Air Filter Buying Guide: Why Size and Efficiency Level Matter

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Be Cautious of High-Efficiency Filters, Which Can Cause More Harm Than Good

One of the most important tasks you can perform to maintain your HVAC system is to regularly change out the air filter. However, it&#;s not just about changing the filter &#; it&#;s about choosing the right filter for your specific HVAC system. Not all filters are created equal, and some high-efficiency filters &#; including HEPA filters &#; may actually do more harm than good for your system.

So, how do you choose the right air filter? HVAC.com dives into the details and explores the factors you should consider when selecting an air filter for your HVAC system.

Finding Your Filter Size

Size matters when choosing the right air filter for your HVAC system. Finding the right size air filter is important to ensure the filter fits properly and performs efficiently. Air filters come in many sizes, and various HVAC systems require filters of varying dimensions &#; you can&#;t simply pick one up at your local big-box store at random.

Using the wrong size filter can lead to a range of issues that affect the efficiency and functionality of the system. Filters that are too small may allow dust and debris to bypass, circulating throughout the system and reducing indoor air quality. Filters that are too large may not fit properly, leading to gaps and bypass airflow, allowing contaminants to enter sensitive components.

It&#;s essential to check the owner&#;s manual for filter specifications before purchasing replacement filters. The manual will tell you the appropriate size filter for your HVAC system. If you cannot locate this information, look at your existing air filter to determine the size you need:

1. Find the existing filter in your HVAC system. It is usually located in the return air duct or near the furnace or air handler.
2. Look for the size information on the side of the filter frame. The size is typically printed on the cardboard edge of the filter.
3. If the size information is not visible, use a tape measure to measure the length and width of the filter. Measure the length (top to bottom), the width (side to side), and the thickness in inches.
4. Write down the measurements. For example, if the measurements are 20 inches by 25 inches, record it as 20" x 25" x 1&#;.

Pleated Vs. Non-Pleated

For the most part, pleated furnace filters are more efficient and last longer than simple fiberglass non-pleated filters. Pleated filters contain much more surface area to catch pollutants/contaminants and have the ability to have smaller pores to capture smaller particles while still allowing full air flow.

Air Filter Efficiency

When it comes to measuring air filter efficiency, a rating system called MERV (Minimum Efficiency Reporting Value) is used. This system rates filters on a scale from 1 to 20, with higher numbers indicating better particle removal efficiency. Filters with a higher MERV rating tend to cost more than less efficient filters.

It&#;s essential to note that air filters with a higher MERV rating may not be appropriate for use with your furnace. Depending on your unit, a high MERV rating can be too restrictive and may affect your HVAC system&#;s overall performance.

• Filters with higher MERV ratings typically have denser materials designed to trap finer particles. However, this density can lead to increased resistance to airflow. If the filter restricts airflow excessively, it can strain the HVAC system, causing reduced efficiency and potentially leading to overheating.
• The higher resistance to airflow caused by a high-MERV filter can force the HVAC system to work harder to maintain the desired temperature. This increased workload results in higher energy consumption, leading to elevated utility bills.
• Strain on the HVAC system caused by restricted airflow can contribute to premature wear and tear on components such as the blower motor and the fan. This can cause more frequent breakdowns and a shortened overall lifespan of the system.
• A filter with too high of a MERV rating may impede the flow of conditioned air into your living spaces. This can result in uneven heating or cooling and decreased comfort levels throughout your home.

To avoid these issues, it&#;s essential to choose an air filter with a MERV rating that aligns with the recommendations of your HVAC system&#;s manufacturer. Most residential HVAC systems work well with filters in the MERV 8 to MERV 13 range. Higher-rated MERV filters should only be used with systems designed to accommodate such filters.

Is a HEPA Filter Worth It?

HEPA stands for High Efficiency Particulate Air filter, and it&#;s made up of densely packed layers of fine mesh. These filters are designed to trap tiny particles like dust, pollen, and even bacteria. In fact, they&#;re so effective that they&#;re often used in hospitals and clean rooms. You&#;ve probably heard of them before, and you may be wondering if a HEPA filter is worth it.

HEPA filters have a very high MERV rating, typically around 17-20. While that may sound impressive, it also means that they can be too restrictive for most residential HVAC systems. In plain English, HEPA filters are too good at their job, and they can make your HVAC system work harder than needed. This can result in decreased airflow, which can lead to issues like frozen coils or even system failure.

While HEPA filters may be worth the investment in certain situations (such as in a hospital or laboratory), they&#;re generally not recommended for use with a standard furnace in a residential home.

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Media Air Cleaners

If you want the power of a high-efficiency filter, installing a media air cleaner may be an option. These units use thicker filters with a MERV rating between 13 and 16, which means they can capture even smaller particles like pollen, mold spores, and pet dander.

While they work with most residential systems, some units may not be able to handle the increased airflow resistance that comes with a high-MERV filter. To avoid damaging your HVAC equipment, it&#;s best to check with a licensed contractor before purchasing a media air cleaner. They will be able to assess your system and recommend the best option for your needs.

For this series on understanding wire mesh for filtration, in part one we discussed different mesh weaves and their roles in filter design.  Here in part two, we will consider factors that degrade mesh performance as it relates to mesh aperture.

When choosing a micronic mesh for a filter application, key variables often overlooked are the durability of the mesh, the flow rate through that mesh &#; and how the two are connected.  Micron mesh filters tend to suffer damage in four different ways: sustained high pressure, repeated pressure fluctuations, corrosion, and operator error when cleaning and servicing the filter.

For each of these sources of damage we will consider filter design missteps and review design best-practices to avoid them.

Sustained Pressure

As particles caught by the micron mesh filter gradually block the screen the back pressure will increase.   In some instances, the operation of the system calls for periodic deblinding before the pressure exceeds a threshold beyond which the mesh suffers damage.  Left unattended, this pressure could simply exceed burst pressure, which is a durability catastrophe.  Before burst pressure is reached, however, less noticeable system failure can occur like exceeding a bypass valve setting, which could allow unfiltered fluid and solids where they are not wanted.  An improved design can raise the damage threshold of the filter, and better maintenance adherence can prevent the mesh from suffering damaging pressures to begin with.

Repeated Pressure Cycling

In a properly designed and maintained filter system, the durability of the micron mesh should not be significantly compromised by normal use and operation.  In addition to considering the effects of constant pressure on a mesh filter, the frequency of pressure changes must be considered as well.  A mesh that might be just fine against a constant pressure of 100 psi could fail quickly from pressure oscillating between 0 and 50 psi.

Pressure oscillations like you get from a diaphragm pump or an automated system repeatedly triggering a backflow cleaning cycle can compromise the mesh by work-hardening the wire far sooner and with less overall pressure differential than a system with a steady pressure.  

This image shows the effects of fatigue on the mesh filter layer of a multi-layer candle-type filter assembly.

Overhandling

Over the course of a filtration session the micron mesh is slowly blocked by particles.  This will reduce the volumetric flow rate through the filter or result in a differential pressure building until the same volumetric flow rate is achieved through brute force.  The added pressure can cause wear and damage to the mesh. Alternatively, the filter will need more frequent cleaning and maintenance, increasing the opportunities for wear and tear to the mesh through handling (or mishandling).  The loss of production time can also have a significant economic impact. 

Corrosion and Abrasion

Micron mesh filters can also fail from corrosion and abrasion.  While these durability issues may not seem tied directly to aperture, they are not entirely independent of it either.  As the wires are polished down by micro-abrasives or eaten away by corrosives, not only are the thinner wires more likely to tear, but the apertures will also widen and let bigger particles through.  Your filtration mesh expert can advise you on wire mesh alloys to stand up to even the most extreme corrosive or abrasive fluid.

Start with Durability in Mind

• Don&#;t make your filter work harder than it must.  Choose a micron mesh with a particle retention no smaller than necessary.
• Instead of selecting a micron filter mesh with the greatest possible flow rating, consider one with the same micron retention size but with a more robust construction, even if it means increased overall surface area of mesh will be needed.  It will last longer and require less maintenance. 
• Establish maintenance protocols that minimize chances of damaging the mesh filter or construct the filter to stand up to some rough handling.
• Reduce the possibility that the wires in the mesh can shift around relative to one another.  This is called fixing the aperture of the mesh, which reduces the likelihood that manufacturing methods, pressure spikes, or operator mishandling will deform the mesh or distort its aperture.  Ways to fix the aperture include:

1. Calendaring, which compresses the mesh between two heavy rolling pins to flatten the knuckles and nest the warp and shute wires together at each wire intersection, so they are less likely to shift around during forming,
2. Heat treating before forming. This reanneals the mesh and decreases the spring tension which makes forming more challenging.  Heat treating after forming decreases stresses in the wires induced by forming.  
3. Sintering, which begins with calendaring to flatten the knuckles, then heat treating just below melting temperature to bond several layers of mesh together. 

Consult a wire mesh filtration expert to learn more about striking the right balance between durability, aperture, and flow rate. 

Gerard Daniel Expertise

At Gerard Daniel, we&#;ve been sourcing, weaving, and distributing wire mesh for 70 years and stock the largest range of inventory in North America. Our application engineers use their deep expertise in woven mesh and components to develop the most effective solution, collaborating with your design, development, and production teams to ensure the lowest cost to manufacture. Please reach out if you would like to learn more about our capabilities.

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