How do I choose the right medium filter for my HVAC system?

The choice of an appropriate medium filter for HVAC system requirements can be made by means of a pragmatic decision-making procedure. To begin with, establish the desired level of particles to be filtered.

Secondly, select the lowest efficiency filter that satisfies that level with respect to the appropriate test method. Next, validate that the rated airflow and pressure drop is reACHed at the design duty.

Additionally, check that the dimensions and seals meet specifications. Finally, confirm that the fan capacity satisfies established limits of replacement efficiency.

The filter that is right for a facility is not necessarily the one that has the greatest efficiency rating. Rather, it is the lowest efficiency filter that meets the requirements set out in the documentation, and stays within the parameters of the system. This definition has implications for both the quality of air and for the costs related to the system.

The following sections of this article provide the guidance on how to choose an HVAC filter step by step. Data from the Deiiang catalogue is referred to as catalogue data. Measurements from the project are referred to as project-reported or illustrative measurements, and there are no arbitrary measurements included in this report.

NdcB8cRAWu8

Define the HVAC System’s Filtration Requirements

The beginning of every selection begins with the facility. A medium filter for HVAC system duty must correspond to the contaminant loading of air quality desired, and also be able to function adequately in the system. Before the Deiiang catalogue is opened, each of these parameters must be documented.

A requirements statement should clearly satisfy the following: what specific contaminant will be filtered, what level of air cleanliness is needed, what airflow will pass through the filter, and what level of filtration will be imposed by the fan ductwork.

What particles and contaminants must the filter capture?

Identify the contaminant source, particle-size range, concentration, and process sensitivity. Particle size alone does not determine the correct grade. A pharmaceutical packaging environment releases paper dust, cardboard fibers, and skin flakes while an office primarily brings in outdoor dust and lint.

Identify whether you are dealing with airborne-particle control, occupant comfort, equipment protection, or product contamination control. Both an office and a packaging facility will have different contaminant makeup depending on the specific airborne particles that can be captured.

As an example: an office environment may need MERV 8–11 for comfort; a cleanroom may need grades F7–F9 pre-filtration before HEPA filtration. Note that these are examples, not hard and fast rules. The design specification will dictate the necessary grade for the medium-efficiency air filter.

What indoor air quality or process targets apply?

The cleanroom iso class is an air cleanliness classification for rooms, and it sets the target that cleanroom HVAC filters must serve. iso 14644-1:2015 Class 6, in particular, specifies a limit of 35,200 particles/m³ at 0.5 µm or larger measured at specific locations [4]. One filter alone does not guarantee this outcome.

Room performance is also affected by airflow patterns, air exchanges, recovery strategies, leakage, occupancy, and validation as the performance of the whole system is tested.

Do not view the MERV rating for HVAC filters as the same for every facility within medical, pharmaceutical, or electronics. The MERV rating needs to correlate to the documented design specification.

Select the Right Efficiency and MERV Rating

The selection of efficiency is an important step for any HVAC medium air filter. If the grade is low by one level, you may not achieve the air quality standard. However, if a filter is chosen to be too high in grade, the pressure drop will increase through the HVAC system without any benefit and will increase energy use.

For an HVAC medium air filter, ASHRAE 52.2-2017 uses the results from particle size effectiveness test for MERV classification [1]. MERV cannot be solely defined as one particle-efficiency number. Installed efficiency varies with sealing within the installation, loading and airflow.

For example, a MERV 13 filter positioned with a 10% bypass can operate essentially at the same efficiency as a MERV 8 filter when placed in ductwork. The test result is about the filter but does not ensure its installed performance.

Table 1: Summary of the MERV rating for HVAC filters against the other efficiency rating systems.

How should I interpret MERV ratings for an HVAC filter?

In understanding MERV rating for HVAC filters, MERV 8 means 70–74.9% efficiency in the 3–10 µm size, while 20–34.9% in the 1–3 µm size. MERV 11 means 85–89.9% in the 3–10 µm range, with 65–79.9% for 1–3 µm. MERV 13 means 90–94.9% and 85–89.9%, respectively.

It is an error to state that MERV 13 achieves an efficiency of over 90% within 1–3 µm range. The correct number for this band is 85–89.9%. Always refer to the table in the relevant section to find the values listed for that specific standard.

Each MERV rating does not have a single specific efficiency value, and the efficiency results do not take any installation bypass into account. In order to use an MERV rating for HVAC filters, it is necessary to use the size range thresholds in the standard and then check the efficiency of the installed unit.

How do ISO and cleanroom classifications affect filter selection?

ISO 16890-1:2016 defines filters for general ventilation based on ePM1, ePM2.5 and ePM10 efficiency, which is a different language from the MERV rating for HVAC filters [2]. For example, an ISO certified ePM1 50% filter will be able to capture at least 50% of particles within the range of 0.3–1 µm. This is not the same as MERV 13.

The testing procedures and types of tests used under each standard varies; therefore, comparison cannot be made directly, but approximation can be done instead. Always request the latest ISO 16890 testing information whenever it is needed for your specific use case.

Both EN 1822-1:2019 and ISO 29463-1:2017 use different kinds of tests to determine the efficiency of a HEPA filter at MPPS, which could vary from 0.3 µm [6][7]. H13 states that efficiency should be ≥99.95% at MPPS while H14 states it should be ≥99.995%.

According to data obtained from the Deiiang catalog, H13 filter has an efficiency level of 99.97–99.99% while H14 filter has an efficiency level of 99.995–99.999% when tested using 0.3 µm method. However, this is based on manufacturer's claim. Therefore, confirmation regarding classification should be sought from relevant standard before making choice.

Verify Airflow, Pressure Drop, and Fan Capacity

Just because the rated airflow is given as a reference does not mean that it is optional to follow such rating in reality; for instance, if a filter has an airflow rating of 2,050 m³/h but there is total duct flow of 3,000 m³/h, then it would still be possible to have multiple filters using that same figure of 2,050 m³/h.

The HVAC filter pressure drop depends on the filters' face area, number of filters and duct layouts.

The HVAC filter pressure drop across a filter under HVAC conditions has two values: the initial resistance when the filter is clean and the final resistance when the filter has to be replaced. The fan must be able to maintain the initial value and still meet the design airflow at the final value.

How can I assess filter pressure drop?

Obtain the initial HVAC filter pressure drop at the air flow determined by the test workflow. For instance, for Deiiang's medium-efficiency bag-type filters, catalogue information shows that the resistance for a 592 mm x 592 mm x 381 mm filter with six bags is below 50 Pa at an airflow of 2,050 m³/h for the F5 Resistance Level, eventually increasing to below 100 Pa at the F8 Resistance Level.

The final resistance level is between 250 and 400 Pa. Therefore, it can be observed that there has been a 5–8 level increase from clean to loaded status. The fan must achieve the desired rate of airflow at the initial value while allowing the airflow to drop at a constant speed.

What airflow and fan limits must the filter meet?

Begin solving the problem with the fan performance curve with the operating point of the filter. Make sure to check that the fan can provide the necessary volume of air at the desired pressure drop.

For a medium-sized filter, make sure three things are confirmed:

• Rated airflow is the same or higher than that required for the position which the filter occupies in the system.
• Starting pressure drops at the rated airflow are within the normal operating range of the fan.
• Final pressure drop is below the maximum pressure drop allowed by the system.

Pressure drop should be compared to the system curve, not a general maximum static pressure. If an H14 terminal filter has been installed downstream then make sure to note this separately.

The Deiiang catalogue indicates that the 592 mm x 592 mm x 292 mm (H14) filter has a maximum flow rate of 2,500 m³/h, an initial pressure drop of less than or equal to 220 Pa, and final pressure drop of between 400 Pa and 600 Pa.

Confirm Filter Size, Construction, and Compatibility

When considering the HVAC filter size and compatibility of air filters, it is important to remember that an air filter 5 mm too deep will not be able to fit into the cage of the air handling unit. Similarly, if the frame type is not compatible with the filter retention method, there is a danger of bypass.

Remember to verify the nominal measurements before ordering filters.

How do I confirm the correct HVAC filter size?

Record the installed filter's nominal and actual dimensions, because HVAC filter size and compatibility is settled by slot opening width and height, slot depth, retaining method, gasket location, and clearance. Measure the slot at its narrowest point.

Nominal and actual sizes differ so it may be wise to be cautious. The nominal measurement for air filters in inches typically refers to the factory standard size (24 inches x 24 inches x 12 inches), but this may not be what the actual measurement is. This same principle applies to all filtration products and each product individual filter manufacturer listing will provide actual dimensions.

Do not make assumptions about HVAC filter size and compatibility from the inch and millimeter listings in the catalog. Deiiang's 592 mm x 592 mm x 381 mm six-bag model is listed with actual dimensions; confirm the slot opening and depth before ordering.

There is a bypass area of approximately 2,400 mm² formed by only a 1 mm space around a 592 mm face filter through geometric means alone. Although this is a geometrical example, it by itself does not predict particle load since both airflow and leakage data are needed.

Which frame, media, and seal details matter?

The type of frame material used dictates hardness and temperature resistance to give rigidity to the device. Deiiang’s fiberglass bag filters, for example, utilize aluminum frames to save the operating temperature of 150 °C and 80 °C for fiberglass and man-made materials respectively as stated in the catalog.

A fiberglass model does not necessarily imply that it can withstand 100 percent humidity. Users must clarify its stated capability for that specific item from the manufacturer. According to Deiiang, some fiberglass models are limited to 100 percent humidity.

Users must consult manufacturers for information on chemical compatibility, gasket material, and outgassing. Deiiang’s products can be incorporated into PU two-component sealant and use either EVA or neoprene gaskets, although they do offer silicone-free gaskets if required.

Match the Filter to Cleanroom and HVAC Standards

The standards in question provide a means of specifying how one will measure performance and what degree of cleanliness one needs to attain within the cleanroom. Confusing these points can result in errors. ASHRAE 52.2 reported a MERV 13 which should not be confused with an ISO ePM1 rating of 50 percent.

The main applicable standards are outlined within Table 2 and there will not necessarily be a standard that relates to all purchases of medium filters used in commercial HVAC air filtration.

Table 2: Relevant standards for selecting medium and high-efficiency filters.

Which standards apply to medium HVAC air filters?

For an HVAC medium air filter in general ventilation, ISO 16890-1:2016 is the current standard [2]. EN 779:2012 is withdrawn and should be cited only as legacy context [3]. ASHRAE 52.2-2017 remains common in North America [1].

Deiiang catalogue data reports F5–F9 grades and MERV 8–13 equivalents for each medium-efficiency air filter in its range. The catalogue does not currently provide model-specific ISO 16890 classifications for every model.

Do not invent a conversion. If the specification requires ISO 16890, request current test data from the manufacturer. Downstream HEPA standards such as EN 1822 or ISO 29463 do not classify the medium pre-filter.

How do cleanroom requirements change the selection process?

Cleanroom HVAC filters must meet the room classification (ISO 14644-1) and the filter classification (EN 1822 or ISO 29463). The medium pre-filter serves to protect and prolong the life of the HEPA filter.

In the case of one particular pharmaceutical packaging project, at the request of the client, the Deiiang company supplied F7 pre-filters prior to H14 terminal filters due to the particle loading, the height of the plenum, and required pressure drop. This exact filter specification cannot be assumed for all cleanrooms.

The Deiiang DOP Integrated Filter has a DOP test port located on the air outlet (downstream from the filter), which is sealed with a rubber plug. The port allows for in-situ leak testing by the DOP method as defined in MIL-STD-282 [9]. Testing depends on the validation protocol for the project.

Balance Filtration, Energy, Maintenance, and Cost

In commercial HVAC air filtration, selection is an economic process in addition to being technical. A higher efficiency medium-efficiency air filter is more expensive per unit, has a higher pressure drop (thus using more energy), and requires more frequent replacements. However, it can help to save costs in cleaning downstream equipment.

How to choose an HVAC filter with the lowest total cost depends on the operating hours, energy costs, and replacement labor. Use the lowest efficiency of filter that achieves the desired results and then weigh the total life cycle costs.

How can I balance filtration efficiency with energy use?

Use the lowest efficiency for filters that achieves the needed specifications. If MERV 8 meets air quality requirements, specify only that amount instead of MERV 13. Higher than necessary filtration results in higher pressure drop and higher energy use.

Do not make a statement about one HVAC filter pressure drop being lower than another unless a respective and equal test has been performed. Compare candidates at the same airflow, dimensions, test method, and loading conditions.

An initial lower pressure drop does not equate to a lower lifetime energy consumption. The complete pressure drop curve and time in operation must also be considered. If there is no full curve with integration over time, do not assume that one filter will consume less lifetime energy as opposed to another filter.

What maintenance plan supports reliable filter performance?

A reference differential pressure should be established during commissioning. Record the differential pressure at appropriate intervals. Set the replacement criteria in accordance with the resistance final established by the manufacturer and design specifications.

Do not use one month and three month inspections or 50 Pa/month to prescribe maintenance intervals for all cases. Deiiang recommends measuring differential pressure at commissioning and the rate of pressure gain to determine remaining life.

The manufacturer’s final resistance should be confirmed on a filter basis as the 250–400 Pa range applies to some Deiiang medium efficiency filters but not to every medium-efficiency air filter type. Change the filter when the pressure drop reaches the level of the final resistance established by the manufacturer.

Compare Candidate Filters and Validate Selection

At this point of choosing a medium filter for HVAC system duty you should have at least two or three candidate filter brands. Ensure that comparison of filters is only made on data measured under identical test conditions. A pressure drop measured at 1,500 m³/h cannot be compared to one measured at 2,500 m³/h directly without correcting for differences in flow rates.

Refer to the checklist provided to help organize the comparison of each HVAC medium air filter, and validate the candidates against the limits set by the system before purchase.

Table 3: Candidate filter comparison using Deiiang catalogue data.

How should I compare HVAC filter candidates?

Align all airflow and the pressure drop data to conform to the design airflow of the installation. The filter may indicate the capability to process 1,700 m³/h but if your installation operates at a flow rate of 2,050 m³/h the pressure drop will be greater than stated by the manufacturer. Obtain specific data on the manufacturer’s pressure drop or conduct an appropriate test at the required air flow rate.

Pressure drop increases as the airflow increases but the exact correlation will depend upon many factors such as the media being used, the geometry of the filter, and the testing conditions. Therefore, it is not valid to generalize and apply a square law. The example of airflow rates of 1,700 m³/h and 2,050 m³/h should not be assumed to be a valid prediction of the expected pressure drop.

Ensure that the MERV rating for HVAC filters that you are comparing meets the required standard. For example, an F5 filter with an efficiency of 45% cannot be rated with the same efficiency as an F8 filter that has an efficiency of 90% if your application requires a filter with an MERV rating of 13. Verify the standard and edition of the test for each claim made regarding efficiency.

What checks confirm a filter is ready for purchase?

Before you work out how to choose an HVAC filter, four criteria must be met:

• Filter dimensions match the dimensions specified in the slots when allowing for the geometry of the gasket compressing the filter.
• The rated airflow must be equal to or greater than the design airflow of the installation.
• The initial pressure drop at the design airflow must be at or below the capability of the fan.
• The ultimate pressure drop must be at or below the maximum specified for the system design.

For cleanroom HVAC filters add one more important requirement: the gasket material must be compatible with the outgassing requirements. Deiiang manufactures silicone-free gaskets that can be used with some pharmaceutical and semiconductor applications. Ensure that you check for a specific model so that you can obtain silicone-free gaskets.

Apply the Selection Method in a Real Facility

The following section will present a hypothetical application as an example using sample data from the Deiiang catalogue. It is important to understand that this is not a report of real project outcomes.

The depicted facility is a cleanroom for pharmaceutical packaging, designed to meet ISO 6 (class 1000), and it uses cleanroom HVAC filters in two stages. The constraints faced involve the low height of the plenum, stringent limits on pressure drop, and requirement for leak testing on an individual basis. The baseline conditions have been taken as iso class 7.

The information related to the project has been recorded in the form of the data as presented below.

Table 4: Cleanroom case study data.

What should a useful HVAC filter case study document?

A medium filter for HVAC system case study must include the baseline parameters, the specific problem, the model chosen, the catalogue specifications, and the result measure taken. The scope of the study must also be noted as well as what is not measured.

For this case study example, the recorded data includes: the assumed baseline iso class, ISO class targeted, model number of the filter, dimensions, airflow rate, initial resistance, media area and the methodology of testing.

Deiiang provides a unit-by-unit documentation for the testing and the traceability of the used filter material which serves to support validation and maintain control in the regulated working environment.

How can engineers verify performance after installation?

Verification after installation is performed when starting up; verify the air flow rate and pressure drop levels meet the corresponding levels in the technical data catalogue for the project. Where the technical data does not state tolerances acceptable levels should be taken. For example, a level for an increase of no more than 10% could be indicated.

The pressure drop increase should be checked over time so that the loading rate can be established. While 20% after 30 days could be used as a guide, actual limits should be set by the project specifications or the manufacturer.

When reaching the final resistance level the loading pressure drop that has been measured should be recorded separately from the catalogue value obtained from the manufacturer. You should keep a record of the person conducting the test and maintaining the instrument used.

Jason Peng, a product designer at Deiiang, was involved in selecting the filter for the case study. His specific role was to view the catalogue and confirm the sealing requirements of the filter.

Frequently Asked Questions

What is a medium filter in an HVAC system?

A medium filter is one that operates between coarse pre-filters and HEPA filters, and the MERV rating for HVAC filters in that band is 8 to 13. It falls within the 8–13 MERV (F5–F9 grades). Deiiang provides medium filters in bag, W-bank and panel style.

How do I choose the right MERV rating?

For an HVAC medium air filter you should only select the lowest MERV rating that satisfies the air quality requirement. For example, MERV 8–11 is most common in commercial HVAC air filtration while MERV 11–13 is typical for cleanroom pre-filters. The Deiiang catalogue gives comparison charts of the F5–F9 grades and compares them to MERV 8, 11, and 13 ratings. Always check the project specification before finalizing selections.

How do I know what filter size to buy?

Start off with measuring filter openings on its height, breadth and depth (in case of narrowest openings). Make sure you compare filter's dimensions that are stated not nominal size. Deiiang supplies a 592×592×381 mm six-module filter and thus it's important to check and verify the dimensions of the filter slot as well as its depth.

Why does pressure drop matter when selecting a filter?

The HVAC filter pressure drop affects both the power consumption of the fan and the available air flow. The fan will require only half of energy with a filter having initial pressure drop of 100 Pa when compared to the filter having the same efficiency with a drop of 200 Pa. Therefore the excessive pressure drop in air filters can negatively affect air volume.

Can I use a medium filter in a cleanroom HVAC system?

It is possible to use medium filters as pre-filters before installing HEPA. By using medium efficient filters at MERV 11–13 (F7–F9), the bulk percentage of particles can be filtered out in order to protect the HEPA filter from the particle load. Therefore Deiiang has both medium efficiency and HEPA filters available in cleanrooms HVAC installation. Always check with the project specification and classifications of the cleanroom.

How often should I ｒｅｐｌａｃｅ a medium HVAC filter?

The replacement process of medium filters should be done when pressure drop reaches final resistance that has been specified by the manufacturer. Most medium-efficiency air filter models have final resistance in the range of 250–400 Pa but this is not always applicable. Therefore an initial baseline needs to be established during commissioning.

References

• [1] ANSI/ASHRAE Standard 52.2-2017, Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size. ASHRAE.
• [2] ISO 16890-1:2016, Air filters for general ventilation — Part 1: Technical specifications, requirements and classification system based upon particulate matter efficiency (ePM). ISO.
• [3] EN 779:2012 (withdrawn), Particulate air filters for general ventilation — Determination of the filtration performance. CEN. Superseded by ISO 16890.
• [4] ISO 14644-1:2015, Cleanrooms and associated controlled environments — Part 1: Classification of air cleanliness by particle concentration. ISO.
• [5] ISO 14644-3:2019, Cleanrooms and associated controlled environments — Part 3: Test methods. ISO.
• [6] EN 1822-1:2019, High efficiency air filters (EPA, HEPA and ULPA) — Part 1: Classification, performance testing, marking. CEN.
• [7] ISO 29463-1:2017, High efficiency filters and filter media for removing particles in air — Part 1: Classification, performance, testing and marking. ISO.
• [8] IEST-RP-CC001, HEPA and ULPA Filters. Institute of Environmental Sciences and Technology.
• [9] MIL-STD-282, Filter Units, Protective Clothing, Gas-Mask Components and Related Products: Performance Test Methods. U.S. Department of Defense.