HVAC UV: In-Duct vs. UV Chamber — Two Installation Modes

Quick answer

UV-C germicidal lamps in an HVAC system are not spread along the whole ventilation network. They sit in one short irradiation section — usually 50–200 cm long — while the duct runs on without UV before and after it. That irradiation section can be built in two fundamentally different ways: either in-duct (lamps pushed through the duct wall) or as a closed UV chamber module (a separate housing flanged inline between two duct segments). The two modes differ in installation cost, maintenance access and lamp layout, and the choice is usually driven by whether you are retrofitting an existing duct or designing a new air handling unit.

A second, often overlooked point: HVAC UV only pays off when the air is recirculated. With a near-100 % fresh-air system every microorganism passes the lamps only once, and a single pass is rarely enough.

The "duct length" misconception

A planner typically works with 20–50 m of ventilation ductwork. When a calculation or specification asks for "duct length", the reflex is to enter the total duct length — not the length of the UV irradiation section where the lamps actually sit.

That is the wrong figure. UV lamps are installed in a chamber or module, mostly 50–200 cm long. The duct simply continues without UV on either side. The number that matters for dose and contact time is the length of the UV zone, not the length of the building's duct network.

The same distinction applies to any inline UV section: an inline process-water reactor length is the irradiated UV tube, not the whole pipe run; a drinking-water reactor length is the certified reactor body, not the entire water main. Wherever a word like "duct" or "reactor" appears, read it as the UV zone.

The two installation modes

Beyond the length question there are two fundamentally different ways UV gets into an HVAC system. The difference is critical for a planner because installation cost, maintenance access and lamp arrangement all change.

Mode A: In-duct (through the duct wall)

• Lamps are introduced from outside, through the duct wall.
• The quartz sleeve reaches into the airstream; the ballast and base stay outside, so the lamp can be serviced without opening the duct.
• Typical for existing ducts (retrofit): a hole is cut and the lamp is flange-mounted. Some retrofit systems use magnetic brackets so lamps can be positioned freely along the ductwork.
• Lamp count is limited by the available mounting positions and the duct perimeter.
• Orientation is usually perpendicular to the airflow (the lamp sits across the stream); a parallel orientation (lamp aligned with the flow) is used for longer UV zones.
• Typical count: a small number of lamps per irradiation section — exact figure is project-specific, set by duct cross-section and target dose.

Pro: retrofit-friendly, serviceable from outside, lower cost. Con: lower achievable lamp density, so lower dose per metre of duct; sheet-metal duct walls reflect UV poorly, which limits irradiance gain.

Mode B: UV chamber (closed module)

• A separate chamber module is flanged inline between two duct segments.
• The full lamp arrangement sits inside the chamber, often with a reflective inner wall (aluminium or PTFE) that raises irradiance.
• Flexible lamp layout: grid, side walls, ceiling, mixed parallel and perpendicular — whatever the chamber geometry allows.
• Pre-filtration (bag or cyclone filters) is often integrated upstream as part of the module.
• Maintenance: open the chamber cover and replace the lamps as a group.
• Typical count: a higher number of lamps than an in-duct section — again project-specific.

Pro: high lamp density and therefore high achievable dose; the reflective wall reinforces irradiance; filter integration. Con: needs space and a flanged intervention; more expensive; not retrofittable without rebuilding the duct.

Decision matrix — which mode when?

How much dose do you need?

The dose an in-duct UV section has to deliver depends on the target organism and the inactivation level. Published design analysis indicates that an in-duct UVGI system should provide an average UV dose of at least roughly 4.6–5.8 J/m² for 90 % inactivation of SARS-CoV-2 / SARS-CoV. A review of EPA field investigations reports delivered doses spanning 2.5–423 J/m², corresponding to disinfection efficacies of roughly 70–100 %, which shows how strongly real-world performance varies with airflow, lamp count and section geometry.

For coil irradiation — a common secondary purpose of HVAC UV — ASHRAE's guidance points to irradiance levels in the order of 50–100 µW/cm² at the coil surface.

The single most important takeaway: a properly designed in-duct UV system first-pass inactivation can reach up to 99 %, and because recirculated air passes the lamps repeatedly, concentration drops further with each pass. ASHRAE and EPA describe a well-designed in-duct UV system as approaching the performance of a 100 % outside-air system or HEPA filtration.

When does HVAC UV pay off at all? (fresh-air share)

HVAC UV only works when the air in the duct is recirculated. At 100 % fresh air every microorganism passes the lamps just once — UV gets no chance at multi-pass decontamination.

The table below is planning orientation, not a regulatory threshold:

Year-round 100 % fresh-air operation is uncommon but real: operating rooms, pharma cleanrooms, some classrooms converted after COVID, smoking zones. In those cases HVAC UV adds little, because every microorganism is single-pass — and a single UV pass is not enough for clinical log reduction. The same recirculation dependency is why upper-room and room-level UV devices exist for poorly ventilated spaces.

Cross-references

• HVAC filter & UV-C recommendation — how filtration and UV-C interact in an air handling unit.
• Room air purifiers (UV-C) — the alternative when fresh-air share is high or there is no central HVAC.
• AG LUV Guideline 100 + DIN/TS 67506 — the German industry standard for mobile UV-C secondary-air disinfection devices. HVAC in-duct units are not directly covered by DIN/TS 67506, but its minimum standards (dose, inactivation rate, photobiological safety per DIN EN 62471) are a useful orientation.

Sources

• ASHRAE Handbook, Chapter 62 — Ultraviolet Air and Surface Treatment.
• US EPA — "What is Upper-Room UVGI? What is HVAC UVGI?" (indoor air quality guidance).
• NIOSH / CDC — Environmental Control for Tuberculosis: Basic Upper-Room Ultraviolet Germicidal Irradiation Guidelines for Healthcare Settings (DHHS/NIOSH 2009-105).
• Nunayon et al. / review — "Ultraviolet germicidal irradiation (UVGI) for in-duct airborne bioaerosol disinfection: review and analysis of design factors" (PubMed Central PMC8021448).
• AMCA — "UV-C for HVAC Air and Surface Disinfection".