Thick and high-MERV filters can cause a significant drop in pressure inside your HVAC system from before to after the filter. Is it a problem?
So, you just purchased some new filters for your home HVAC system. Did you check the MERV rating to see if it is right for your system? Do you know what could happen if you didn’t get the right ones? What if they cause too much pressure drop? Will it burn up your HVAC system?
- MERV and Pressure Drop Confusion
- MERV and Pressure Drop - What Makes the Difference?
- Dirty Filters and Furnace Danger
MERV and Pressure Drop Confusion
There seems to be a lot of misunderstanding about the MERV rating of HVAC air filters and its effect on the pressure drop within the system. This has resulted in confusion and apprehension on the part of the homeowner who wants to provide good quality indoor air, but not at the expense of that expensive HVAC system.
Do a quick search online using “MERV” in your question and you’ll see statements such as, “Higher MERV means higher pressure drop.” – OR – “Higher MERV filters can destroy your HVAC system.” These statements are presented as set-in-stone, hard facts that are not to be refuted. The truth is, there are many differences between HVAC filters that make statements like these tantamount to comparing apples with oranges - they are both fruits, but that’s where the similarities end.
In case you haven’t heard of MERV before, it stands for Minimum Efficiency Reporting Value. It is a measure of a filter's ability to trap particles in the air. They begin at MERV 1 and go as high as MERV 20, with filters rated MERV 16 through 20 being mainly used in hospitals, research laboratories, computer labs, etc. The higher the MERV number, the smaller the particles the filter can trap – and the more of them.
Higher MERV But Lower Air Resistance Can Happen
Since filters with a higher MERV can trap smaller particles, it is often assumed that they will automatically have a greater resistance to air flow. While it is true that the more resistance to the flow of air results in a greater drop in air pressure on the other side of the filter, it isn’t always true that higher MERV means higher pressure drop.
Here’s one example: We compared three filters of a popular brand and the same size (20”x25”x1”) with MERV 5, MERV 11 and MERV 13 ratings. This chart shows their stated resistance to air flow in inches of water column (IWC/i.w.c.) was at various amounts of airflow in cubic feet per minute (CFM).
The higher the number, the more restricted the airflow
While these numbers are fairly close, at least in this example the filter with a MERV of 11, which is more than twice the MERV 5, had a lower resistance to air flow. Why? Because the pleated design creates more surface area, which is a less restrictive design.
Note that when the MERV 13 is added, it is more restrictive than either. Again, it has a pleated design. A flat design with a MERV 13 would be too restrictive. Most manufacturers recommend ratings of 0.5 or lower for the IWC test.
MERV and Pressure Drop - What Makes the Difference?
There are a number of factors that determine what the resistance to airflow of a new, clean filter (Initial Resistance, or IR) and the resulting pressure drop will be:
1. The material used to make the filter. Spun fiberglass almost always has a lower IR than other materials. They usually have the lowest MERV of all filters as well. Other materials used for filters have different MERV and air flow (IR) properties. Manufacturers are in continual competition to try to provide the best of both worlds in this area.
2. Surface area of the filter. This will be determined by both the frame size of the filter (usually close to the size of the return duct), and whether or not the filter has a pleated surface. Pleats look like an accordion, and can more than double the “face” area of a filter. This allows greater airflow without having to increase the size of the actual filter. This is one way a higher MERV filter can have a lower pressure drop than one with a lower MERV rating.
3. Thickness of the filter. The majority of residential HVAC systems use a 1” thick filter, but filters are available at 2”, 4” and 5” thick. A thicker filter of the same MERV as a thinner one will usually have a larger face area, a lower resistance to air flow, and less pressure drop.
Do I Really Need to be Concerned with the MERV/Pressure Drop Issue?
While it’s helpful to know something about the relationship between MERV and pressure drop, it isn’t generally something that you should lose any sleep over. Here’s why: Reputable HVAC manufacturers like Carrier, Rheem and Lennox make their systems to high standards that allow a certain amount of leeway in the choice of filters. Most of them will specify using a certain size filter, and they may also suggest a minimum MERV and a maximum MERV or IR (resistance to air flow of a new filter).
Where to find MERV and IR: This information is printed on the edge of the frame of the filter and can be seen before you purchase it.
If you don’t have, or can’t find, the owner’s manual for your HVAC system, here are the recommendations of three agencies. They specify a minimum MERV of a filter to use, but not the IR.
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends a minimum MERV of 6 as their general rule for home HVAC systems. However, if you are trying to filter out COVID-19 virus, they call for a MERV 13 or as high as your system can accommodate.
The US Department of Energy recommends at least a MERV 8.
Leadership in Energy and Environmental Design (LEED) also recommends a minimum of MERV 8.
The primary concern that some people have with the pressure drop issue is that of wondering if they are going to damage their HVAC system by using a filter with a high MERV. While that can happen, it most likely will not be a result except under extreme circumstances involving a cheap blower motor and the concern addressed below.
Frankly, that can happen with about any filter higher than a MERV 5 if it gets dirty and clogged.
However, short of filter neglect, most modern HVAC systems are designed to be able to work with MERV filters in the range of 10 to 13.
Dirty Filters and Furnace Danger
The higher the MERV of a filter, the smaller and the more particles it will remove from the air that passes through it. Since it collects more particles, it’s likely to get dirty more quickly than some other filters. That will result in more resistance to air passing through it. That will definitely increase the pressure drop and place unwanted strain on your HVAC system.
This is your main concern. Check your filter at least once a month for dust buildup. Most of us don’t have gauges to check the pressure drop, but we can remove the filter and hold it up to a light or a window. If the light is significantly blocked, install a new filter.
Other Signs your MERV is Too High / Your Filter Too Restrictive
Your HVAC equipment will give off a few clues that the filter is too restrictive and creating unacceptable pressure drop.
Your efficiency will drop – measured by higher energy costs: Systems working too hard burn more fuel or use more electricity to get the job done.
Airflow through the vents is lower: Is the new filter causing low airflow? Take it out and run the system for a couple minutes. Does the airflow significantly improve? Just don’t leave it out for more than a day, as we advise in our FAQ post, Can I Temporarily Run My Furnace Without a Filter.
Restrictive filters are one of many causes for low airflow, but if you’ve just installed a high-MERV filter and now air isn’t flowing very well, this is likely the reason in this instance.
Rooms will be too hot or too cold: This is a symptom of low airflow. If the house was comfy all over, but you upgraded your filter and now rooms distant from the furnace or air handler aren’t getting enough warm or cooled air, low airflow caused by a restrictive filter should be considered.
Our Complete Recommended MERV Rating Guide is a good source for understanding the best filter for your HVAC system.
We’ve also posted one of the popular Pick HVAC FAQ guides on Signs the Filter Needs to be Changed that addresses related topics.