Ozone Removal by Residential HVAC Filters: For
Better or for Worse
Paper # 1184
Ping Zhao, Jeffrey A. Siegel, Richard L. Corsi*
Civil Engineering, 1.304B CTR for Energy Studies, University of Texas, Austin, 78758.
*Email: corsi@mail.utexas.edu; Phone: (512) 475-8617.
ABSTRACT
HVAC filters have a significant influence on indoor air. In addition to removing
particulate contaminants in HVAC air, filters accumulate a particle layer that can react
with ozone. Ozone-particle cake reactions serve as a sink for ozone and a source of
secondary carbonyls. The location of filters in air distribution systems can lead to these
by-products being distributed throughout a building. Two experiments were performed to
determine the ozone removal efficiency, η, on filters that were loaded with particles for
one month (#1) and three months (#2). On both filters, η dropped rapidly during the first
30 minutes and then stayed approximately constant at 0.1 for filter #1 and 0.14 for filter
#2 for 4 hours at an airflow of 20L/min. In this paper, we describe a model that predicts
ozone reactions on particle-laden commercial and residential HVAC filters. The filter is
divided into sections and a well-mixed reactor model is applied to each section.
INTRODUCTION
Heating, ventilation, and air conditioning (HVAC) systems play an important role in
providing comfort in residential, commercial, and institutional buildings. However, they
also play a potentially significant role as conveyers and as sources of indoor air
pollution1-4. A higher incidence of sick-building syndrome has been observed among
office workers in buildings with HVAC systems5. The reasons for this observation are not
entirely clear, but may include direct (primary) emissions of volatile organic compounds
(VOCs) from HVAC components, growth and release of biological agents, and/or
emissions of secondary pollutants resulting from heterogeneous chemistry that occurs on
surfaces of the HVAC system.
Ozone, a chemical oxidant and common indoor and outdoor pollutant, is of particular
interest for surface reactions with HVAC components. By-products of ozone-surface
reactions are likely to include carbonyls that might be irritating to the upper-respiratory
system of building occupants. Morrison et al.6 observed that exposure to 100 ppb ozone
increased the emission rates of aldehydes from selected HVAC materials such as duct
liners, duct sealing caulks and neoprene gaskets. In a laboratory study of ozone
interactions with naturally-loaded filters from commercial buildings, Hyttinen et al.7
observed the consumption of ozone in almost all of the filters. In their field experiments,
the reduction in ozone concentrations varied from 8 to 26%; the highest ozone reduction
was obtained in an HVAC unit with three stages of filtration. In a separate study, Halas
and Beko8 observed poor air quality downstream of filters exposed to ozone or air as
compared to filters exposed to nitrogen. In their study, 90% of participants were
dissatisfied with the air quality downstream of filters exposed to ozone, compared to 35%
dissatisfied before the filters were exposed to ozone. They also showed a relatively high
regeneration of the initial ozone removal efficiency after the samples were treated with
clean air, nitrogen and heat. They hypothesized that VOCs inside the bulk particle
volume could slowly diffuse to the external surface following reactions of surface sites
by ozone.
In this study, we explored ozone interactions with filters removed from residential HVAC
systems. We first constructed a mathematical model of ozone reactions within a filter.
And then we presented a series of experiments that validated the model.
MODEL DEVELOPMENT
Figure 1 shows a schematic of ozone reactions on an HVAC filter. Room air at a
concentration of Cx and an air flow rate of Q flows through an HVAC filter. The filter has
a thickness  in the direction of air flow. After air flows through a distance ∆x along the
filter, the concentration of ozone in the air is Cx+∆x.
Figure 1: Schematic of the filter model
∆x
y (height of the filter)
Q, Cx
Q, Cx+∆x
x
x+∆x
x
 = (thickness of the filter)
A mass balance on the incremental element ∆x leads to Equation 1.
Equation 1. An ozone mass balance through an incremental slice of a filter.
C
VT
 QC x  QC x  x  v d As C
t
where:
 = porosity of the filter (volume of air/bulk volume of filter)
VT = total (bulk) volume of the differential filter slice (shaded in Figure 1) (m3)
C = ozone concentration in air inside the slice (µg/m3)
C x = ozone concentration in the air entering the incremental slice (µg/m3)
C x  x = ozone concentrations in the air exiting the incremental slice (µg/m3)
Q = air flow rate through the slice (m3/s)
v d = ozone deposition velocity on filter particles (m/s)
As = total surface area in the incremental slice (m2)
For comparison with our experimental results, we ignore the effect of continuous particle
deposition. Also, a constant ozone concentration upstream of the filter, constant air flow
rate, no other deposition except onto the particle surfaces, and well-mixed air in the
incremental slice are all assumed. Based on these assumptions, Equation 1 can be
reduced to Equation 2.
Equation 2. Rearrangement and deduction of Equation 1.
C
U C vd As


C
t
 x VT
where:
Q
, where AT is the cross-sectional area of the slice) (m/s)
AT
x = thickness of the filter slice (m).
Taking the limit of Equation 2 as x goes to zero yields Equation 3.
U = face velocity (
Equation 3. Reduction of Equation 2 after taking the limit as x goes to zero.
C
U C vd As


C
t
 x VT
Equation 3 can be simplified by combining variables as shown in Equation 4.
U
Equation 4. Reduction after letting  =
(effective air velocity) (cm min-1) and  =

v d As
(effective deposition) (min-1).
VT
C
C
 
 C
t
x
Thus, the solution of ozone concentration through the filter requires knowledge of 
and  , and appropriate boundary conditions. Since not all parameters in  can be
determined by direct measurement,  is estimated through Equation 4 by using ozone
concentration data from a series of experiments, as shown in Equation 6.
Equation 6. Estimation of  by sequential ozone concentration measurement.
C n 1, j  C n, j  C n 1, j 1  C n, j 1
 
C n, j 1  C n, j  C n 1, j 1  C n 1, j

C n 1, j  C n, j  C n 1, J 1  C n, J 1
2t
2x
4
C n , j = ozone concentrations in the air entering (position j) the incremental slice at time n
(n=1, 2, 3 ….. n unit time) (µg/m3)
C n 1, j = ozone concentrations in the air entering (position j) the incremental slice at time
n+1 (n=1, 2, 3 …… n unit time) (µg/m3)
C n , j 1 = ozone concentrations in the air exiting (position j+1) the incremental slice at time
n (n=1, 2, 3 ….. n unit time) (µg/m3)
C n 1, j 1 = ozone concentrations in the air exiting (position j+1) the incremental slice at
time n+1(n=1, 2, 3 ….. n unit time) (µg/m3)
t = time step (1min)
x = thickness of the HVAC filter (cm)
It is difficult to measure ozone concentrations within the volume of an HVAC filter, only
the top and bottom surfaces are considered here. Therefore,  is the total filter effective
deposition and is a function of time.
EXPERIMENTAL METHODOLOGY
Experimental Systems
The solution to the model described above is dependent on the knowledge of two
parameters,  and  . Figure 2 depicts the experimental system used to determine  .
Figure 2 Schematic of Experimental System to Measure β
O3 Generator
Vacuum Pump
Mixing Fan
Covered
Sample Port
O3
Analyzer
Top chamber
HVAC Filter
Bottom chamber
O3
Analyzer
Covered
Sample Port
Bubble Flow Meter
A dual section electro-polished stainless steel chamber (28.3 L per section) was separated
by a test filter. Small fans were used to mix the air in each section. All air passed through
the HVAC filter was pre-filtered through a PTFE (Polyetrafluoroethylene) filter with pore
size of 2.0 µm in order to keep particles from depositing on the sample filter. Thereafter,
room air through a vacuum pump with fixed flow rate was mixed with ozone generated
by an ozone generator (Prozone, Model PZ 6 Air). Mixed air was then conveyed through
the chamber system (into the top chamber, through the filter, out of bottom chamber).
The air flow rate through the filter was measured with a bubble flow meter (Sensidyne,
Model GilibratorTM 2) at the outlet of the bottom chamber, as shown in Figure 2.
Analytical Measurement
Two fiber glass filters were tested in this study. Both filters came from residential HVAC
systems after use for one month (#1) and three month (#2). The temperature and relative
humidity of each experiment were approximately 23oC and 50%, respectively. Ozone
concentrations upstream and downstream of the filter were continuously monitored and
recorded by two calibrated UV ozone analyzers (2B Technologies, Model 202) with
sampling intervals of 10 seconds. Each experiment lasted for 4.5 hours.
The value of  were determined by direct measurements of Q , AT and indirect
measurement of  through observing a volume change due to soaking a filter in clean
water in a glass cylinder. The porosity of test filters was measured separately. Five pieces
of filter were tested for filter #1, and 6 pieces of filter were tested for filter #2. The size of
specimen was measured by ruler. Then, we completely soaked each piece in a 250 ml
glass cylinder with a known amount of clean water and recorded the volume reading after
10 minutes.
The porosity of each test specimen was determined by use of Equation 5.
Equation 5. Determination of the porosity of test filters.
Va  Vb
Vbulk
Vbulk = total (bulk) volume of the filter specimen (m3)
  1
Vb = water volume before soaking the filter (m3)
V a = water volume after soaking the filter (m3)
RESULTS AND DISCUSSION
Ozone Removal
The ozone concentration upstream of filter #1 increased while conducting the experiment
and was approximately 700 ppb by the end of the experiment. The ozone concentration
upstream for filter #2 also increased during the experiment and was approximately 250
ppb by the end of the experiment. The ozone concentration downstream of the filter was
always less than ozone concentration upstream for each experiment, which means that
some ozone was removed when passing through the filter. In this paper, ozone removal
efficiency, η was used to describe the ozone removal across the filter. Ozone removal
efficiency, η is the ratio of the ozone concentration difference across the filter to the
ozone concentration upstream of the filter.
For each filter, η started at a high value and then dropped rapidly within the first 30
minutes of each experiment (Figures 3). Thus, reaction sites on particle surfaces were
consumed rapidly. In Figure 3a, η is characterized by considerable noise for the first 30
minutes of operation. We suspect that this might be due to variations in the flow rate.
After 30 minutes of experiment, η becomes smaller and almost constant during the rest of
the experiment. This may come from the fact that reactive species inside the bulk particle
volume slowly diffuse to the external surface following reactions of surface sites by
ozone8. Thus, η was never zero during our experiments and there may be a balance
between ozone consumption and reactive species diffusing out of bulk particles.
Additionally, a steady η value (0.1) for filter #1 (1 month in use) exposed to higher ozone
concentration is less than the steady η value (0.14) for filter #2 (3 months in use) exposed
to a lower ozone concentration. In actual buildings, particles will constantly deposit onto
filters, and η would likely be greater than what we report here.
Figure 3a : Ozone Removal Efficiency for Filter # 1
Ozone Removal Efficiency, h
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0
30
60
90
120
150
180
210
240
270
Tim e (m in)
Figure 3 b: Ozone Removal Efficiency for Filter #2
Ozone Removal Efficiency, h
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0
30
60
90
120
150
180
210
240
270
Tim e (m in)
Porosity
Table 1 shows the measured porosity for each filter. No difference in porosity was
observed between filter #1 and filter #2. This may due to that fact that the deposited
particles are not a substantial contributor to the bulk volume, which is almost entirely
associated with the filter fibers.
Table 1. Porosity Measurement for Filters
Filter
(#)
Description
1
1 month use
2
2 month use
∆x (cm)
2.75
2.75
2.75
2.75
2.75
2.75
2.75
2.75
2.75
2.75
2.75
Vbulk(cm3)
354.8
511.5
500.5
453.8
475.3
466.5
488.1
441.7
497.2
473.1
476.4
Vmaterial(ml)
1
2
2.2
2
2
2.5
2.4
3
3.2
3.2
3
Porosity
1.00
1.00
1.00
1.00
1.00
0.99
1.00
0.99
0.99
0.99
0.99
Model
Since all porosity measurements were very close to 1, we used this value as the porosity
in the model. Therefore,  was determined to be 21.54 cm min-1.Values of  were
calculated based on Equation 6 for both filters and are shown in Figures 4a and b. has a
The time dependence for  and η were similar, because  was kept constant during the
experiment and nothing else changed in the model except for  . The steady value of 
(0.8) for filter #1 was smaller than  (1.2) for filter #2.
Figure 4a : Characterization of Effective Deposition Change
for Filter #1 as a Function of Time
Effective Deposition,  (min -1)
3.50
3.00
2.50
2.00
 = 2.5579t -0.1325
1.50
R2 = 0.787
1.00
0.50
0.00
0
30
60
90
120
150
Tim e (m in)
180
210
240
270
Figure 4b: Charaterization of Effective Deposition Change
for Filter #2 as a Function of Time
Effective Deposition,  (min -1)
2.3
2.1
1.9
1.7
 = 2.2468t-0.0582
1.5
R2 = 0.6148
1.3
1.1
0.9
0.7
0.5
0
30
60
90
120
150
180
210
240
270
Tim e (m in)
CONCLUSION
Two experiments involving ozone removal on HVAC filters were performed. Ozone
concentration was observed to decrease across each filter. Ozone removal efficiency
dropped rapidly and then remained almost constant during the rest of each experiment. A
model was developed to predict ozone removal in filters and model parameters were
determined through experiments.
ACKNOWLEDGEMENTS
Charlie Weschler provided valuable guidance when formulating the ideas for this
research. Charlie Perego and Robert Montgomery fabricated the apparatus used for the
experiments.
REFERENCES
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Conference on Indoor Air Quality. 1993, 6, 279-384.
4. Batterman, S.; and Burge, H. International Journal of Heating, Ventilating, AirConditioning and Refrigerating Research. 1995, 1(1), 61-80.
5. Mendell, M. J.; Smith, A. H. American Journal of Public Health. 1990, 80, 11931199.
6. Morrsion, G. C.; Nazaroff, W. W.; Cano-Ruiz, J. A.; Hodgson A. T.; and Modera, M.
P. Journal of Air and Waste Management Association. 1998, 48, 941-952.
7. Hyttinen M.; Pasanen P.; Salo J.; Bjorkroth M.; Vartiainen M.; and Kalliokoski P.
Indoor and Built Environment. 2003, 12(3), 151-158.
8. Halas, O.; and Beko G. Ventilation filters as sources of pollution, Master Thesis,
2003, Technical University of Denmark.
Key Words
Ozone
Residential HVAC filter
Removal
Model
Deposition