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Increasing Filter Surface Area for Better HVAC Filtration

Lots of customers are jumping on the idea to use their central HVAC as a whole-house air cleaner.  Why not?  It’s a very expensive piece of equipment, it has a fan and an air filter, and it circulates air throughout the house.  The only thing is that you can’t snatch out your 1” MERV 2 filter and throw in a MERV 12 one.  At the least, the unit will protest by whistling, or delayed starts and stops, and at the worst you could cause damage it or cause a fire.  Whoa!  Let’s make the conversion to a whole-house air cleaner SAFELY.

We’ve written another article on how to increase filtration with your HVAC and why it’s dangerous to increase MERV without increasing filter surface area.  Doing this increases the pressure drop, throwing a restriction into the airflow of the system.  Basically, HVAC systems are not designed to have more than 0.25 inches water gage pressure drop over the filter (manufacturers recommend on average a 0.1 inch water gage over the filter), and adding a high MERV filter in the same slot as a low MERV filter will drastically increase the pressure drop.  

Now, you can train that dog to hunt–just put in some extra equipment!  What we’re talking about is more filter surface area.  Here’s a great rule of thumb to keep in mind: when the filter surface area in a system is doubled, the pressure drop over the filters will typically reduce by more than 50 percent. (Duct Dynasty: Confronting Restrictive Air Filters)  Another rule of thumb is to keep the filter face velocity between 200-250 feet per minute.  This allows the air enough time to interact with the filter and effectively remove even fine particles. Here’s a great video explaining the concept; even though the instructor is using an app that HVAC techs use (measureQuick), he explains the filter face velocity concept beautifully.  The difference between his velocity range (250-500 feet per minute) and ours (200-250 feet per minute) is that he’s working with commercial systems, so according to residential IAQ gurus, you’ll want to keep it between 200-250 feet per minute..  

Ok, so I need to add more surface area to my filters–what options do I have?  

1. Adding another return grille: You can examine the layout of your current HVAC return duct and air handler to see if there is room to add another return grille, where you can place another filter.  This may be the least expensive initial cost, however, over time thicker filters (see next option) may cost less.  In order to calculate the required surface area of the additional filter, start with the cfm of your unit (see our article to determine, and divide it by 1) 200 ft/min and then 2) 250 ft/min.  This will give your max and min surface area in feet squared.  To convert these numbers to square inches, divide by 144.  Then, subtract your current filter’s square inches to get the max and min square inches of the additional filter, and look at common filter sizes to fall in this range.
2. A thicker filter: although the cross-sectional area of the face of the filter may remain the same, increasing from a 1” to a 4” filter adds a lot more surface area with those deep pleats! They are also called media filters. These thicker filters usually also require less frequent filter replacements. You will want to check the rated cfm and clean filter pressure drop for each model you consider.  Here are some systems that fall into this category:
   1. If you have a 1” filter return filter grille on a wall somewhere, and there is enough room behind it (it doesn’t immediately narrow down), you can substitute a 4-5” thick filter for the 1” filter very easily using this type. See how the lip of the filter is designed to fit in the 1” grille, but you have a lot deeper filter behind it?  Genius!
   2. Again, if you have deep space behind your 1” return grille, you can consider an Electronic Air Cleaner, which can increase MERV with the same surface area.  For example, the Clean Comfort® brand AE14-G Series Electronic Air Cleaner, $600, claims "At rated airflow, the electronic air cleaner achieves a MERV 14 rating. With the fan running on low speed, the air cleaner increases efficiency up to a rating of MERV 16.  The static pressure drop of the electronic air cleaner is as low as 0.16” compared to 0.22" or higher for a typical 1" MERV 8 furnace filter.”
3. Install a cabinet air filter:
   
   1. Honeywell F100 Air Cleaners:  At 6.25 inches wide, the cabinet is not super-wide, and it comes in 7 dimensions.  For the 20x25” filter, the cabinet and filter are $168, and replacement filters are $40 each (when buying a 2-pack of MERV 11 filters).  It’s recommended to replace the filter at least every 6 months, so $80 a year for filter changes is not bad!  The clean filter pressure drop is 0.25 inches water gage for 2000 cfm.
   2. Aprilaire also makes a media filter.  Their model 1210 is 20x25” and costs $120.  MERV 11 filters for this unit cost $50 per filter (with a 2-pack).   The clean filter pressure drop is 0.22 inches water gage for 2000 cfm.
   3. IQAir PerfectPro 2025 is a thick filter with nanofibers arranged for “hyperHEPA” filtration. It can drastically reduce the PM2.5 and PM10 in your home, as this test/review found, but the pressure drop at 2000 cfm (5 tons) is 0.32 inches water gage, which is very high!  The price tag is also quite steep at  $1,995.  
   4. Trane CleanEffects Air Cleaner is an electrostatic filter.  It uses ions to charge the incoming air to make particles more “sticky” on the filter.  There are 3 parts to the air cleaner: a prefilter which should be vacuumed every 1-3 months, a field charger (with metal pins that generate ions) that should be cleaned by a technician, and a set of “reusable collection cells” which also get vacuumed by the homeowner.  The pros: CleanEffects has the highest efficiency of particle removal with the lowest pressure drop, it has no filters to “replace”, and it’s Asthma & Allergy Friendly™ certified by the Asthma and Allergy Foundation of America. However, the cost for this device is about $2000 installed, and maintenance (cleaning) is key because its efficiency drops when it gets dirty.  These units are only sold and installed by licensed HVAC technicians.
4. A “V-Bank” of filters: Instead of placing one filter perpendicular to the air flow, you can get more surface area by adding 2 (or more) filters in the shape of a V.  This device would usually be installed in an HVAC room (it’s not a filter grille for a wall). The only problem with this arrangement is that filter loading is not always even; air (like any fluid) will always seek the lowest pressure/resistance, and at the pointy ends where air becomes compressed, the filter load is lighter.  There are several manufacturers that sell this type of filter arrangement.
   1. IQAir also makes a double-v-bank filter, meaning that it technically has up to 4x the filter surface area of a typical single, perpendicular filter installation (see picture below).  This enables the device to exceed MERV 16, according to independent testing. The PerfectPro X 25x30 is compatible with a 5-ton HVAC system, the price tag is quite steep at  $3,395, but you do get 3 years out of the filters. The pressure drop is similar to other systems at 0.22 in H2O at 2000 cfm. 
   2. For a more economical V-bank filter system, AirScape SFB-V Series are MERV-13 V-Bank inline filter boxes.  However, their largest unit is slightly undersized for our 2000 cfm system above. The SFB-V-16x25 costs about $374 and has a min-max airflow of 1667-2778 cfm, corresponding to 300-500 feet per minute face velocity and 0.12-0.29 pressure drop.  Therefore, we would recommend you stay on the lower side and use this unit only in a 4 ton system (1600 cfm) to keep the face velocity down.

Source: IQAir Whole House Air Purifiers

If you want cleaner air with less filter changes, there are many options out there (we didn't include cabinets by Koch, GeneralAire and others simply because pressure drop information was not available).  You’ll want to take into consideration ease of obtaining (purchasing) the filters, and also placement of the cabinet for ease of changing/cleaning them.  Keeping up with filter changes is a worthwhile, minor chore if it means less dust in the house, less cleaning, and less allergens to potentially infect or annoy your family.  

How can I get more filtration with my current HVAC system?  It’s a tug of war!

At a staff meeting one day, one of our team members related how the HVAC company which installed the central AC system in his new home recommended using the lowest MERV filters available.  I was shocked!  Well, after thinking about it some more, I hypothesized they were waiting for his evaporator coil (the part that transfers absorbs heat from the air by transferring it to the cold refrigerant) to plug up so they could sell him a new system.  In this day and age of availability of every type, face size and thickness of filter, a good HVAC company should be able to work with your existing system to get good filtration.  Period.

If you’ve never heard of MERV, it is an acronym that stands for minimum efficiency reporting value, developed by the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) in 1987.    The range is from 1 to 20, and designates with what efficiency the filter removes small particles between 0.3 and 10 micrometers in diameter. (check out this post for more information on MERV).   Generally you’ll want to get the highest rating possible (more filtration) for your system, without causing too much pressure drop, because in general, increasing the MERV increases pressure drop across the filter, while HVAC equipment manufacturers want you to stay with a low pressure drop of around 0.10 inches of water column (i.w.c.) across the filter.  That’s the tug of war–but why aren’t HVAC installers figuring out how to give consumers, the ones who pay for new or upgraded systems, both?  It’s like selling a delicious drink in a cup with a straw that’s too small to get it out at any satisfying rate (like a coffee stirrer).  Sure, you could take out the straw and lid and risk getting it all over yourself as you drink it.  However, even fast food chains and gas stations figured this out years ago: larger straw and cup= convenient way to drink, more satisfaction, more sales.

In this case, though, the consumer is left to bow to the advice of greedy or ignorant HVAC installers, or do research to figure it out himself.  Yes, there is a way to get both high MERV and low pressure drop!  You just need to install a larger filter.  It sounds simple, right?  Yet, because many installers are trained to recommend standard size, 1” filter frames, you’ll probably have to do the math and specify one yourself.  Don’t get scared yet!  We’re here to help with that calculation.  

Here’s a diagram of the typical HVAC system so you know what we’re talking about/aiming for:

Image source: RemoveandReplace.com

The part we’re talking about is outlined in blue.  The filter can be installed on the side of the HVAC closet door, in a ceiling or a wall.  In the diagram the air is flowing through the filter, up through the air handling unit, through the evaporator coil, and out to various room registers/grilles.  The whole system “sucks” air through that filter, and if it’s too small, it’s like sucking a Big Gulp through a coffee stirrer–the pressure drop or suction pressure is too much!  Making the “face” of the filter larger will allow the velocity of the air through the filter to drop, which makes the pressure drop go down.  

So, what is the magic size of filter that makes the pressure drop go down?  That depends on the size of your HVAC system.  This very helpful article from industry expert Allison Bailes gives the secret requirement:  

Filter Area = 2.0 square feet (or more) for each 400 cfm of air flow

Since most filters are measured in inches, we can convert that formula to:

Filter size (sq. inches) = System Air Flow(cfm) x 288/400    OR  

Filter size (sq. inches) = 0.72 x System Air Flow (cfm)

Like in any interesting math problem, this one has a formula with some knowns and some unknowns.  The unknown is the filter size, and the known is the System Air Flow.  To find the system air flow, you can do several things: 

• Look at the HVAC air handler information specifications.  If you don’t have the system specs, go to the air handler, take a photo of the sticker with the model number on it, and search for this model’s manual online.   For example, I replaced my air handler recently with a variable-speed unit.  It will shift fan speeds according to the heating or cooling load, with maximum 1200 cfm, 640 cfm intermediate, and 400 cfm minimum.  Since the pressure drop will be maximum at the maximum air flow, I’m going with 1200 cfm.
• Approximate the air flow using the system tonnage: cooling units are often measured in the US by “tons”.  According to HVACtrainingsolutions.net, 350 to 400 CFM per ton of cooling is required for proper air conditioning system operation. We’ll use 400 cfm to be conservative.  If you know you have a 3 ton system, then 1200 cfm is the maximum airflow.  This lines up with the specifications on my unit.  (This equivalent of 400 cfm per ton can vary because of relative humidity, dry-bulb temperatures, wet-bulb temperatures, air density, mass flow rate, and elevation; if you want to “get technical”, check out this article!)

After you determine the cfm of your system, plug it into the filter size formula above.  In my case, 1200 cfm x 0.72 = 864 square inches of filter.  Yikes!  My own filter (24”x24”) was undersized by a third, and when I measured the pressure drop at maximum fan speed (1200cfm) it was 0.25 inches water gage, which was fairly high for a clean filter. However, if I “upgraded” to a 24x36” filter that size fit my requirements exactly (864 square inches).  The problem is that I don’t have room for such a big filter.

If you find that space or filter availability for bigger filters is a problem, you can solve it in a different way: add another return with another filter.  Many homes have 2 returns, such as one upstairs and one downstairs.  In this way, you’re getting the area and the filtration you need.  Adding a second return lowers the airflow per return, and also changes the air circulation in your home.  At the minimum, high MERV and high airflow will not be a problem!  In my case, the easiest thing to do was look at the return air duct and add another grille in the only place I could: my bedroom.  I ended up adding a 20x20 return air grill there, which lowered the pressure drop to 0.09 inches water gage for a clean filter, which eased the work of the fan unit and gave me more filtration.

This is the dilemma homeowners often face: accept the “expert” opinion of their contractor, or start doing their own research and demand equipment or installations that at least safeguard the equipment they are installing!  Many installers mean well, but by not using standard equipment like manometers (pressure-sensing devices) they have no idea what the pressure drop over the filter is.  They also don’t know what pollutants like dust, human and pet dander, and microbes are allowed  into the new system by specifying low-grade MERV filters.  Their ignorance or bad advice costs homeowners BIG when the air, and consequently the system, stays dirty.  Just like we sometimes must do with our health and doctors, we hope that you take this information to your HVAC company and specify what you need to win the tug-of war and keep you and your family healthy!