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Professional air filtration for EV charging devices, protecting against dust and sand ingress to ensure stable, reliable operation of charging piles and station infrastructure.

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Medium Efficiency Filters (F5-F9): Why They Are the “Workhorse” of HVAC Systems

Abstract

In a multi‑stage air filtration system, medium efficiency filters (F5‑F9) handle the heaviest filtration duty. They are neither as coarse as pre‑filters (which only capture large particles) nor as expensive and restrictive as HEPA filters. Instead, they offer a balanced combination of reasonable cost and moderate pressure drop while efficiently removing fine particles such as PM2.5, bacteria, and pollen – the very pollutants that are most harmful to human health and equipment operation. This article systematically explains the efficiency classification, construction types, key parameters, typical applications, and selection & maintenance guidelines for medium filters, helping engineers and facility managers fully understand and maximize the value of this “workhorse”.

1. Definition and Role of Medium Efficiency Filters

1.1 What Are Medium Efficiency Filters?

Medium efficiency filters sit between coarse (G1‑G4) and HEPA (H10‑H14) grades. According to international standards, they typically cover the following range:

Standard System	Grade Range	Typical Efficiency Description
EN 779 (withdrawn)	F5‑F9	Counting efficiency for 0.4μm particles: 40‑95%
ASHRAE 52.2	MERV 9‑16	Efficiency for 0.3‑10μm particles: 30‑95%
ISO 16890	ePM2.5 40% – ePM1 >80%	Capture capability for PM2.5/PM1

1.2 The “Workhorse” Role in Multi‑Stage Filtration

In a typical HVAC multi‑stage filtration system, medium filters occupy the middle position:

Outdoor air → Pre‑filter (G4) → Medium filter (F5‑F9) → HEPA (optional) → Supply air

Why are they called the “workhorse”?

Stage	Task	Load Share
Pre‑filter	Captures large particles (>5μm), protects downstream	About 30% of mass (mostly coarse dust)
Medium filter	Captures fine particles (0.3‑5μm), main purification task	About 70% of particle count load
HEPA	Captures sub‑micron particles for special cleanliness needs	When no HEPA is used, medium filter is the final filter

In most commercial buildings, general hospital areas, data centers, etc., the medium filter is itself the terminal filter, directly determining the air quality delivered to the occupied space.

1.3 Core Value of Medium Filters

Best price/performance ratio: Optimal balance between efficiency, initial cost, and operating energy
Health protection: Effective removal of PM2.5, bacteria, pollen, and other health‑relevant particles
Equipment protection: Prevents fine particles from entering ducts, coils, and fans, extending system life
Energy contribution: 50‑70% lower pressure drop compared to HEPA filters, significantly reducing fan energy consumption

2. Efficiency Classification and Standards

2.1 EN 779 (Historical Reference)

Although EN 779 was withdrawn in 2022, many products on the market still use this labelling; understanding it is still useful:

Grade	Average Arrestance	Average Counting Efficiency (0.4μm)	Typical Application
F5	—	40‑60%	Basic medium
F6	—	60‑80%	General medium
F7	—	80‑90%	High medium
F8	—	90‑95%	Sub‑HEPA
F9	—	≥95%	Near‑HEPA

2.2 ISO 16890 (Current International Standard)

ISO 16890 is the most authoritative standard today, using ePM1, ePM2.5, and ePM10 to directly reflect a filter’s ability to capture health‑relevant particles:

ISO Grade	ePM1 Efficiency	ePM2.5 Efficiency	Approx. EN 779 Equivalent	Typical Application
ePM2.5 50%	—	≥50%	F5‑F6	Basic medium
ePM1 50%	≥50%	≥65%	F7	High medium
ePM1 65%	≥65%	≥80%	F8	Sub‑HEPA
ePM1 80%	≥80%	≥90%	F9	Near‑HEPA

Important note: Because of different test methods, efficiency values measured by ISO 16890 are usually lower than those labelled under EN 779. A filter claimed to be F7 may only achieve ePM1 55% under ISO 16890. Procurement should always be based on an ISO 16890 test report.

2.3 Correspondence with ASHRAE MERV

MERV	Efficiency 0.3‑1.0μm	Efficiency 1‑3μm	Efficiency 3‑10μm	Approx. ISO Grade
9	—	—	50‑70%	ePM10 60%
10	—	50‑65%	—	ePM2.5 55%
11	—	65‑80%	—	ePM2.5 70%
12	—	80‑90%	—	ePM1 50%
13	≥90%	≥90%	≥90%	ePM1 55%
14	75‑85%	≥90%	≥90%	ePM1 65%
15	85‑95%	≥90%	≥90%	ePM1 75%
16	≥95%	≥95%	≥95%	ePM1 85%

3. Construction Types of Medium Filters

Type	Principle	Advantages	Limitations	Typical Applications
Bag	Multiple bags in parallel, large filtration area	Very high dust‑holding capacity, long life, slow pressure‑drop rise	Bulky, not suitable for tight AHU spaces	High airflow, dusty environments; cleanroom pre‑filtration
Pleated (Mini‑pleat)	Media folded into compact pleats	Compact, low pressure drop, flexible installation	Lower dust‑holding capacity than bag	Data centers, commercial buildings, space‑constrained AHUs
Box (Mini‑pleat rigid)	Deep pleats, sturdy frame	High efficiency, robust construction	Higher cost	Pharmaceutical, hospitals, high‑cleanliness applications
Replaceable‑cartridge	Cartridge replaced, frame reused	Less waste, environmentally friendlier	Higher initial investment	Large central HVAC systems

Comparison Table for Selection:

Parameter	Bag F7	Pleated F7	Box F9
Filter area (592×592)	≈4‑6 m²	≈2‑3 m²	≈3‑4 m²
Initial pressure drop (Pa @ 2.5 m/s)	80‑120	60‑100	100‑150
Dust‑holding capacity (g @ 250 Pa)	400‑600	200‑300	250‑400
Recommended change interval (normal environment)	12‑18 months	9‑12 months	12 months
Relative unit price (reference)	1.0	0.8	1.3

4. Key Performance Parameters

4.1 Filtration Efficiency

Medium filter efficiency is usually expressed as counting efficiency (percentage of particles captured at a given size). For F7 and above, ePM1 efficiency (0.3‑1.0μm) is the key metric.

Requirement	Recommended Minimum ISO Grade	Typical Efficiency
General PM2.5 control	ePM2.5 50%	≥50% for PM2.5
Commercial building IAQ	ePM1 50%	≥65% for PM2.5
Hospitals, data centers	ePM1 65%	≥80% for PM2.5
Cleanroom pre‑filtration	ePM1 80%	≥90% for PM2.5

4.2 Initial Pressure Drop and Final Resistance

Grade	Typical Initial ΔP (Pa @ rated flow)	Recommended Final Resistance (Pa)
F5	50‑70	150‑200
F6	60‑80	180‑220
F7	80‑120	200‑250
F8	100‑150	250‑300
F9	120‑180	300‑350

Impact of ΔP on energy: For a 50,000 m³/h AHU, every 50 Pa increase in pressure drop raises annual electricity cost by approximately 8,000‑12,000 RMB (at 0.8 RMB/kWh).

4.3 Dust‑Holding Capacity

Dust‑holding capacity determines replacement intervals. Medium filters typically have capacities of 200‑600 g (using standard dust).

Ambient Dust Level	Recommended Capacity	Expected Change Interval
Low (office)	>200 g/m²	12‑18 months
Medium (mall, hospital)	>300 g/m²	9‑12 months
High (factory, roadside building)	>400 g/m²	6‑9 months

4.4 Flame Retardancy

For critical facilities such as hospitals and data centers, medium filters should meet flame‑retardant requirements:

Standard	Class	Application
UL94	HF‑1	General commercial HVAC
UL94	V‑0	Hospitals, data centers
UL900	Class 1	Most stringent, critical facilities

4.5 Other Parameters

Temperature resistance: Typically ‑10°C to 70°C; customisable for special conditions
Humidity resistance: Up to 90% relative humidity without condensation
Microbial inhibition: Optional antimicrobial / anti‑mould treatment

5. Applications and Selection Guide

5.1 Typical Applications

Application	Recommended Grade	Recommended Type	Key Requirements
Commercial office	F7 (ePM1 50%)	Bag / pleated	Low noise, low ΔP
Shopping mall	F7 (ePM1 50%)	Bag	High dust‑holding, long life
General hospital ward	F7‑F8 (ePM1 50‑65%)	Bag	Flame retardant, optional antimicrobial
Hospital operating room (pre‑filter)	F8‑F9 (ePM1 65‑80%)	Box	Low particle shedding, silicone‑free
Data center	F7‑F8 (ePM1 50‑65%)	Pleated	Very low ΔP
Pharmaceutical cleanroom (pre‑filter)	F9 (ePM1 80%)	Bag / box	Silicone‑free, chemical‑resistant
School, kindergarten	F7 (ePM1 50%)	Pleated	Low noise, antimicrobial
Industrial plant	F5‑F7	Bag	High dust‑holding, washable option
Metro, airport	F7‑F8	Bag	High dust‑holding, flame retardant

5.2 Selection Decision Tree

Start → Required indoor PM2.5 target? → ≤35 μg/m³ → F5‑F6 (ePM2.5 50%) → ≤25 μg/m³ → F7 (ePM1 50%) → ≤15 μg/m³ → F8 (ePM1 65%) → ≤10 μg/m³ → F9 (ePM1 80%) + possibly HEPA → Is installation space tight? → Yes → Pleated → No → Bag (higher dust‑holding) → Flame retardancy required? → Yes → Specify UL94 HF‑1 or V‑0 → No → Standard product → Special requirements (silicone‑free, antimicrobial, chemical‑resistant)? → Yes → Custom → No → Standard product

5.3 Common Selection Mistakes

Mistake	Consequence	Correct Practice
Choosing too high efficiency	High ΔP, high energy use, premature clogging	Select “right‑fit” efficiency based on actual cleanliness need
Ignoring pressure drop	Increased fan energy, higher PUE	Compare ΔP within same efficiency class; choose lowest
Only looking at purchase price, not lifetime	Frequent changes, high total cost	Calculate TCO; balance unit price and service life
Neglecting pre‑filtration	Medium filter clogs quickly	Install G4 pre‑filter upstream

6. Maintenance and Replacement

6.1 Scientific Change Scheduling with ΔP

Install a differential pressure gauge and replace based on pressure drop:

ΔP Condition	Action	Remarks
Initial +50%	Record, normal	—
2× initial	Schedule replacement (1‑2 weeks)	Still usable, but energy already increased
2.5× initial	Replace soon (within 1 week)	Energy increase significant
3× initial	Replace immediately	Severe clogging, may affect airflow

6.2 Replacement Interval Guidelines by Environment

Environment	Pre‑filter (G4)	Medium (F7)	Medium (F9)
Urban office	6 months	12‑18 months	—
Mall / hospital	3‑6 months	9‑12 months	12 months
Data center	6‑12 months	12‑18 months	—
Factory / workshop	1‑3 months	6‑9 months	9‑12 months

6.3 Maintenance Points

Record initial ΔP immediately after installing a new filter as the reference baseline.
Regular inspection: Read ΔP monthly and plot trend.
Check seals: Inspect gaskets between filter and frame at every change.
Clean surroundings: Remove dust from the filter housing during replacement.

7. Energy Saving Potential of Medium Filters

7.1 Benefit of Low‑Pressure‑Drop Products

For a 50,000 m³/h AHU, operating 8,760 h/year, electricity cost 0.8 RMB/kWh:

Filter	Initial ΔP	Average Operating ΔP	Annual Electricity Cost (est.)	Saving vs. Baseline
Standard F7 bag	120 Pa	180 Pa	≈85,000 RMB	—
Low‑ΔP F7 pleated	80 Pa	130 Pa	≈72,000 RMB	13,000 RMB/year

Payback period: Low‑ΔP F7 pleated filters have about 30% higher unit cost, but the annual electricity saving recovers the difference in 6‑12 months.

7.2 Contribution of Pre‑Filters

Installing a G4 pre‑filter reduces the pressure‑drop rise rate of the medium filter by 40‑50%, extends its replacement interval by 30‑50%, and reduces the time the medium filter spends in high‑ΔP (high‑energy) condition.

8. Case Studies

Case 1: Commercial Complex in Shanghai

Background: 150,000 m² floor area, 4 AHUs, originally used F5 bag filters. Indoor PM2.5 often exceeded 50 μg/m³.

Problems: Insufficient filtration efficiency led to tenant complaints; also F5 pressure drop rose quickly, requiring replacement every 4‑6 months.

Optimisation: Upgraded to F7 bag filters (ePM1 55%) and changed from fixed‑interval to ΔP‑based replacement.

Results:

Indoor PM2.5 reduced to below 25 μg/m³
Medium filter replacement interval extended from 5 to 11 months
Annual media cost slightly increased, but energy cost slightly decreased; overall TCO unchanged

Case 2: Data Center in North China

Background: 5MW IT load, AHUs used F7 bag filters without pre‑filtration.

Problems: Filters replaced every 6 months; rapid pressure‑drop rise caused high fan energy consumption.

Optimisation:

Added G4 pleated pre‑filters upstream
Replaced main filters with low‑ΔP F7 pleated filters (75 Pa vs original 120 Pa)

Results:

Main filter replacement interval extended to 14 months
Fan energy consumption reduced by 9%, saving ≈120,000 RMB/year in electricity
Total annual O&M cost reduced by 25%

9. Frequently Asked Questions (FAQ)

Q: Can medium filters replace HEPA filters?

A: No. Medium filters have limited efficiency for particles

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