What MERV Rating Can My Furnace Handle?
HVAC systems have air filters (installed in the return air) to trap particles in the air thereby preventing them from damaging the furnace/air handler and also being circulated back to the house.
In short, air filters are there to clean the air and that is why they should be changed often. Ideally, air filters should be changed after 3 months.
All air filters are however not the same. Some will allow some particles to pass through while some will trap almost everything. Depending on what size of particles the air filters can trap or let through, they are classified in a chart known as the MERV rating.
The Minimum Efficiency Reporting Value (MERV) rating indicates the effectiveness of an air filter (from 1 to 20) at trapping particles suspended in air. Filters with a higher MERV rating are more effective and vice versa.
MERV ratings were designed by ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) and the ratings are respected across the industry irrespective of brand.
It is not necessarily true to say that air filters with a higher MERV rating are the best to use. Every MERV level/rating is useful, you only need to know where to use that specific filter.
So, what MERV level should you use? Or rather, what MERV rating can your furnace handle?
The best MERV rating for a home is between 8 and 13. Air filters with a MERV 8-13 will remove most of the particles in the air and will not overwork your furnace/air handler. These filters therefore offer a great balance between temperature regulation, filtration and affordability.
If you have no pets or allergies and your house is far from smoke pollution, a MERV 8 filter is good. A MERV 11 filter offers better filtration than a MERV 8 filter and is the best if you have pets and/or suffer mild-allergies. Use MERV 13 filters if you have severe allergies, but they will overwork your HVAC system.
If you want to know the MERV rating that your furnace can handle, check the maximum allowed MERV rating in your furnace’s user manual. Using a filter with a higher rating than the recommended value forces it to work hard, reducing not only its efficiency but also its lifespan.
Although filters with a higher MERV rating are the most effective, they can be bad for your furnace. These filters have a high pressure drop and clog easily, restricting the flow of air to the furnace. The furnace therefore has to work extra hard which increases wear and tear thereby reducing its lifespan.
MERV Ratings Applications Chart
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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.