Virginia will hold every generator in a new data-center air permit to a Tier 4-equivalent limit starting November 4.

In Virginia, the diesel generators behind a data center's backup power are now judged one unit at a time. A state rule taking effect November 4 says a new air permit for a data center cannot be issued unless every engine-generator set in it is limited to what a Tier 4-equivalent unit achieves.

What's confirmed

The rule. Virginia's State Air Pollution Control Board published a final regulation in the Virginia Register on October 5, 2026 (Vol. 43, Iss. 4), adding Article 7, "Emission Standards for Data Center Engine-Generator Sets (Rule 5-7)," to 9VAC5-50 as sections 9VAC5-50-610 through -630. Its effective date is November 4, 2026. The board adopted the amendments on August 18, 2026. [1][2]

Where it comes from. The rule implements House Bill 507, enacted as Chapter 397 of the 2026 Acts of Assembly and approved April 8, 2026, which added § 10.1-1322.6 to the Code of Virginia. Because it only conforms the regulations to that statute, the board used the exemption for changes "where no agency discretion is involved." [1][2][3]

Who it covers. The rule applies to "each data center that applies for a permit on or after July 1, 2026," anywhere in Virginia. "Data center" takes the state tax-code definition: a facility whose primary services are storing, managing, and processing digital data and that houses, among other things, equipment for at least one megawatt of electrical power and cooling capacity. [1][4]

The standard. For a data-center permit, including coverage under a general permit, DEQ "shall not issue such permit unless the emission limit for each gen-set established by such permit is equal to or less than the emissions achieved by a Tier 4 equivalent gen-set." [1][3]

What "Tier 4 equivalent" means. It is defined by controls, not by a certification label: selective catalytic reduction (SCR) or equivalent for nitrogen oxides, a diesel oxidation catalyst or equivalent for carbon monoxide, and a diesel particulate filter or equivalent for particulate matter. DEQ can still set stricter limits, and still makes its usual BACT, LAER, RACT, NSPS, and NESHAP determinations. [1]

The guidance that came first. DEQ's revised permit guidance for diesel engine-generator sets, APG-576, took effect April 9, 2026. It sets SCR, oxidation catalyst, and particulate filter controls as the presumptive best available control technology for both emergency and non-emergency gen-sets at data centers, for applications received on or after July 1, 2026. Presumptive controls for gen-sets at other kinds of facilities did not change. [5][6]

What the guidance says about operation and testing. APG-576 states that the traditional "limited operating hours" basis for not requiring SCR on emergency data-center gen-sets "is no longer appropriate," and that DEQ expects future applications to ask that most backup gen-sets be permitted as non-emergency units. It sets a presumptive NOx limit of 0.60 g/hp-hr for data-center gen-sets. Initial compliance testing calls for three one-hour CO and NOx runs per selected unit and a visible-emissions evaluation, with engines above 90 percent capacity unless DEQ approves load-band testing, typically on 20 to 25 percent of each gen-set type. [5]

Existing and replacement units in the guidance. APG-576 says modified gen-sets previously permitted as emergency units without SCR, and staying emergency units, keep the older presumptive controls, but "any new or replacement gen-sets included in a project" are subject to the SCR-based presumptive controls. It also keeps older presumptive controls for gen-sets that serve as secondary or tertiary backup or that power non-server loads. [5]

The debate. Data Center Knowledge reported in May that Compass Data Centers CEO Chris Crosby said Compass has stayed with Tier 2 generators because SCR systems can slow or shut down engines if emission controls fail during an outage. A Southern Environmental Law Center attorney called the move an improvement while noting that Tier 4 units may also run for non-emergency purposes. The Data Center Coalition pointed to a state legislative (JLARC) finding that the generators often run 10 to 30 minutes a month. [7]

FRG operational interpretation (not a legal or compliance finding)

The practical effect is that a new or expanded generator plant at a Virginia data center now starts from an assumption: aftertreatment on every unit. That decision used to be negotiated during permitting. It now belongs in early design and budget. Space for SCR and particulate filters, reagent storage and handling, and the way a control fault is handled while the site is on generator power all need to be settled before the application goes in. The testing requirements also mean someone has to supply load above 90 percent on the units that get tested, so commissioning plans should account for load-bank time.

The guidance and the statute do not line up perfectly in the sources reviewed. APG-576 keeps older presumptive controls for some existing emergency units and for secondary backup, while the statute and rule set a per-gen-set limit for any data-center permit applied for since July 1. How DEQ applies the statutory standard to a permit application that mixes existing, modified, and new units is a question for the DEQ regional office before filing, not an assumption.

The same rule shapes the other end of a generator's life. When Tier 2 units are retired, swapped for Tier 4-equivalent sets, or pulled during a site exit, buyers and receiving sites will ask what the equipment is permitted for. Keep the air permit and registration number, engine certification and tier documents, hour-meter logs, and any aftertreatment maintenance records with each unit. Do not assume a removed unit can be reinstalled at another Virginia data center under a new permit without meeting the Tier 4-equivalent limit. Check existing permit conditions before removing or replacing a permitted unit; DEQ publishes a list of issued data-center air permits that can help confirm which permit covers a site.

If you are planning a generator replacement, a fleet consolidation, or a Virginia site exit, Facility Recovery Group can help inventory what is installed, organize the documentation that goes with it, and scope removal or recovery of units you no longer need. Bring your gen-set list and permit number to the first conversation.