Performance and costs of particle air filtration in HVAC supply airstreams Page: 4 of 8
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For environmental tobacco smoke particles, which are almost entirely submicron in size, the predicted
indoor concentration reductions from use of low to moderate efficiency filters [ASHRAE 30% and 45%
(MERV 7 to 9)] are very small (5% to 12%). Increasing the filter efficiency to ASHRAE 85% (MERV
13) yields a predicted concentration reduction of 61%. Further increases in filter efficiency bring modest
additional benefits, up to a 75% reduction in concentration with use of a HEPA filter.
Figure 3 shows the predicted reductions in outdoor fine-mode particles. The predicted benefits of low to
moderate efficiency filters, ASHRAE 45% (MERV 9) and lower, are again quite small. The reduction in
concentrations from a filter with an ASHRAE 85% rating (MERV 13) is 80% with base-case assumptions
(1 h- of mechanical outside air ventilation, 0.25 h-1 of unfiltered infiltration, and 4 h-i of recirculation).
Upgrading to a HEPA filter brings only a modest additional benefit, with a predicted reduction in
concentration of 95%. If there is no infiltration, the predicted reductions in indoor concentration are two
to six percentage points higher. If the building does not recirculate air, the concentration reductions are
significantly smaller and the benefits of increasing filter efficiency above ASHRAE 85% (MERV 13) are
Figure 4 displays the predicted energy and total costs of using ASHRAE 25% to ASHRAE 95% (MERV
7 to 14) filters in HVAC supply airstreams. Cost are provided per 1000 ft3/min (470 L/s) of filtered
supply air, which would typically serve 5 -7 persons in a U.S.-style office building with air recirculation
or 10 to 15 occupants in European buildings without air recirculation. Monthly energy costs range from
$3.26 for a filter with an efficiency rating of 30% (MERV 7) to $5.94 for a filter with an efficiency of
95% (MERV 14). With the labor cost schedule for filter replacement, total monthly costs range from
$3.90 to $11.10. There is a general tendency toward higher energy and total costs with higher efficiency
filters; however, the costs of using different filter products of the same efficiency vary widely.
Consequently, use of more efficient air filters does not always increase costs. For example, monthly costs
per 470 L/s (1000 ft3/min) of filtered supply air were as high as $5.60 for a 25% filter efficiency (MERV
7) and as low as $4.20 for a filter with a 60% efficiency (MERV 11).
The costs displayed on Figure 3 are based on the manufacturers' rated air flows and average of the initial
and recommended final pressure drops for the filters. With the assumed inlet particle concentrations and
hours of usage, the predicted filter lifetimes range from two to 27 months. Energy consumption and
energy costs will be smaller if the filter is replaced with a smaller final pressure drop; however, the costs
of purchasing filters and the labor for filter replacement will increase. Using the data available from one
major filter manufacturer, a limited sensitivity analyses explored the trends in total life cycle cost as final
pressure drops decreased from the manufacturer's recommended values. In general, total predicted
monthly costs increased or decreased less than 10% as the final pressure drop at filter replacement was
decreased by up to 0.4 inch of water (100 Pa). In a few cases, total monthly costs decreased by 30%.
Consequently, more frequent filter replacement could save energy and sometimes also slightly decrease
Filter selection and costs
Two results of this modeling may be surprising to some readers. First, since filters are more efficient in
removing larger particles from airstreams, one might expect filtration to decrease indoor concentrations of
cat and dust mite allergens by a larger percentage than concentrations of ETS and outdoor fine mode
particles. Except for the lowest efficiency filters, the findings are inconsistent with this expectation. The
large losses of the large allergen particles by deposition on indoor surfaces are the explanation. Because
these allergen particles are removed from indoor air at a substantial rate by deposition on surfaces, the
filtration increases the total particle removal rate by only a moderate percentage. The rates of loss of the
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Fisk, William J.; Faulkner, David; Palonen, Jari & Seppanen, Olli. Performance and costs of particle air filtration in HVAC supply airstreams, article, June 1, 2003; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc738204/m1/4/: accessed October 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.