A MERV (Minimum Efficiency Reporting Value) rating is used to measure the effectiveness of air filters in removing particles from the air. The MERV scale typically ranges from 1 to 20, with higher numbers indicating better filtration efficiency.
The choice of a “good” MERV rating or the “best” MERV rating depends on your specific indoor air quality needs and your HVAC system’s compatibility. Here’s a summary:
1. Low MERV Ratings (1-4):
- Filters with low MERV ratings are less efficient at capturing small particles but allow for high airflow.
- These filters are suitable for HVAC systems with minimal filtration needs, where the primary goal is to protect the system’s components.
2. Moderate MERV Ratings (5-8):
- Filters in this range offer improved particle capture efficiency, including capturing larger dust particles, pollen, and pet dander.
- They strike a balance between filtration effectiveness and system airflow and are suitable for most residential applications.
3. High MERV Ratings (9-12):
- Filters in the high MERV range are highly efficient at capturing a wide range of particles, including fine dust, mold spores, and some allergens.
- They provide excellent indoor air quality and are suitable for homes where occupants have allergies or respiratory issues.
4. Very High MERV Ratings (13-16):
- Filters with very high MERV ratings provide superior filtration by capturing even smaller particles, including bacteria and some viruses.
- They are often used in hospitals, laboratories, and environments that require ultra-clean air but may not be suitable for all residential HVAC systems due to their potential to restrict airflow.
5. Ultra-High MERV Ratings (17-20):
- Filters in this range offer the highest level of filtration efficiency and are used in critical environments like cleanrooms and pharmaceutical manufacturing facilities.
- These filters can significantly restrict airflow and may not be suitable for most residential HVAC systems without system modifications.
Choosing the Right MERV Rating:
- The “good” or “best” MERV rating depends on factors such as your specific indoor air quality needs, the type of HVAC system you have, and the manufacturer’s recommendations.
- For most homes, a MERV rating between 8 and 12 is considered sufficient for maintaining good indoor air quality without significantly impacting system performance.
- Consult with an HVAC professional to determine the optimal MERV rating for your HVAC system and specific air quality concerns.
MERV Ratings Applications Chart
Are High MERV Filters Bad for Furnace?
One would be forgiven for thinking that high MERV filters are the best furnace filters to use in their home. After all, these filters are highly effective and remove just about any particle from the air.
The truth of the matter however is that high MERV filters can actually turn to be very bad for your furnace. But how is that possible you ask?
You see, furnace filters are installed in the return air duct just before the return air (air from the house) enters the furnace. As such, return air has to go through the filter before being heated or cooled in the furnace or cooling coil.
High MERV air filters have very fine pores which only allow air to pass through them. As a result, these filters restrict the flow of air from the return air duct to the furnace forcing the furnace blower to work extra hard to keep up with your house’s airflow demands.
The first impact of such a scenario is extra high energy bills. Since the furnace blower motor has to work harder, it will consume more power (just like driving your car uphill. Its gas mileage drops significantly).
Having the furnace blower motor running for long also means that it is highly likely to overheat (although it is equipped with a limit switch to turn it off in case of that). Anyway, having the blower running longer than it should increases wear and tear effectively reducing its lifespan.
As you can see, high MERV ratings are not necessarily the best filters for furnaces. They have their use (in sensitive industries like laboratories or pharmaceutical industries) but an air filter with a MERV rating of 14 and above should not be used in homes.
Apart from increasing your electricity bills and reducing the lifespan of your furnace, high MERV air filters are very expensive. Since they clog easily, they also need to be changed more often than filters with lower MERV ratings.
Clearly, you really don’t need to use air filters with high MERV ratings. They will do you more harm than good.
Pre-Filters vs Final Filters
I mentioned that the best MERV rating for residential air filters is between 8 and 13. The question you may be asking yourself now is “But why have low MERV rating air filters in the first place”?
You see, air filters can be used as pre-filters or final finals. Filters with a lower MERV rating can be used as pre-filters while those with a higher MERV rating can be used as final filters.
That is however done in commercial applications. But why do we need pre-filters and final filters?
As mentioned, filters with a lower MERV rating only trap the big particles and are quite cheap. On the other hand, filters with a higher MERV rating can trap even the smallest particles but are quite pricey.
A building (especially industries) can therefore have both low and high MERV air filters. Low MERV filters act like pre-filters while high MERV filters are used as the final filters.
Pre-filters are used to remove the large particles from the air instead of them clogging the final filter. As a result, you don’t need to replace the more expensive final filters as often as you would, without the pre-filter.
What MERV Rating is Best for a Furnace?
Ideally, the best MERV rating for a furnace filter is between 8 and 13. These filters will remove most of the particles from the air and are not pricey, and neither will they restrict airflow to the furnace.
Having said that, I must also add that MERV 8 to 13 is quite a big range. So, is MERV 8 better than 11? Or is MERV 11 or 13 better?
If you are shopping for a furnace filter to use in your home, I would recommend that you choose a MERV 8, 11 or 13 filter based on your needs. Let us see how different these filters are.
MERV 8 Air Filter
A MERV 8 air filter will remove the basic particles from your return air like dust, pollen, mold, dust mites and pollen. This filters will remove at least 70% of particles that are between 3 and 10 microns in size and 20% of particles that are between 1 and 3 microns in size.
MERV 8 air filters are recommend for use in houses that have no pets or persons who suffer from allergies. The house also need to be located in a place that isn’t prone to smoke pollution (near industries and other smoke producing activities).
MERV 11 Air Filter
A MERV 11 air filter is more efficient than a MERV 8 air filter especially if you own pets and/or a member of the family suffers from allergies. They will trap all the particles trapped by MERV 8 filters and also pet dander, auto-emissions and fine dust.
MERV 11 air filters will trap at least 85% of particles between 3 and 10 microns, at least 65% of particles between 1 and 3 microns and at least 20% of particles between 0.3 and 1 microns.
In my opinion, a MERV 11 air filter is better than both MERV 8 and 13 air filters. It filters out most of the particles you would want removed from the house and is not too expensive and neither does it overwhelm the furnace.
MERV 13 Air Filter
Most HVAC technicians do not recommend MERV 13 air filters for use in residential applications. They have a higher efficiency than both MERV 8 and 11 filters but are more expensive and also have a higher pressure drop so they overwork the furnace blower and hence low energy-efficiency.
MERV 13 air filters should be used in a houses with people who suffer from severe allergies. They filter everything filtered by MERV 8 and 11 filters and more, including virus carriers.
A MERV 13 air filter is the highest you should use in your home’s HVAC system. I wouldn’t recommend going higher than that.
Just to appreciate how effective they are, MERV 13 air filters can remove up to 95% of particles from 3 to 10 microns, 85% of particles from 1 to 3 microns and 50% of particles from 0.3 to 1 micron.
What is Pressure Drop in HVAC?
When shopping for a furnace filter, a phrase you are likely to come across from the pros is “pressure drop”. So, what is pressure drop in HVAC and how does it affect your air filter?
Pressure drop in HVAC refers to the reduction of pressure as the return air goes through the air filter. It is basically the air resistance created by the filter.
It could also mean the difference in pressure between 2 points, in this case between the return air and the supply air. Air pressure is higher in the return air than in the supply air.
All air filters have a pressure drop. Filters with a lower MERV rating have a lower drop while those with a higher MERV rating have a higher pressure drop due to how tightly woven they are.
As you can see, to get a good filter you have to balance filtration efficiency, cost and factor in pressure drop. While higher MERV filters have a high filtration efficiency, they have a high pressure drop and are also expensive.
On the other hand, air filters with a lower MERV rating have a low pressure drop and are cheaper but have a low filtration efficiency.
That is why MERV 8 to 13 air filters are the best for use in homes. They offer an excellent balance between filtration, pressure drop and cost.
And basically that is a simple guide on how to select the best MERV rating for your air filter. I hope by now you know what MERV rating you need for your furnace filter.