitudinal seams, and duct wall penetrations. Rotating shaft openings now require bushings or bearings to minimize leakage. The owner or the owner’s representative now chooses which duct sections are to be leakage tested. The leakage class was changed to 4 cfm/100 sq ft for all tested ducts, both round and rectangular. This differs from the Sheet Metal and Air Conditioning Contractors’ National Assn.  HVAC Air Duct Leakage Test Manual, which lists 6 cfm/100 sq ft for rectangular and 3 cfm/100 sq ft for round ducts.

90.1-2010 added several miscellaneous requirements including:

  • Designers must calculate pump head requirements.
  • A table was added showing minimum sizes for chilled water (including glycol) and condenser water pipes. For most engineers this will require pipes above 6 in. to be larger than the common practice of 4 to 5 ft. per 100 ft. of friction loss.
  • Hydronic systems with more than 10 hp of total pump system power must be variable flow, and be capable of reducing flow rates to 50% or less of the peak flow rate.
  • Chilled water systems must use variable frequency drives or other means to reduce pumping power to not over 30% at flow rates below 50% of the peak flow rate, and the differential pressure setpoint must be reset based on valve positions.
  • Insulation with a thermal resistance of at least R-3.5 is required behind radiant heating panels.
  • Single-zone DX systems ≥110,000 Btuh must reduce their airflow rates to 67% prior to reheating.
  • Single-zone chilled water systems ≥ 5 hp must reduce their airflow rates to 50% prior to reheating.
  • Minimum prescriptive efficiencies were added for variable refrigerant flow systems and computer room air conditioners.

Domestic water booster systems are now prohibited from including pressure-reducing valves (PRVs) that all of their discharge water passes through. This does not prohibit using PRVs for zones or individual floors, but it does prohibit pumping all of the water to a high pressure and then reducing it at the booster pump. Booster pump systems also must include a pressure sensor to start and/or control their speed. The sensor must be mounted remotely in the system, or logic must be included in the package that senses flow and simulates a remote sensor input. When there is no domestic water demand, booster pumps must automatically stop.

Saving even more energy by going beyond 90.1-2010

The Advanced Energy Design Guide (AEDG) series of free downloadable guidelines are developed by ASHRAE and funded by the Dept. of Energy. They provide additional guidance on reducing building energy use associated with HVAC systems. The overall intended energy savings of the 30% AEDG documents are similar to 90.1-2010, but some recommendations may result in additional energy savings over 90.1-2010. ASHRAE is now working on the 50% energy savings AEDG series, and several of those guides have been published. “The AEDG for Large Healthcare Facilities” is scheduled to be published in January 2012.

ASHRAE 189.1 Standard for the Design of High-Performance, Green Buildings is currently being revised. It will reference 90.1-2010 and will include additional suggestions that in some cases provide more energy savings than 90.1-2010.

One strategy that can exceed the requirements of 90.1-2010 with good economic return is using condenser energy recovery for reheat (not just domestic preheat). At Midwest utility rates, these systems typically provide a 3- to 5-year return on investment for facilities with high minimum air change rates, such as hospitals and laboratories.

Boldt is a principal and the director of engineering for KJWW Engineering. He is a member of ASHRAE, ASHRAE SSPC 90.1, and the 90.1 mechanical subcommittee. He is a co-author of “The AEDG for Small Healthcare Facilities” and “The AEDG for Large Healthcare Facilities.”

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