MERV 4 Air Filters | Washable HF Series Two-Stage Foam Filters

Shop MERV 4 Air Filters

Standard sizes shown above. Custom-cut HF Series foam filters are available to your specified dimensions. View the complete size reference table below for all 14 standard sizes.

What Are MERV 4 Air Filters?

The MERV (Minimum Efficiency Reporting Value) rating scale, defined by ASHRAE Standard 52.2, measures how effectively an air filter captures particles of specific size ranges. MERV 4 is the second-lowest efficiency level on the scale, targeting particles 10 microns and larger — the category that includes household dust, pollen, dust mite debris, carpet and textile fibers, lint, and coarse industrial particulate. A MERV 4 filter captures these particles at 75% or higher dust arrestance efficiency (synthetic dust test per ASHRAE 52.1) while maintaining the low static pressure drop that residential blower motors, electronics cooling systems, and airflow-sensitive HVAC installations require.

MERV 4 is not intended for removal of fine particles such as tobacco smoke, mold spores, or respiratory aerosols — those applications require MERV 8 or higher. What MERV 4 does exceptionally well is capture the coarse particulate load that accumulates on heat exchanger coils, fan blades, circuit boards, and cooling fins in residential and light commercial HVAC systems. By removing this coarse dust before it can accumulate on system components, a MERV 4 washable filter extends equipment service life, maintains heat exchanger efficiency, and protects electronics from the thermal resistance caused by dust accumulation — all while maintaining airflow volume that higher-MERV-rated filters would restrict.

The HF Series two-stage foam filter is American Metal Filter Company’s MERV 4 product line for applications that demand washable construction, low air resistance, and long service life. Unlike disposable glass-fiber or synthetic-media MERV 4 filters, the HF Series eliminates recurring media replacement cost — a standard warm-water rinse restores the two-stage aluminum mesh and foam media pack to full filtration performance. The all-aluminum frame with mitered pop-rivet corners provides structural durability through decades of wash cycles and HVAC service, maintaining the critical face-dimension tolerances (±1/8”) required for proper fitment without air bypass gaps.

Two important care requirements distinguish the HF Series from all-metal washable filters: first, do NOT coat HF Series filters with any adhesive — AMFCO explicitly states that adhesive coating is not advisable for this filter type, as it clogs the 25 ppi foam cell structure and impairs washability. Second, do NOT expose HF Series filters to direct sunlight or ultraviolet light — UV radiation degrades polyurethane foam media over time. Store and install away from UV sources including skylights, sun-facing windows, and outdoor equipment with exposed filter sections.

Technical Specifications — HF Series MERV 4 Foam Air Filters

All specifications confirmed from AMFCO HF Series product data and WooCommerce product records (verified April 2026). Specifications apply to all standard HFA sizes unless noted.

Standard Size Reference — HF Series MERV 4 Foam Filters

All 14 standard HF Series nominal sizes are listed below in both 1-inch and 2-inch depths. Actual face dimensions follow the 1/2-inch undercut rule: actual height and width each equal nominal minus 1/2 inch. Depth is 7/8” actual for 1-inch nominal and 1‑3/4” actual for 2-inch nominal.

1-Inch Nominal MERV 4 Foam Filters (7/8” actual depth)

2-Inch Nominal MERV 4 Foam Filters (1‑3/4” actual depth)

Need a size not listed? Custom-cut HF Series foam filters are available to your specified height, width, and depth. Contact us with your exact required dimensions.

Two-Stage Construction: How the HF Series MERV 4 Foam Filter Works

The HF Series achieves MERV 4 efficiency through a progressive-density two-stage media pack that places the coarser aluminum mesh stage upstream of the finer foam stage. This sequential arrangement is what distinguishes the HF Series from single-stage all-metal washable filters and single-stage foam filters — and it is why the HF Series delivers both higher efficiency and longer service intervals than simpler designs.

Stage 1: Aluminum Mesh Prefilter

The first stage consists of an expanded aluminum grid corrugated to increase surface area, combined with layers of corrugated aluminum screen wire assembled in a criss-cross pattern for structural strength. This aluminum mesh stage captures larger airborne particles — dust, lint, pet hair, pollen, coarse industrial particulate — before the airstream reaches the foam stage. By removing large particles at Stage 1, the aluminum mesh protects the 25 ppi foam from rapid loading, which would otherwise quickly increase static resistance and reduce the service interval between wash cycles.

Stage 2: 25 ppi Open-Cell Polyurethane Foam

The second stage is open-cell polyurethane foam with nominal 25 pores per inch (±5 ppi tolerance), the grade that balances particle capture efficiency with low air resistance. Pre-filtered airstream from Stage 1 passes through the foam cell structure; particles are captured through mechanical interception and inertial impaction. The 25 ppi density achieves MERV 4 rated efficiency while keeping static pressure drop low — the characteristic that makes the HF Series suited for electronics cooling and airflow-sensitive HVAC installations where higher-MERV filters would degrade cooling effectiveness or stress blower motors.

Exit Support Grid

An expanded aluminum support grid on the air-exit side supports the two-stage media pack against airflow pressure forces at rated 300 fpm, preventing media deflection or downstream displacement during HVAC operation. Aluminum construction ensures corrosion resistance through years of wash cycles and HVAC service.

All-Aluminum Frame

The .030” thick aluminum perimeter frame provides structural rigidity while maintaining critical face-dimension tolerance (±1/8”) for proper fitment in standard HVAC housings. Precision-mitered 45-degree corners secured with pop rivets create true square corners essential for proper seating without air bypass gaps. Drain holes in three corners provide structural drainage; HF Series filters may be installed in any orientation without drainage consideration.

Applications for MERV 4 Washable Foam Air Filters

Installation

HF Series MERV 4 foam filters install in any standard HVAC panel filter housing accepting the nominal size. The HF Series may be installed vertically or horizontally in any orientation without performance impact.

Before Installing

• Verify the housing accepts the nominal size and that interior dimensions accommodate the actual dimensions (±1/8” on face; ±1/16” on depth)
• Confirm system operating temperature does not exceed 250°F at the filter location
• Confirm the filter location is not exposed to direct sunlight or UV light
• Confirm no adhesive overspray or chemical vapor will contact the foam media
• Clean the housing interior of accumulated debris from the previous filter service

Installation Steps

1. Shut down your HVAC system before accessing the filter section
2. Remove the existing filter and clean the housing interior
3. Insert the HF Series filter in either vertical or horizontal orientation
4. Press firmly into place verifying all four frame edges seat against housing walls with no air bypass gaps
5. Secure using the housing’s retention mechanism and close the access panel
6. Restart the system and record the installation date for wash interval tracking

Cleaning and Maintenance

The HF Series foam filter is rated for “frequent and moderate” washing (AMFCO classification). A straightforward warm-water rinse is the only procedure required to restore full MERV 4 filtration performance.

Washing Procedure

1. Remove the filter from the housing
2. Hold the filter with the dirty (air-inlet) face directed away from you
3. Rinse with a moderate-to-heavy stream of warm water from the clean (exit) side through to the dirty (inlet) side
4. Continue rinsing until water runs completely clear through the foam media
5. For heavily loaded filters, allow a brief warm-water soak before rinsing
6. Shake off excess water and air dry before reinstalling in moisture-sensitive systems

What NOT To Do

• No adhesive coating — ever. AMFCO explicitly states adhesive coating is not advisable for HF Series filters.
• No UV or sunlight exposure. UV light degrades polyurethane foam over time.
• No high-powered steam cleaning. Not necessary; not recommended.
• No chemical dips. Not necessary; not recommended.
• No vigorous scrubbing that deforms the aluminum mesh stage.

Washing Frequency

• Residential / light commercial HVAC: every 1–3 months
• Electronics cooling / computer rooms: every 3–6 months
• Industrial / machinery ventilation: monthly or more frequently
• Best indicator: visual inspection — wash when the aluminum mesh prefilter stage is visibly loaded with dust, or when system airflow noticeably decreases

Frequently Asked Questions About MERV 4 Air Filters

What is a MERV 4 air filter?

MERV 4 is the second-lowest efficiency level in the MERV (Minimum Efficiency Reporting Value) rating scale. A MERV 4 filter captures particles 10 microns and larger — including household dust, pollen, dust mite debris, lint, carpet fibers, and spray paint particles — at 75% or higher dust arrestance efficiency. MERV 4 is the appropriate rating for general residential HVAC systems, electronics cooling, machinery ventilation, and any application where low air resistance is as important as particle capture efficiency.

How does the HF Series two-stage foam air filter work?

The HF Series uses two sequential filtration stages. Stage 1 is an aluminum mesh prefilter — expanded aluminum grid plus corrugated aluminum screen wire in a criss-cross pattern — that captures larger airborne particles (dust, lint, pollen, pet hair) before the airstream reaches Stage 2. Stage 2 is 25 ppi open-cell polyurethane foam that captures smaller particles through mechanical interception and inertial impaction. This progressive density approach keeps large particles out of the foam stage, extending service intervals between wash cycles.

What are the actual dimensions of HF Series MERV 4 foam filters?

HF Series filters use a 1/2-inch undercut on height and width — actual dimensions equal nominal minus 1/2” on each face dimension. A 20×20×1 nominal filter has actual dimensions of 19.5” × 19.5” × 7/8” (±1/8” on face dimensions, ±1/16” on depth). A 20×20×2 nominal filter measures 19.5” × 19.5” × 1‑3/4”. The same 1/2-inch undercut rule applies to every standard size in the HF Series.

Can HF Series MERV 4 foam filters be coated with adhesive?

No. AMFCO explicitly states that coating the HF Series filter with any adhesive is not advisable. The two-stage foam and aluminum mesh construction provides effective mechanical particulate capture without adhesive treatment. Adhesive coating can clog the 25 ppi foam cell structure, increase static resistance, and impede subsequent washing. Do not apply Filter Coat, tackifier, or any other adhesive treatment to HF Series filters.

Can HF Series MERV 4 filters be exposed to sunlight or UV light?

No. Do NOT place HF Series filters in direct sunlight or ultraviolet light. UV radiation degrades the polyurethane foam media over time, causing foam breakdown, reduced filtration efficiency, and potential media fragmentation. Store and install the filter away from UV sources — including direct sunlight through windows, skylights, outdoor equipment with sun-exposed filter sections, and any UV-emitting equipment nearby.

What is the maximum operating temperature for HF Series MERV 4 foam filters?

The HF Series MERV 4 foam air filter is rated for continuous operation at 250°F maximum. This covers the requirements of most commercial and residential HVAC applications with elevated supply air temperatures. Note that this is lower than the all-metal HI Series aluminum filters (275°F) or the galvanized HP Series frames (325°F), because the polyurethane foam media has a lower temperature ceiling than all-metal filter media.

How do I clean an HF Series MERV 4 foam filter?

Remove the filter and rinse with a moderate-to-heavy stream of warm water from the clean (air-exit) side through to the dirty (air-inlet) side until water runs completely clear through the foam media. High-powered steam cleaning and chemical dips are unnecessary and not recommended. Shake off excess water and allow the filter to air dry before reinstalling in moisture-sensitive systems. Do NOT apply adhesive coating after washing — this is specifically not advisable for the HF Series foam filter.

How often should I wash an HF Series MERV 4 foam filter?

Washing frequency depends on the application and particulate loading. General residential and commercial HVAC: every 1–3 months. Electronics cooling and computer room applications with lower dust loads: every 3–6 months. Industrial or machinery ventilation with higher particulate loads: monthly or more frequently. Visual inspection is the most reliable indicator — wash when the aluminum mesh prefilter stage appears visibly loaded with dust, or when system airflow noticeably decreases from the clean-filter baseline.

What is the difference between MERV 4 HF Series and HI Series (aluminum mesh) filters?

Both are washable MERV 4 filters made by American Metal Filter Company (AMFCO), but they use different media. The HF Series uses two-stage foam and aluminum mesh — designed for low air resistance in electronics cooling, computer rooms, and airflow-sensitive HVAC. The HI Series (HIA) uses all-aluminum screen wire mesh — designed for general HVAC and higher-temperature industrial applications (275°F vs 250°F for HF Series). The HF Series foam provides higher dust arrestance (75% at 300 fpm) with lower pressure drop; the HI Series all-metal construction handles higher temperatures and can be coated with filter adhesive (HF Series cannot).

Why Choose HF Series MERV 4 Washable Foam Filters?

The HF Series MERV 4 foam air filter is the engineering choice for HVAC applications where permanent washable construction, low air resistance, and long service life matter more than high-efficiency fine particle capture. With 75% dust arrestance at 300 fpm for 1-inch depth — rising to 85.4% for 2-inch depth — the HF Series delivers verified MERV 4 performance through the two-stage aluminum mesh and 25 ppi foam media pack that makes this filter uniquely suited to electronics cooling, computer room air conditioning, residential HVAC, and machinery ventilation applications.

Every HF Series filter is manufactured by American Metal Filter Company with the all-aluminum .030” frame, mitered pop-rivet corners, and two-stage media pack that have characterized AMFCO washable filter construction for decades. UL Classified 900 certification, 250°F maximum continuous operating temperature, and vertical-or-horizontal installation flexibility make the HF Series a practical, durable specification for building engineers, facility managers, and residential HVAC installers who value permanent washable construction over recurring disposable media programs.

Available in 14 standard nominal sizes from 12×24 to 24×24 in both 1-inch and 2-inch depths, with custom cut-to-size capability for non-standard housings. Remember: no adhesive coating (specifically not recommended for foam), no UV/sunlight exposure (degrades foam), and wash with warm water for lasting performance.