By Jim Rosenthal, CAFS
A MERV 13 filter and a box fan? It is definitely not a new idea. It has been floating around for years. However, in the era of Covid-19, it is time to take all of the tools out of the toolbox. This simple, inexpensive air cleaning device can remove substantial amounts of particles from the air and reduce exposure to airborne Covid19.
Here is how to make it:
- Purchase a standard 20″ Box Fan. Mine cost $20 at a local home improvement store.
- Apply gasketing to the edges of the air intake part of the fan.
- Securely fasten a MERV 13 filter to the air intake side of the fan. The air flow arrow on the filter needs to be pointing to the fan. You can tape the filter to the fan. I used large rubber bands.
- Turn on.
Total cost – less than $50.
So how does our “Box Fan with a MERV 13 Filter” air cleaning device perform? Here are the particle counts from a tool room in a factory (door closed). Particle counts were taken with a Lighthouse 3016-IAQ Handheld 6-channel particle counter. All counts are per cubic foot.
|Room Count||After the Fan and Filter||Removed by Filter||% Removed|
Here are the results from an employee breakroom. No one is allowed to eat in the room, but it does have moderate traffic from people using the refrigerator and microwaves.
|Breakroom||After Fan and Filter||Removed by Filter||% Removed|
|10 um||100||100||0||No change|
So how does this information fit into the fight against the spread of Covid-19? Science has made tremendous progress in understanding this virus. But there is still much more that we do not know. The current thinking is that particles containing viable Covid-19 viruses start at about one micron. Particles less than 5 microns settle out of the air very slowly. They can remain airborne for hours. These particles are called aerosols and are one of the major methods for transmission of Covid-19. Consequently, the one to five micron particle size range is a good target for filter removal.
So how did we do in our tests with our “Box Fan with a MERV 13 filter” cleaning device? In the tool room, the device removed 59.7% of the one micron sized particles and 86.4% of the five micron sized particles. In the breakroom, the device removed 74.6% of the one micron sized particles and 80% of the 5 micron sized particles. In short, the results show meaningful reductions in the target particle size range. The “Box Fan with a MERV 13 Filter” should be a useful tool in the fight against Covid-19.
One thing we need to make clear – this device is not a HEPA air purifier. It does not have a Clean Air Delivery Rate (CADR) which is the standard for measuring the effectiveness of air cleaners. Purchasing and using a HEPA air purifier is the better option. However, their cost may be prohibitive. In this case the “Box Fan with a MERV 13” filter is a reasonable option.
Another concern about the box fan and filter approach is the resistance of the filter on the fan motor. The strain caused by this could lead to the premature failure of the fan.
One way to measure the resistance and also the performance of the “Box Fan with MERV 13″ filter device is to measure air flow. Here are the air flow measurements in feet per minute – 24” from the fan:
|No Filter||780 feet per minute|
|1″ MERV 13 Filter||320 feet per minute|
|2″ MERV 13 Filter||400 feet per minute|
|4″ MERV 13 Filter||460 feet per minute|
The resistance with a 1″ high efficiency filter is pretty substantial – 59%. But the device still delivers 320 feet per minute of cleaned air. That looks like a pretty good trade-off. And the downside is buying another $20 box fan.
On the other hand, one can decrease the resistance and increase the flow by changing the depth of the pleated MERV 13 filter. Filter resistance is determined by the amount of filter media in the filter. As the amount of filter media goes up, resistance (or static pressure) goes down. Air flow through the filter also goes up. A 2″ pleated filter has about 2 times the media area of a 1″ pleated filter. A 4″ pleated filter has about 4 times the media area.
By using a 4″ MERV 13 filter instead of a 1″ filter, resistance is cut down and the flow rate increases from 320 fpm to 460 fpm. Increased media area also means longer filter life. So a good option to increase the performance and preserve the motor of the “Box Fan with a MERV 13″ air cleaner is to go to a 4” filter.
And how do you use this air cleaner? Where do you place it in a room? Where do you direct the flow? Here are my thoughts.
The best place to use the device is in a room cleaning the indoor air. It has to be at least 4 feet away from all walls – the closer to the center of the room the better. You should position it so as much of the indoor air passes through the filter as possible.
Another option is to place it in an open window – facing in. Obviously, there are limitations on doing this. Many buildings do not have windows that are large enough to fit the fan. Many areas of the country are too hot or too cold to bring in large quantities of uncooled or unheated air.
But there are some advantages of placing the fan in the window. You substantially increase the amount of ventilation air coming into the indoor space. Proper ventilation is one of the keys to making buildings safer from the transmission of Covid-19. The “Box Fan with the MERV 13” can filter the air before it enters the space thereby reducing allergens and other pollutants found in outdoor air. The fan in the window also positively pressurizes the indoor space so that contaminants are pushed out – preferably through another window.
My recommendation would be to run the fan on “medium” speed. As we see by the air flow numbers, this speed moves a lot of air through the filter. “Medium” speed also puts less stress on the motor and keeps fan noise at an acceptable level.
There have been some questions raised about directing air flow at occupants of indoor spaces. The room air could contain airborne viruses and they could be carried in the airstream to other individuals. This scenario is highly unlikely with this device. Remember the air has just passed through a MERV 13 filter. It is probably the cleanest and safest air in the room. Besides many people in the room would probably like to have the air blowing on them to help keep them cool.
Unfortunately, this pandemic has brought all of the “miracle” indoor air cleaners out of the woodwork (plus many new ones). Virtually every day I read about some device that ionizes, polarizes, synthesizes, oxidizes, neutralizes, etc., etc. particles in the air. These products claim to eliminate all of the bad particles without harming humans. To my knowledge none of them has been tested in peer reviewed studies to be safe or effective. In other words, if it sounds too good to be true – it isn’t true. Also, consider this, if the “miracle” air cleaning device claims to attack and destroy any living organism in the air, why would it not attack human cells and tissue?
The “Box Fan with a MERV 13” air cleaner is effective and safe to use. It is inexpensive, easy to assemble and based on sound filtration technology. In short, it is a useful tool to help fight Covid-19 in indoor environments and is highly recommended.