Virginia Tightens Air Permitting for Data Center Backup Generators: Tier 4-Equivalent Emissions Limits Now Required

Summary — Virginia has adopted new emissions requirements for data center backup generators, requiring applications submitted on or after July 1, 2026, to meet Tier 4-equivalent standards. The rules may affect generator selection, permitting, project costs and ongoing compliance obligations. Developers and operators should also monitor further DEQ guidance, particularly regarding how the requirements will apply to existing facilities that add or replace generators.

Virginia has adopted new air emissions requirements for data center backup generators, marking a significant shift in how the Commonwealth regulates one of the fastest-growing sources of stationary diesel engine emissions. Data center developers and operators should assess how these requirements affect pending and future air permit applications, control technology selection, and ongoing compliance obligations.

House Bill 507 (HB 507), enacted during the 2026 General Assembly Session as Chapter 397 of the 2026 Acts of Assembly and codified at Virginia Code § 10.1-1322.6(B), prohibits the Virginia Department of Environmental Quality (DEQ) from issuing an air permit, including coverage under any general permit, for any data center application submitted on or after July 1, 2026, unless the emission limit established by the permit for each engine-generator set (gen-set) is equal to or less than the emissions achieved by a Tier 4-equivalent gen-set.

The State Air Pollution Control Board has also adopted implementing regulations, published in the Virginia Register on October 5, 2026, that will become effective November 4, 2026. See 9VAC5-50-610 through 9VAC5-50-630, 43:4 Va. Reg. Regs. 280-81 (Oct. 5, 2026). These implementing regulations largely mirror the statutory requirements without providing additional substantive standards or implementation procedures.

The implementing regulations build on DEQ’s revised generator permitting guidance, effective April 9, 2026, which identifies Tier 4-equivalent controls as presumptive Best Available Control Technology for data center diesel generators for applications received on or after July 1, 2026. See Va. Dep’t of Env’t Quality, Guidance Memo No. APG-576, Diesel Engine-Generator Set Procedure for Writing New and Modified Permits (rev. Apr. 9, 2026) (“Generator Permitting Guidance”).

Virginia’s New Emissions Requirements

The statute defines a “Tier 4 equivalent” generator as one equipped with, or using equivalent technology to achieve, three categories of emissions control:

• Selective catalytic reduction (SCR) (or equivalent) to reduce nitrogen oxides (NOx);
• Diesel oxidation catalyst (DOC) (or equivalent) to control carbon monoxide; and
• Diesel particulate filter (DPF) (or equivalent) to reduce particulate matter.

The standard is performance-based: it does not necessarily require a generator bearing a particular Tier 4 certification, so long as equivalent technologies can achieve emissions performance equivalent to that of a Tier 4-equivalent generator. Importantly, the regulations do not establish specific numerical emission limits for demonstrating Tier 4-equivalent performance. However, DEQ’s Generator Permitting Guidance identifies presumptive BACT emission limits and associated control technologies for data center generators. It remains unclear whether DEQ will rely on the existing guidance when implementing the new regulations or issue additional guidance addressing Tier 4-equivalent emissions performance, including how equivalency must be demonstrated.

DEQ also retains authority to impose more stringent requirements under other state or federal law, so Tier 4-equivalent performance is not necessarily the ceiling for every project.

The requirements apply statewide to data center air permit applications submitted on or after July 1, 2026, including general permit applications. Neither the statute nor the implementing regulations expressly clarifies how the requirements apply to modifications of existing facilities. In particular, it remains uncertain whether a permit modification involving the addition or replacement of a generator could trigger Tier 4-equivalent emission limits for previously permitted generators that are not themselves being modified (i.e., whether existing emission limits are considered “established by such permit” under various modification scenarios). Notably, DEQ’s existing Generator Permitting Guidance recognizes the costs and complexities associated with retrofitting existing generators, and provides for less stringent presumptive BACT requirements in certain circumstances. However, the new rules for data center generators do not expressly provide similar flexibility, leaving uncertainty as to how DEQ will address existing generators when implementing the new requirements.

Air Permitting Implications

For new data center air permit applications, the Tier 4-equivalent standard will be a central element of permitting strategy. Applicants should confirm early whether each engine-generator set proposed for the facility can achieve the required emissions limits.

A generator selected before the new requirements took effect may not achieve the limits applicable to a later permit application. In that case, applicants may need to add SCR, DOC, and DPF controls, or other technology capable of achieving equivalent performance, before DEQ will issue the permit.

Because the standard is performance-based, applicants relying on alternative or equivalent technologies should be prepared to support that equivalency in the permit application. Applicants should also be mindful that DEQ may impose more stringent limits where other state or federal requirements apply.

Although the new requirements may increase the upfront costs of generator installations, the resulting emissions reductions may also alleviate certain permitting and compliance challenges. In recent years, data centers have encountered difficulties complying with synthetic minor permit conditions that impose restrictive operating hour or fuel throughput limits. The substantial reduction in emissions associated with Tier 4-equivalent performance may provide greater operational flexibility, including the ability to accommodate larger generator fleets or prolonged, unexpected power outages. Reduced reliance on such operational limits could also simplify compliance demonstrations, particularly where exceedances of operating restrictions would otherwise require facilities to reconstruct actual emissions using generator-specific emission rates at varying loads and conduct stack testing to substantiate those rates in an effort to mitigate potential penalties.

Compliance Demonstration and Ongoing Obligations

The new requirements are likely to create practical compliance challenges beyond the permit application stage. Existing data centers that add Tier 4-equivalent generators may need to manage two sets of obligations at the same facility: one set of monitoring, recordkeeping, and reporting requirements for previously permitted generators, and another for new generators subject to Tier 4-equivalent limits. For example, SCR-equipped generators may carry conditions related to reagent (e.g., urea) usage, catalyst temperature, and aftertreatment system performance that do not apply to older units. Tracking those differing obligations generator by generator, often across facilities with large fleets, will require careful attention to compliance systems and recordkeeping practices.

DEQ may also impose more extensive or recurring stack testing requirements to verify that the controls are achieving the required emission reductions. Other jurisdictions have taken this approach, issuing permits for SCR-equipped data center generators that require initial testing of a representative generator, followed by annual testing of different generators each year to confirm that the SCR systems continue to perform as intended. If DEQ follows a similar approach, operators should anticipate the cost, scheduling, and operational coordination associated with testing multiple units over the life of the permit, as well as the compliance consequences if a tested unit fails to demonstrate the required performance.

In this context, a manufacturer’s Tier 4-compliant label or performance representation does not by itself establish that an installation satisfies the emissions limits and other conditions of the facility’s DEQ air permit. Because aftertreatment controls depend on proper operation, including reaching adequate operating temperatures, emergency generators that run infrequently or at low loads during routine maintenance and readiness testing may present particular challenges in demonstrating ongoing performance.

Finally, given how limited the statute and implementing regulations are, data center developers and operators should closely track how DEQ begins to implement the new requirements, including through permit conditions, guidance, and its treatment of individual applications. The modification question discussed above is particularly important to monitor: how DEQ treats previously permitted generators when a facility adds or replaces units could significantly affect expansion plans, retrofit costs, and compliance obligations at existing sites. In the meantime, records of the permit application submission date, generator specifications, control technologies, and compliance demonstrations will be particularly important if applicability or compliance is later questioned.

Timing and Transition Issues

Applicability turns on when the air permit application is submitted. Applications submitted on or after July 1, 2026, including those seeking general permit coverage, are subject to the new limits, so project teams should confirm the submission timing for each permitting action.

Facilities planning expansions or additional generator capacity should evaluate whether new or amended permit applications will bring those generators within the new requirements and should coordinate early with DEQ where applicability is unclear

Compliance may also affect equipment costs and project schedules, so developers should ensure that procurement decisions and project contracts reflect the applicable permitting requirements and clearly allocate compliance responsibility.

Practical Considerations for Data Center Developers and Operators

Data center developers, operators, and their environmental consultants should consider the following steps:

• Confirm permitting applicability. Determine when the applicable DEQ air permit application was or will be submitted and whether the new emissions requirements govern the project.
• Evaluate control technology. Confirm whether planned generators achieve Tier 4-equivalent limits or will require SCR, DOC, DPF, or equivalent controls.
• Prepare compliance support. Assemble manufacturer data, emissions information, and other documentation needed to support the permit application, particularly where equivalent technologies are proposed.
• Plan for operating obligations. Develop operation, maintenance, monitoring, recordkeeping, and reporting procedures to satisfy permit conditions, including separate tracking for previously permitted generators and new Tier 4-equivalent generators at the same facility, and budget for initial and periodic stack testing that DEQ may require to confirm control performance.
• Monitor regulatory developments. Track implementation of the regulations effective November 4, 2026, and any further DEQ guidance, including how DEQ treats previously permitted generators when a facility adds or replaces units.
• Coordinate with project teams. Ensure procurement and contracts reflect the permitting requirements and allocate responsibility for emissions compliance.

Looking Ahead

Virginia’s Tier 4-equivalent standards reflect increased regulatory scrutiny of the air emissions associated with data center backup power. As data center development continues across the Commonwealth, these requirements will play a central role in air permitting and long-term environmental compliance.

Developers and operators that address these issues early, through careful control technology selection, well-supported permit applications, and robust compliance programs, will be better positioned to obtain permits efficiently and maintain compliance as Virginia’s requirements evolve.