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HCAC filter being removed from an HVAC system

The Best HVAC Filter?-Time to Change Your Filter Technology

Most people spend approximately 90% of their time inside, either at home or work. Also, air pollution can be much higher in indoor air than it is outdoors, and the health effects of air pollution are becoming more obvious. So indoor air quality is becoming more of a concern, and so it is natural to consider if our HVAC systems could help purify the air.

An HVAC (home ventilation and air conditioning) filter is the filter on the inlet to the HVAC system. It is sometimes called a “furnace filter”.This is the filter in the return air duct, the “return” refers to bringing the indoor air back to the HVAC unit. From this duct the air enters the air handler unit, which has heating and cooling elements in it (please see diagram below). These filters are sometimes known as a “furnace filter”.

The best HVAC filter is a HEPA filter, but not all HVAC systems can take one without remodelling as there needs to be enough air flowing past the heating and cooling coils in the HVAC system.

There is a minimum air flow needed to stop the cooling coils in the HVAC unit when in use from forming ice, which can damage them and the air conditioning system. Also, when the heating elements in the HVAC unit are in use, the air flow needs to stop them from overheating and cracking.

As the density of the filter increases, the more effective it is at removing fine particles, but the more difficult it is for the fan to push enough air through the filter. So to protect the HVAC system, a thin porous filter allowing a high airflow is often used. These filters do little to improve air quality in the range of particle sizes that damage human health. A MERV 13 filter, for instance, may remove less than 50% of fine particle from the air each time the air goes through it.

So in effect currently most people tend to have filters that protect the HVAC system but not themselves!

Now that we know that ultrafine particles when breathed in enter the bloodstream and have been found in the brain, it could be a time for a rethink. Especially as fine particle air pollution has been linked to 12% of the total annual death toll in the United States.

There is a now an HVAC filter system that allows very efficient small particle whole house filtration with your existing HVAC system. We outline this below-in the HVAC Filter Efficiency section.

Construction of an HVAC Filter

An HVAC filter is a flat panel of composed of fibrous material that can filter out airborne contaminants from the air. The MERV-Minimum Efficiency Reporting Value indicates the filtration efficiency on a range of 1 to 20.

Fiberglass filters(Merv 2-4) These can be recognized as they are blue or green and can often almost be seen through. They are made from spun fibreglass. They are cheap $5-10 but they have a very limited ability to trap pollutant/contaminants.

Washable filters-these can be recognized by the plastic frame. They can be electrostatic as well (Merv 8) ie self-charge with static electricity as the air flows through them. As this type of air conditioning filter age, they gradually lose their self-charging ability.

Pleated filter(Merv 6-13) These are a type of media filter. They disposable and made of polyester, polypropylene, acrylic fibers, or cotton paper. Cotton paper filters have a greater ability to filter particles and can act to an extent like electrostatic filters by self-charging. Some of these filters have a permanent electrostatic charge-increasing their efficiency for capturing small particles. Cost $5-20 depending on the efficiency of filtration-well worth it for the lower frequency of needing changing (see below). These have a variable ability to trap airborne particles but even Merv 13 filters may not be adequate if you are wishing to reduce the health impacts of pollution.

Electrostatic filters-these have paper or cotton as the main media which spontaneously develops a positive charge. This helps clean the air as negatively charged particles stick to the positively charged filter. This electrostatic effect is particularly effective at reducing small particle air pollutants. So these filters are especially useful for cigarette or forest fire smoke. They come in disposable or washable forms.

None of the above air conditioning (AC) filters are really adequate for removing particles from the air to protect your health, but they do protect the air conditioner. The following AC filters are capable of removing smaller particles but only the HEPA filter is really very effective, although some MERV 16 filters are fairly good.

HEPA Filters (Merv 17-20) Some HVAC systems are rated for HEPA filters. HEPA filters are used in most air purifiers as they are extremely efficient at removing particles from the air. These really increase resistance to airflow in the HVAC system, and their use in a system not rated for HEPA filters may damage the system. They are expensive $75-100 and usually need changing every 1-3 months.

Electronic Air Cleaner-this uses ionizing elements, the mechanism of this type of air purifier system is explained in greater detail in this post, but we include a short explanation here. In an HVAC system, the air goes through an HVAC filter first as the electronic air cleaner is not efficient enough to replace a filter. This will at least take out large particles such as pet dander, house dust mite, and mold spores. The air then goes past wire electrodes which are negatively charged, and particles in the air are ionized. These negatively charged particles then pass close to positively charge plates to which many of them are attracted to and then stick to the plates. As the particles stick to the plates they become covered in the particles and so stop attracting further particles. So the plates need cleaning once per month. Some people would find this too much additional maintenance. So simply opting for a more maintenance free setup with a HEPA filter would be attractive. The HEPA filter would need changing periodically but you would be doing that anyway, there would be no need to wash electronic air cleaner plates every month. However, the electronic plates are good at capturing finer particles-better than most HVAC filters but not as well as HEPA filters. So if your system cannot take HEPA filters, the electronic air cleaner will help provide additional air cleaning. They cost about $1000-$1500 to install. We would suggest this money could be better spent remodelling your HVAC system to increase its filtration efficiency.

All filters will become blocked with particles from the air. The dirty air filter then needs changing for a clean HVAC air filter, or washing so that an adequate flow of air can reach the air handler unit of the air conditioner. The only reusable filter is the washable air filter the others are disposable filter types apart from the electrostatic filter which with cleaning should last long term.

Smart HVAC Filters

The blockage of the filter and the rise in pressure as the air is forced through the filter is used by a new generation of smart HVAC filters. These have a sensor on the filter which then connects to your mobile phone. When the pressure rise indicates that the filter needs changing, the mobile phone app will alert you and ask you if you want to reorder a replacement. An example of this type of filter is the Filtrete Smart Replenishable AC Furnace Air Filter. If your air has fewer particles than an average house and so the filter needs changing less often than average, this could save you money as you will be replacing the filters only when they really have reached end of life. However, they are considerably more expensive than other brands without the bluetooth sensors, so you could even end up spending more on filters. Obviously you could try one pack and see if it works out cheaper for your HVAC system.

Apart from the expense, you may appreciate a longer period between filter changes if you have fewer particles in your air than the average home. Less maintenance is always welcome, and you may also like having the filter remind you to change it!

Smart Owner Testing HVAC Filter?

You may think that instead of paying for a smart HVAC filter that you will be a smart owner by using a particle meter to check your HVAC filter. You would then only replace the filter when the particle count of the increases. This seems very logical but has two problems-

  1. As the filter becomes blocked, even if the particle count does not increase the pressure across the filter will. This could potentially damage the AC fan/blower as it is having to work harder to pull the air through the filter. This increased work may also increase electricity costs. As a hobbyist, you could buy a pressure sensor to measure the pressure in the HVAC system and only change the filter when it rises above the specification for your system. You could then use much cheaper filters. You could also connect it to a small $30 computer such as a basic Raspberry Pi so that you are automatically alerted by text message if the pressure exceeds a certain threshold. It is only a matter of time until someone writes the software for this and it is freely available.
  2. You will not know whether the particles most important for health, the ultrafine particles (UFP), are being filtered out efficiently. This is because particle counters that can count these particles are very expensive, approximately $4000. So it is probably best to stick to the manufacture’s specifications. Even though we cannot be certain that the manufacturers check UFP performance for their filters. If they do, we have never seen it released to the public. We can only hope for the best and assume that they do!

Size of HVAC Filters

HVAC filters come in thicknesses of 1 to 5 inches. They come in many sizes. Common sizes are 30 x 20 inches, 25 x 20 inches, 25 x 16 inches, and 20 x 20 inches.

The larger the filter size, the more air can be processed per unit time for any given filter density and fan power. That is the bigger the filter, the more air flow can go through the system if everything else stays the same. The thicker fibrous media pleated filters, thicker than 1 inch in general, need changing less often.

Position of Filter in HVAC System

The HVAC filter filters the air entering the air handler unit where the fan, heating, and cooling coils are and which need protecting from dust.

This diagram illustrates the position of the filter in a HVAC system-

Diagram showing that the filter is positioned before the air handling unit containing the fan, heating elements and cooling coils.
The filter is positioned before the air handling unit containing the fan, heating elements and cooling coils.

Changing the Filter

The HVAC filter should be changed every 1-12 months depending on filter type, thickness, how much your HVAC system is on and how dusty the air in your home is.

Assuming a normal amount of dust in your environment, the following table gives an indication of the frequency of filter changes that your HVAC system will need.

Type of FilterMonths Until Filter Will Need Changing
Fiber Glass1
HEPA Filter1-3
Pleated Fabric 1 inch thick3
Pleated Fabric 4 inch thick6
Pleated Fabric 5 or more inch thick12
Table of filter type and usual interval between changes

Here is a demonstration of how to change the filter-

The filter can be behind a grill in a wall or in the ceiling, depending on the design of your system.

As you can appreciate having seen a filter changed, many filters do not have airtight seals around the edges. Up to 30% of the air can under suction from the fan get through these small channels around the filter. So even if a filter is 90% effective if it is only filtering 70% of the air it is only removing 90% of 70% of particles ie 63%. So a leak can make a very big difference to overall system efficiency.

HVAC Filter Efficiency

An HVAC/air conditioning system filter needs to be dense enough to keep particles out of the HVAC machinery, but not so dense that it puts a strain on the fan trying to move air through the system. So the mesh of the filter needs to be loose enough for a good quantity of air to flow through the furnace and over the air conditioning coils. If not, it will put a strain on the fan trying to circulate the air around the system-which can burn out.  As the fan will work harder to pump the air around the system, it will need to use more electricity so increasing costs. A low airflow may also lead to ice forming on the air conditioning coils. Liquid coolant may then return to the cooling system and damage it. The heat exchanger of the furnace also needs a minimum airflow to take the heat away into the air, so with a low airflow, it may overheat. It can then crack and release carbon dioxide into the HVAC system-this can be fatal.

So it is important not to put in a very efficient filter (high MERV rating-see below) which your HVAC system cannot cope with.

Another problem with the reduced air flow is that you will not feel as comfortable as less conditioned air is delivered to each room. 

On the other hand, the problem with very low resistance filters are not suitable for air purification – they let too many particles through. So conventionally a balance has to be struck, although there is a recent development outlined below which may give you the best of both worlds. 

As any HVAC filter gathers dust, it clogs up the pores in the filter. So a dirty filter will cause an increased resistance to air flowing through the system. The clogged air filter then needs replacing with a clean filter to restore the air flow the system needs. The frequency of maintenance of the system is a factor in deciding what is the best HVAC filter for your system. Most people will want to decrease the frequency of maintenance, but the tradeoff is that filters with longer times between replacement tend to be more expensive.

The filtration efficiency of HVAC filters is measured in grades of MERV-Minimum Efficiency Reporting Value. This grading system was introduced in 1987 by the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE). The higher the MERV rating, the greater the efficiency of the filter in filtering out fine particles. As seen from the diagram below, a MERV 5 filter will never even filter 50% of particles, no matter what the size of particle. All filters have the most difficulty filtering particles at about 0.1-0.3um. HEPA filters that are found in air purifiers will filter 99.97% of any particle size, whereas a MERV 12 filter will only filter 35-60% of the most difficult to filter particles. If you can increase your HVAC system filter to one of the best MERV 16 filters, this will take out 95% of even the most difficult particles and close to 99% of the rest.

Diagram to give a general idea of the efficiency of filters at various particle sizes (for more precision see this study)
Diagram to give a general idea of the efficiency of filters at various particle sizes (for more precision see this study)

Various corporations have developed their own filter efficiency ratings. The Micro-Particle Performance (MPR) Rating system was produced by 3M to grade the ability of the filter to filter out particles less than 1 um. The range for this scale is 300-1900. Home Depot has its own rating system, “the Home Depot Air Filter Performance Rating” scale 4-10. 10 being the highest filtration performance.

So the higher the MERV rating the more efficient the filter but the more expensive it is. Also, the greater the difficulty for air passing through the filter and the greater the possibility of it damaging your HVAC system. Filters in the MERV range 7-12 are thought to strike the best balance between cost, obstruction to airflow, and filtration efficiency. In general, MERV 13 is thought to be the highest MERV value safe for most residential furnaces.

However, a new development may allow many existing HVAC systems to have a higher quality of air filtration-MERV 16. iQair has developed an HVAC filter which reduces PM2.5 (less than 2.5um diameter) and ultrafine particles (less than 0.1um) by 95%. Due to the pleat design of the filter (Nanomax) it has a surface area presented to the air 6 times that in a normal HVAC filter. So it does not produce as much resistance to airflow and is suitable for many existing HVAC systems. It is the same size as a standard HVAC filter. Also, it only needs changing once per year. It only comes in 2-inch thickness. If you want excellent filtration from your existing HVAC system, this may be your best choice. Before using it, you would need to ask an HVAC engineer or technical support at iQair whether it would be suitable for your particular HVAC system.

The other option is to remodel the filter section next to your furnace. The only whole house system with a MERV 16 rating is the iQair Perfect 16 system. This starts at $2700. Another option would be to replace your HVAC system for one that can handle HEPA filters. This could be expensive.

HVAC Filters Can Come With Activated Carbon to Deal With Chemicals and Odors

Activated carbon absorbs chemicals very effectively and safely. For instance, activated charcoal has been given to patients to swallow if they have taken an overdose-to absorb the substance taken.

So if you have a problem with chemicals or odors in your home, you can buy HVAC filters with activated charcoal on it. The amount of charcoal on the filter is small, a few ounces at most. This is nothing like the 5-10 pounds in some air purifiers which service a single room but will be of some help.

Air Flow Through the HVAC Filter is Usually Intermittent-Governed by Temperature Not Air Quality-You Would Need to Override This and Have the Fan on All the Time to Purify the Air

The HVAC/Air Conditioner system is only on and air going through the HVAC filter when the temperature is outside the range set on the thermostat. This is usually only 12-25% of the time. There is usually not a constant stream of filtered air entering the room. Fortunately, most HVAC units do have an option to have the fan on constantly, which you would need to use if you wish to purify all the house air with an HVAC system.

Particles are very difficult to effectively remove from the air. For proper air purification room air needs to be going through a HEPA filter continuously while people are in the room. A HEPA filter is particularly efficient at removing airborne particles as it has a much finer mesh than a usual HVAC filter.

Your HVAC System May or May Not Push Enough Air Through the Filter When the System is On to Purify the Air

To adequately purify the air in a room, there should be 5 air changes per hour through the filter. It does not matter how good the filter is, if not enough air is going through it. A fairly typical HVAC fan will output 1000 cubic foot/minute of air, even if all of this goes into the living space this would only be ideal for a 1500 square foot room area. This assumes an 8-foot ceiling height. You could get around this by closing the vents in rooms that you were not using, so the filtered air has to go into rooms that you are using. If you also have air purifiers, you could also close off the vents in those rooms with air purifiers. So you may prefer to have air purifiers in some rooms and using the HVAC system for other areas The other option is to replace the fan in the air handling unit with a more powerful one.

HVAC Filters and Protecting Health

The smallest particles form 90% of particles in the air, and they can reach into the deepest parts of our lungs. There are approximately 2,000,000 of these particles per cubic litre of normal room air. The potential most damaging particles are the smallest of these particles Nano-particles of 20-50nm that can get into our bloodstream. From there they can reach our hearts, blood vessels, brain, the placenta of an unborn child they are filtered out by our kidneys. Of course a few are left in our internal organs and blood vessels and this is thought to be implicated in various diseases.

It is very difficult to effectively purify the air even with HEPA filters. So trying to do so with a HVAC filter that not a top MERV 16 filter is unlikely to succeed to an extent that will protect your health. As you can see in the filter efficiency diagram above, the EPA specification for a MERV 16 filter could still allow a significant proportion of particles to stay in the air. So if the filter is not certified as HEPA, you really should see a graph from the manufacturer of filter efficiency across the whole range of particle sizes. Ideally, it should show at least 85% efficiency in the particle size range 20-60nm.

There is one study that modelled the effect of HVAC filter efficiency on PM2.5 particulate air pollution and calculated the increase in life expectancy. It analysed 22 US locations. It found that for old “vintage” houses life expectancy would be increased by a month in going from a MERV 5 filter to a HEPA filter. The effect was less marked for existing houses with a gain of approximately 0.35 of a month and for new houses no gain in life expectancy. There was a continuous increase in benefit with increase in filter efficiency throughout the range of filter efficiency. However, a MERV 16 filter was very nearly as good as a HEPA filter.

The average percentage of the time the HVAC system was assumed to be on in this study was 20% for vintage homes, 13% for existing homes and 16% for new homes. So if the system were run all the time the effect may be expected to be 5-8 times greater. Also, they only considered the PM2.5 coming from outdoor pollution and not any element coming from indoor pollution. This only represents 50% of the particulate pollution in the home the other 50% coming from sources in the home such as cooking. So the effect of using an HVAC system continuously to purify air may be (2 x 5 to 2x 8) that is 10-16 fold higher. That would equate to an increased life expectancy of 1 year for people in a “vintage house” and 4 months for people living in an “existing house” as defined by this study. Assuming a couple live in a “vintage house” this could translate to a combined 2 year gain in life expectancy. ICER values a quality life year (QALY) at $50,000-150,000 so this couple could gain $100,000 in health gains from using their HVAC system as a whole house air conditioner. So $3000 to remodel your HVAC system may be a reasonable investment.

Another study showed that going from MERV 5 to MERV 12 filters in the HVAC system reduced PM2.5 particle count in the home by 50%. Increasing from MERV 12 filter to a MERV 14 filter made little difference.

The above paragraph only considers at gain in QALYs due to avoiding death there would be additional QALY gain from avoiding the illnesses that have been linked to air pollution such as respiratory disease, heart attack, stroke and dementia.

Now we would add that these figures are approximate and make some assumptions, and so should be taken as simply illustrative of a general way of looking at this issue.

The strict medical view would be that although there is extensive evidence of an association between particle pollution and many illnesses, there is no trial evidence of evidence of a benefit from reducing particle exposure with HVAC systems. Actually, there is a study that did show less breathlessness in asthma following servicing an HVAC system but not changing the HVAC filter. However, there has not really been a significant number of studies on using very high quality filters in an HVAC system and the effect on health. There are studies using air purifiers with high-quality filters, and these do seem to benefit health. Another study showed that people in homes with air conditioning units had a lower hospitalization rate for heart attacks and strokes during times of increased particulate pollution.

Conclusion

HVAC filters are used to keep particles out of the HVAC system, the fan, heating element, and cooling coils, whilst supplying enough air-flow to keep these components working. So currently the filters in HVAC systems are there to protect the HVAC system and not the occupants of the house. There is so much evidence now about the health effects of air pollution that this approach probably needs to be reviewed. Especially vulnerable are children, pregnant women and those in late middle age and older. So we wonder if more focus should be on protecting the occupants of the house rather than the fan, heating and cooling elements of the HVAC system?

a) If You Want to Improve your HVAC System a Bit From a Health Point of View Without Too Much Hassle

Simply increase the filtration efficiency, MERV rating, of your current filter will help. Most HVAC systems can take a MERV 12 or 13 filter but you will need to check with an HVAC technician that yours can. If a technician says that yours cannot then go through option b) below. There is not much point in paying money for a remodel and only aiming for MERV 13 standard. If you are going to the expense and trouble of remodelling the system you should aim for a MERV 16 or even HEPA system.

The filter should be US manufacturer eg Filtrette, Honeywell, FilterBuy, Nordic Pure, Aerostar…

You may wish to try one pack of smart HVAC filters to see if these lower the frequency with which you need filter changes, the filter will remind you to change it. With a smart filter you may or may not save money. They are more expensive to buy but if your air has little dust in it then you may end up saving money as you are not changing the filters as often.

Also leaving the HVAC system on more when you are at home will help. Most HVAC systems are only on for 20% of the time so 80% of the time that you are at home you are breathing unfiltered air. By switching your HVAC system onto continuous fan you could be breathing filtered air 100% of the time that you are at home.

b) If You Really Want to Explore the Options for Improving Your HVAC System to the Have the Maximum Beneficial Effect on Your Health

To protect the occupants, HVAC systems can be modified with new technology. For instance, specialized HVAC filters can be installed. Some of these will then need less maintenance than your current filter!

However, we wish to point out that the EPA recommend using mobile air purifiers in individual rooms for fully purified air. This approach is the best for the rooms that you spend most of your time in but for a whole family you would need an effective air purifier in each room and all hallways. So you may wish to use your HVAC system to lower the general particle count in the house. Then you can always follow the EPA advice by having air purifiers in the rooms in which you will spend most time.

If you are interested in using your HVAC system as a giant whole house air purifier, please read this post as there are several considerations-

  • The HVAC filter should ideally be of HEPA quality or at least MERV 16. If MERV 16 ask manufacturer for a graph showing at least 85% efficiency in the 20-60 nanometer diameter range of particle size. This is the particle range in which inhaled particles can enter the bloodstream most easily and reach your internal organs including your brain.
  • The HVAC system will need to be operated with the fan running all the time-most systems have this option.
  • The airflow even when the system is on may or may not be high enough for air purification. This would need checking with an HVAC engineer-you should aim of at least 4 air changes per hour for the whole house. If you install a more powerful fan will the system be too noisy? You may want to shut the ventilation off to rooms you seldom go into so that the purified air is saved for rooms in which you spend most time.

So currently the ideal filter for your HVAC system is often a compromise. From a health point of view, a HEPA filter is the best choice. However, this will break most HVAC systems which need to have a higher air flow and so a less efficient, thinner and less dense filter. There is technology at the next step down, MERV 16 which may suit you very well. You may wish to look at the Nanomax filter and this system which can be installed in homes or offices and read this article with a few other options.

If you wish to change your HVAC filter for a more efficient filter, you should consult a qualified HVAC engineer as a self install could break your system.

Long term the HVAC industry may wish to focus on designing HVAC filters and systems which protect the occupants of the house rather than just the HVAC system itself!

Related Questions

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