ASHRAE 90.1-2019 Mechanical Requirements: Economizer and Energy Recovery Rules

ASHRAE 90.1-2019 §6.5.1 requires an economizer on fan-cooling units at or above 54,000 Btu/h in every climate zone except 0A, 0B, 1A and 1B, and §6.5.6.1 requires exhaust air energy recovery when a system’s design supply airflow exceeds a table value set by climate zone, outdoor air percentage and operating hours. The economizer threshold is the 2019 edition as published (the base row shown in ASHRAE’s addendum ci); the energy recovery rules follow the 2019 text as modified by addendum bz. Both addenda are free from ASHRAE and were approved 2022-04-29. Confirm the edition and addenda your jurisdiction has adopted before sizing equipment.

When is an economizer required under ASHRAE 90.1-2019?

An economizer lets a cooling system use cool outdoor conditions in place of mechanical cooling. Table 6.5.1-1, “Minimum Fan-Cooling Unit Size for which an Economizer Is Required,” sets the threshold by climate zone. In the 2019 edition as published:

Addendum ci (2022) lowers the threshold to 33,000 Btu/h in those zones for individual fan-cooling units located outside the envelope of the building; other locations stay at 54,000 Btu/h. Section 6.5.1 also carries exceptions for particular system types and conditions. Read them in the standard before documenting an economizer path.

When is exhaust air energy recovery required?

Section 6.5.6.1.2 of 90.1-2019, as modified by addendum bz, requires each fan system serving spaces other than nontransient dwelling units to have an energy recovery system when its design supply fan airflow exceeds the value in Tables 6.5.6.1.2-1 and 6.5.6.1.2-2. The value depends on the climate zone and the percentage of outdoor air at design airflow. The addendum text splits the tables by operating hours, fewer than 8,000 hours per year versus 8,000 or more, but prints the same table number for both, so confirm the table numbers in the published standard. Read the tables in the standard for the airflow values.

Where energy recovery is required, §6.5.6.1.2.1 sets a minimum enthalpy recovery ratio of 50% at the cooling design condition. A 50% ratio means the outdoor-air supply enthalpy changes by 50% of the difference between outdoor air and entering exhaust air enthalpies at design conditions. At the heating design condition the system must reach a 50% enthalpy recovery ratio where active humidification is provided, or a 50% sensible energy recovery ratio where it is not.

Section 6.5.6.1.2 lists exceptions, including laboratory systems meeting §6.5.7.3, systems serving spaces that are not cooled and are heated to less than 60°F, and systems expected to operate fewer than 20 hours per week at the table’s outdoor air percentage. Nontransient dwelling units (§6.5.6.1.1) have their own rule: energy recovery ventilation at an enthalpy recovery ratio of at least 50% at the cooling design condition and at least 60% at the heating design condition, with exceptions by climate zone and unit size.

What other mechanical rules should be checked against the standard?

This guide does not cover demand control ventilation, pump controls or setback rules. Read the following in the adopted edition:

• §6.4 (mandatory provisions) and §6.5 (prescriptive provisions) for ventilation controls, pumps and setback
• §6.5.3.8: occupied-standby mode, which ties zone airflow to occupant sensors (DOE 90.1-2019 webinar)
• §6.8: minimum equipment efficiencies

Healthcare facilities also need review against ASHRAE Standard 170, which sets ventilation requirements for those spaces. Single-family residential buildings are outside ASHRAE 90.1; the IECC residential provisions govern them.

Frequently asked questions

What cooling capacity triggers an economizer under ASHRAE 90.1-2019?

54,000 Btu/h per fan-cooling unit in climate zones 2A through 8, with no requirement in zones 0A, 0B, 1A and 1B (Table 6.5.1-1). Addendum ci lowers the threshold to 33,000 Btu/h for fan-cooling units located outside the building envelope.

When does a system need energy recovery?

When the design supply fan airflow exceeds the value in Tables 6.5.6.1.2-1 and 6.5.6.1.2-2 for its climate zone, outdoor air percentage and operating hours, unless an exception applies. The minimum enthalpy recovery ratio is 50% at the cooling design condition.

Sources

• ASHRAE, Addendum ci to ANSI/ASHRAE/IES Standard 90.1-2019 (Table 6.5.1-1).
• ASHRAE, Addendum bz to ANSI/ASHRAE/IES Standard 90.1-2019 (§6.5.6.1).
• ASHRAE 90.1 itself is paywalled; read §6 of the adopted edition before specifying.