ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 6

While upper-room/air UV-C "is very effective in areas
ventilation,"12

concentrations reduced further with each subsequent pass.

in spaces with no or

For coils, ASHRAE recommends irradiance levels

weak air circulation, ceiling fans can compensate for the

of 50 µW/cm2 to 100 µW/cm2.12 Perhaps a better way to

lack of sufficient mechanical air movement and improve

achieve a desired dosage is to convert 100 µW/cm2 to the

inactivation rates (see sidebar, "Improving Upper-Room

more easily understood and specifiable 7.5 lamp watts

UV With Ceiling Fans," on Page 11).

(as printed on the lamp surface) per square foot of coil

with no, or minimal,

HVAC air-stream disinfection. In HVAC air-stream

surface area.13 By making this conversion, frontline engi-

disinfection, UV-C fixtures installed in air-handling-unit

neers easily can verify that submittals conform to their

(AHU) plenums, air-distribution systems, or HVAC duct-

specifications.

work inactivate microorganisms "on the fly." Simplified,
the germicidal dose is determined by the UV-C inten-

Environmental Factors

sity, exposure time, and target pathogen's susceptibility

There are many operational and site-specific conditions

to UV-C.

that impact inactivation or kill rate, including:

In addition to the amount of germicidal energy absorbed

■	

The target pathogen and its susceptibility to UV-C. The

by a pathogen over a specific amount of time, a variety

amounts of UV-C energy needed to inactivate indi-

of other factors are taken into consideration when UV-C

vidual bacteria, viruses, and spores have been identi-

is applied for HVAC air-stream disinfection. These site-

fied through decades of research.14

specific considerations will be explored in the next section.

■	

The volume and velocity of air traveling through the

HVAC coil/surface irradiation. The most common

HVAC system, which will impact the length of residence

type of germicidal-UV system, HVAC coil/surface irra-

time. A higher volume of air and/or faster-moving air

diation continuously targets bacteria, viruses, mold, and

requires greater intensity (more UV-C lamps) and/or a

biofilm that proliferate on coils, air filters, duct walls, and

longer run of duct to increase residence time. Said differ-

drain pans, preventing them from becoming reservoirs

ently, as velocity increases beyond the typical 500 fpm,

for pathogen growth.

UV intensity must increase with it. Conversely, less UV
intensity is required for air velocities below 500 fpm.

A coil/surface-irradiation system can eliminate up to
30 percent of airborne pathogens on a first-pass basis, with

■	

The length of the plenum/duct-the longer the plenum
or duct run, the better, as residence time and, thus,

Germicidal
Engineering Control

dose are increased.

Recommended Application
■	

* Goal: greater than 50 µW/cm2
Upper-room/air
systems

from the surface-irradiation standard 36-in. centerlines

* Wall-mounted
* Installation height: above 7 ft
in occupied spaces

to 15-in. to 18-in. centerlines) increases UV-C fluence.
■	

susceptibility to UV-C, air-stream-disinfection measures

* Duct-run length: greater
than 24 in.

can be more effective on the upstream side of a coil.
Although on-the-fly disinfection can be accomplished

* Air velocity: less than 500 fpm

downstream of coils, it typically requires an increase

* Minimum exposure time: greater
than 0.24 sec
* Upstream of coil
HVAC coil/surface
irradiation

in UV-C intensity (more lamps).
■	

The lamps-using more or higher-output lamps will

* Downstream of coil

increase the total dose-that is, the microwatt-seconds

* Lamps spaced every 30 to 40 in.
of coil height

per square centimeter (µW-s/cm²). Lamps with 360-degree

TABLE 1. Germicidal-UV application.

6

Temperature-because cold air reduces the output of
UV-C lamps and high relative humidity affects pathogen

* Lamps spaced every 14 in.
of coil height

HVAC air-stream
disinfection

Fixture spacing-decreasing lamp-row spacing (e.g.,

2020 A M CA i n m o t i o n

irradiation allow more UV-C energy to saturate a plenum,
increasing UV-C fluence.

w w w. a m c a .o r g
http://www.amca.org
ASHRAE Journal Supplement - AMCA InMotion - October 2020

Table of Contents for the Digital Edition of ASHRAE Journal Supplement - AMCA InMotion - October 2020
