The 2 main types of filters for use in HVAC systems are HEPA and MERV filters. So, how different are they? And which one is the best to use in your HVAC system?
All HEPA filters have the same rating as a MERV 17 filter or higher. They remove 99.7% of particles sized 0.3 microns (most penetrating particle size). HEPA filters perform even better than the 99.7% rating since they are rated at their worst performance.
An HVAC system can use both HEPA and MERV filters together. The lower MERV filters are way cheaper compared to HEPA filters and are therefore used as pre-filters while the more expensive HEPA filters are used as final filters.
The pre-filters will remove the big particles before they get to the final HEPA filter. This system helps to extend the life of the HEPA filters.
HEPA filters are replaced after 6 to 12 months. On the other hand, MERV filters should be replaced after 1 to 3 months. Thicker MERV filters (4-inch) can however last for up to 12 months.
Using a low-rated MERV filter as a pre-filter can extend the life of a HEPA filter for up to 3 years. That will definitely bring your HVAC costs low.
MERV filters are the filters used in home HVAC systems. HEPA filters are in most cases not compatible with residential furnaces/air handlers. More on that later in this post.
The advantage of HEPA filters is that ALL of them have their worst effectiveness at 99.7% against particles with a size of 0.3 microns. However, be careful of filters labeled HEPA-type or HEPA-like. They are not “true HEPA” filters and are not certified as such in the US.
It is however not true to say that HEPA filters are better than MERV filters. The highly rated MERV filters (17-20) are highly effective and are actually HEPA filters.
The advantage of MERV filters is that they offer cheap and less effective filters and expensive highly effective filters. The low-rated filters can be used as pre-filters while high-rated filters can be used in hospitals, laboratories, and other industries where contamination should be avoided.
For perspective purposes, the highest-rated MERV filter (MERV 20) will remove 99.99997% of air particles sized 0.3 to 1 micron. Also, the lowest rated MERV filter (MERV 1) will remove less than 20% of particles sized 3 to 10 microns from the air.
HEPA vs MERV Chart
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Let us now look at both of these filters in more detail.
HEPA is an acronym standing for Highest Efficiency Particulate Air. For an air filter to be categorized as HEPA, it must remove at least 99.7% of 0.3 micron-sized particles from the air.
It is therefore true to say that MERV 17-20 air filters can also be categorized as HEPA filters if they can prove that they can remove (at their worst performance) 99.7% of particles with a size of 0.3 microns.
Particles with a size of 0.3 microns are the standard for certifying HEPA filters. But why is that the case?
It turns out (from research of course) that 0.3 microns are the most penetrating particle size (MPPS). Even more than smaller-sized particles. Who would have thought?
You see, HEPA filters are usually made of sheets of fibers of glass randomly arranged and put together. The fibers have tiny pores with diameters of between 0.5 and 2 micrometers (roughly 0.00002 and 0.00008 inch).
The random arrangement of fiber sheets in HEPA filters changes the way air moves through the filter. It creates twisted pathways through which the air is forced through. That is unlike in low-rated MERV filters which work like a kitchen sieve.
The large particles are trapped by the filter just like in regular filters. However, smaller particles are forced to twist and turn as the air moves through the pathways. That movement causes them to collide with the fibers and air molecules and that is how they end up being trapped.
What are True HEPA Filters?
You have probably heard about true HEPA filters. For there to be true HEPA filters, there must be false HEPA filters, right? Not Exactly!
In the United States, HEPA filters as I mentioned are expected to remove at the very least 99.7% of particles with a size of 0.3 microns from the air being filtered. In Europe however, the requirement is only 87%.
The European (and even some American) HEPA filters are therefore cheaper than the certified American HEPA filters and even have the HEPA label. Needless to say, consumers will naturally prefer to buy those ones.
To counter that effect, American manufacturers of certified HEPA filters started labeling theirs as “true HEPA”. Now when customers see that, they know that is the real deal.
If you are on the market for a HEPA filter and you see one labeled as “HEPA-type” or “HEPA-like” be very careful. True or absolute HEPA filters will have the correct rating (99.7%) printed on them and will also likely have the “true HEPA” label on them.
MERV stands for Minimum Efficiency Reporting Value. It is a standard developed by the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE).
MERV ratings indicate (on a scale of 1 to 20) how effective a filter is in removing particles from the air. The higher the rating the more effective the filter is and vice versa.
MERV filters are used as pre-filters in industries, in residential furnaces as well as in hospitals, laboratories, and pharmaceutical industries among other commercial uses, depending on their rating.
It is the low-rated (MERV 1-4) filters that are mainly used as pre-filters. As I mentioned earlier these filters will only trap the large particles (about 20% of particles sized 3 to 10 microns) and let the small ones pass through.
MERV filters with a higher rating (16-20) will trap 95% and above of particles with a size between 0.3 and 1 microns. That is why these filters are used in sensitive areas like hospital theater rooms where contamination is highly avoided.
However, filters with a high MERV rating are not necessarily the best filters to use. That is especially the case for residential HVAC systems. In fact, MERV 14 filters and above shouldn’t be used in home furnaces and air handlers.
The best MERV rating for home HVAC systems is between 8 and 13. Filters in that category provide an excellent balance between filtration, air temperature regulation, and furnace efficiency.
Although filters with a higher MERV rating (just like HEPA) filters have excellent filtration capabilities, they also have a very high pressure drop. Pressure drop is the reduction in pressure created by the filter.
As air flows through the filter, it encounters restriction which reduces its pressure. All filters have some sort of pressure drop but the pressure drop increases with an increase in MERV ratings.
Although low-rated MERV filters have low pressure-drop, they are poor at filtration. They therefore cannot be used in home furnaces.
Highly-rated MERV filters restrict the flow of air from the return air duct to the furnace, forcing the blower to work extra harder. That not only increases your electricity bill but also shortens the furnace’s lifespan due to wear and tear.
And that is why the 8-13 MERV filters offer the sweet spot. They remove most of the particles from the air and their pressure drop is not too bad.
For a house that has no pets and nobody suffers from allergies or asthma, a MERV 8 filter will suffice. If your house has pets and/or you suffer from mild allergies or asthma, a MERV 11filter will work well. Only go for a MERV 13 filter if you suffer from severe allergies.
HEPA vs MERV Filters: Applications
HEPA filters are mostly used in vacuum cleaners and air purifiers. You however need to be careful about these appliances if you want to experience the real benefits of a HEPA filter.
All the air pulled in by these appliances must go through the filter before being released back to the house for them to be HEPA-compliant. It is not uncommon to have a vacuum cleaner where the air being released back to the house goes round the filter.
HEPA filters are unlikely to be used in residential HVAC systems. Just like the high-rated MERV filters, they have a very high pressure drop and the furnace blowers will struggle to pull air through them resulting in premature failure of the system.
What actually happens (in most homes) is that the furnace will have a MERV 8-14 filter and then there will be a stand-alone air purifier fitted with a HEPA filter. The HEPA filter in the air purifier will therefore remove the finer particles that will have escaped through the MERV filter.
And basically those are the difference between HEPA and MERV filters. I hope by now you know which filter to use for your HVAC system.