H1 Stack Height Assessment UK (EA D1 Screening)

H1 Stack Height Assessment for EA Permits, D1 Screening and Detailed Dispersion Modelling

An H1 stack height assessment determines the minimum chimney height needed for emissions to disperse safely and meet the Environment Agency's H1 environmental risk criteria. Assessments are required for new bespoke permits, variations, and where existing flues are altered, replaced or relocated. We deliver D1 screening for low-complexity sites and full ADMS-based detailed assessments where buildings, terrain or sensitive receptors mean screening is not appropriate. Outputs are written for the EA, with clear assumptions, model setup and a defensible recommended height. Where modelled emission rates need source data, we pair H1 outputs with our MCERTS stack emissions testing and stack emissions monitoring service.

When an H1 Stack Height Assessment Is Required

• Bespoke EPR permit applications discharging to air
• Permit variations that change emission rate, temperature, velocity or stack location
• Replacement or shortening of an existing flue
• MCP and Specified Generator installations where building downwash or terrain effects apply
• Planning conditions requiring justification of proposed chimney height

D1 Screening vs Detailed Modelling

EA D1 is a screening calculation suitable only for simple, isolated stacks with no significant building or terrain influence. Where buildings within 5L of the stack, complex topography, sensitive receptors or multiple discharge points are present, D1 will under-predict required height and the EA expects a detailed assessment using ADMS or AERMOD with building downwash (PRIME) and appropriate meteorological data.

Methodology and Inputs

• Source data: emission rates, exit velocity, temperature, internal diameter, operating hours
• At least 3 years of representative hourly Met Office MIDAS data for the nearest suitable station
• OS terrain data and building heights/footprints within the influence zone
• Background concentrations from Defra mapped data and any relevant AQMA designations
• Receptor grid plus discrete receptors at residential, ecological and amenity locations
• Comparison of predicted process contribution against EAL/AQAL screening thresholds

What the Report Delivers

• Recommended minimum stack height with sensitivity test on +/- 1 m increments
• Process contribution and predicted environmental concentration tables for each pollutant
• Building downwash assessment and figures showing receptor-by-receptor effect
• Clear EA-ready conclusion: significant or insignificant against H1 criteria
• Modelling files retained for EA review or duly-made checks

What We Need From You to Scope an H1 Assessment

• Site location and a layout plan showing the proposed discharge point
• Plant/process description and operating regime (hours, loads, fuels)
• Proposed or measured emission rates, exit temperature, velocity and stack diameter
• Nearby buildings within the stack-influence zone (heights and footprints)
• Any sensitive receptors, AQMA, SAC/SPA or amenity considerations
• Permit reference and any prior modelling, screening or EA correspondence

What affects stack height

• Emission rate and pollutant type (mass flow and toxicity / EAL of each pollutant)
• Stack diameter, exit velocity and volumetric flow
• Exhaust temperature and resulting plume buoyancy
• Nearby buildings within the stack-influence zone (downwash effects)
• Terrain and topography surrounding the site
• Nearby sensitive receptors (residential, ecological, amenity)
• Process operating pattern (continuous, peaking, intermittent)
• Dispersion risk in the local meteorological context

Worked example: combustion plant on a constrained site

A site installing a new combustion plant may need to demonstrate that emissions are released at a height that supports adequate dispersion to nearby receptors. In a typical case, Alkali reviews the proposed plant, stack details, site layout, surrounding buildings, nearby sensitive receptors and the permit context, then advises whether D1 screening is sufficient or whether a detailed ADMS-based H1 assessment — and, where appropriate, full air dispersion modelling — is required. The output is a defensible recommended stack height with the supporting evidence the Environment Agency expects to see in the permit application.

How H1 links to air dispersion modelling

H1 D1 screening is appropriate only for simple, isolated stacks with no significant building, terrain or receptor influence. Where any of these factors are present, more detailed air dispersion modelling using ADMS or AERMOD is required, typically with PRIME building downwash treatment and at least three years of representative met data. In practice, most industrial sites move beyond D1 screening into a full modelled H1 assessment, which is delivered alongside the modelling report so the application is duly-made on first submission.

Related permitting and modelling services

H1 stack height assessments are commonly paired with ADMS air dispersion modelling where D1 screening indicates further work, and form part of broader H1 environmental risk assessments for new permits. The same calculations underpin Medium Combustion Plant permit applications and full environmental permit applications, with downstream evidence delivered through our MCERTS stack emissions testing across the UK. To scope an assessment, request a fixed-fee quote or discuss your permit requirements with a senior consultant.