GEP stack height: why you can't just build a taller stack

A stack tall enough makes any ground-level problem disappear — which is exactly why the Clean Air Act closes that door. Emission limits must come from controlling pollution, not dispersing it higher, so the credit a stack's height earns in modeling is capped at Good Engineering Practice (GEP) height:

(a) The plan must provide that the degree of emission limitation required of any source for control of any air pollutant must not be affected by so much of any source's stack height that exceeds good engineering practice or by any other dispersion technique … This section does not require the plan to restrict, in any manner, the actual stack height of any source.

Source · 40 CFR §51.118(a) — quoted word-for-word from the current regulation text (eCFR, 2026-08-01 edition). The Guideline states the same prohibition for modeling: "The use of stack height credit in excess of Good Engineering Practice (GEP) stack height or credit resulting from any other dispersion technique is prohibited in the development of emissions limits" — App. W §7.2.2.1(a).

Note the two-sided design: you may build a stack as tall as you like — the rule caps only the height you may model when emission limits are being set. A 200-meter stack next to a small building is modeled at its GEP height, and the extra 100 meters earns nothing.

The definition: greater of three things

Good engineering practice (GEP) stack height means the greater of: (1) 65 meters, measured from the ground-level elevation at the base of the stack: (2) … For all other stacks, Hg = H + 1.5L where: Hg = good engineering practice stack height … H = height of nearby structure(s) … L = lesser dimension, height or projected width, of nearby structure(s) … (3) The height demonstrated by a fluid model or a field study approved by the EPA State or local control agency, which ensures that the emissions from a stack do not result in excessive concentrations of any air pollutant as a result of atmospheric downwash, wakes, or eddy effects created by the source itself, nearby structures or nearby terrain features.

Source · 40 CFR §51.100(ii) — quoted word-for-word from the source document. (Clause (2)(i), Hg = 2.5H, survives only for pre-1979 stacks whose permits actually relied on it.)

The GEP formula height, Hg = H + 1.5L (§51.100(ii)(2)(ii)) — L = lesser of the nearby structure's height or projected width

The formula — which the U.S. Environmental Protection Agency (EPA) calls its refined formula height — is downwash physics turned into law: a building disturbs the flow above and behind it to roughly its height plus 1.5 times its lesser dimension, so a stack top above H + 1.5L sits above the building's wake. The 65-meter floor means small buildings never drag GEP below a useful minimum, and the fluid-model route (3) exists for sources that need credit beyond the formula — at the price of demonstrating "excessive concentrations" as defined in §51.100(kk): a downwash-caused excess of at least 40 percent that contributes to exceeding an ambient standard.

"Nearby" is defined, not eyeballed

Which buildings count toward the formula is itself codified:

…that distance up to five times the lesser of the height or the width dimension of a structure, but not greater than 0.8 km (1/2 mile)

Source · 40 CFR §51.100(jj)(1). For fluid-model demonstrations, terrain features reach farther — up to 10× the feature height, capped at 2 miles, with a 40%-of-GEP prominence test (§51.100(jj)(2)).

So a tall structure 300 meters away may control your GEP height while a taller one a kilometer away is irrelevant — the 5L radius is why building inventories for downwash analysis are assembled per stack, not per plant.

What it means inside the model

GEP cuts both ways in an AERMOD analysis, per the Guideline (App. W §7.2.2.1(b)):

Stack below formula height → "air quality impacts associated with cavity or wake effects due to the nearby building structures should be determined" — this is the building-downwash analysis, with building dimensions processed through BPIPPRM and the PRIME algorithm inside AERMOD (the subject of this guide's Downwash pages).
Stack at or above formula height → downwash "should also be considered," because the excessive-concentration definition (40% excess) can still bite; AERSCREEN offers a screening estimate, AERMOD the refined one.
Stack above GEP → modeled at GEP height, not actual height, when emission limits are being developed (§51.118 / App. W §7.2.2.1(a)).

In PlumeSmart

Building geometry entered with a project is processed through BPIPPRM, which computes the GEP formula values alongside the direction-specific building dimensions AERMOD's downwash algorithm needs — so the same building data answers both the GEP question and the downwash analysis.