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The Advantages of Filter Grilles for Your HVAC System

The Advantages Of Filter Grilles

Oh, the controversies of duct design.  Standard versus high-efficiency filters.  Radial versus trunk and branch.  Central versus dedicated return vents.  Flex versus sheet metal ducts.  But there’s another duct debate that centers around the location of the filter.  In our HVAC design practice, we have a clear preference.  The advantages of filter grilles have led us to design duct systems with the filters at the intakes.

The advantages

So, what are those advantages?  Here are four:

  1. More filter area.  We like high-efficiency (high-MERV) filters.  To use them without the unintended consequences that sometimes accompany them, you have to have enough filter area.  It’s often easier to do that at the intakes of the return ducts instead of at the air handler, where there’s rarely enough space.
  2. Easier access for filter changes.  The return grilles are always in the conditioned space.  (OK, that’s not completely true.  Sometimes installers do stupid things, but let’s ignore that.)  The air handler can be in a cramped, dirty attic or a nasty, snake-infested crawl space with rotting possum carcasses.  Where would you rather go to change the filter.
  3. Clean return ducts.  Filtering the air before it enters the return ducts keeps those ducts clean.  You’ve got to prevent bypass with tape (as shown below) or gaskets, and the return ducts have to be as airtight as possible, but that’s doable.
  4. Less expensive.  It depends on what you install,  but putting 2 inch thick MERV-13 filters at the return grilles can be less expensive than using 4 to 6 inch thick filters at the air handler.
Clean MERV-13 filter in dining room, taped into filter grille
A MERV-13 filter taped into the filter grille in a central return vent

Filter grille advice

When we design the ducts for our clients, we specify filter grilles sized to have low pressure drop with MERV-13 filters.  To make them effective and convenient, we do or advise the following:

  • Use MERV-13 filters that are 2 inches deep.  The 1 inch filters generally have higher pressure drop.  Yes, thicker filters can help, too.  The downside, though, is that they’re more expensive and will require more space in the ceiling or wall.
  • Make them all the same size.  It’s much more convenient to have only size of filter to buy and change.
  • Seal the holes, joints, and seams in the filter housing.  Those holes can pull in air behind the filter, which means it won’t get filtered.
  • Use tape or gaskets around the filter.  Eliminate all the bypass routes and make sure all the air gets filtered.

If you’re ever faced with the choice of where to put the filter, remember these advantages of filter grilles.  And once you’ve got all these HVAC debates settled in your mind, we can talk about this doozy:  Air-sealed ducts versus that ducts that breathe.  If a house needs to breathe, shouldn’t ducts also breathe?*

 

Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has a book on building science coming out in the summer of 2022. You can follow him on Twitter at @EnergyVanguard.

 

* In case you’re new here, let me point out that no, a house does NOT need to breathe.  And neither do ducts.

 

Related Articles

The Path to Low Pressure Drop Across a High-MERV Filter

Do High-MERV Filters Always Reduce Air Flow?

The Central vs Dedicated Return Vent Debate

 

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This Post Has 23 Comments

  1. Does this eliminate the need for a filter at the air hander or should they be used in tandem? If used in tandem, does it reduce the frequency of replacement at the air handler?

    1. Cheryl: A high-efficiency filter in a filter grille eliminates the need for a filter at the air handler. Putting high-efficiency filters in both places would be difficult to do because of the resistance to air flow, but you could do it if you sized them properly. But using high-efficiency filters in both places would be overkill.

      What makes more sense is to use a lower-efficiency filter at the filter grille and then a high-efficiency filter downstream from the grille. That way you’d have a prefilter to remove a lot of the larger stuff. The more expensive high-efficiency filter would last longer, too. I was talking to a Californian at a conference two weeks ago, and he said his MERV-16 filters need to be changed frequently during wildfire season. I suggested a MERV-8 (or thereabouts) prefilter to stretch out the usage of his more expensive filters.

    2. Yes,it does eliminate the need for a filter at the air handler unit. Make sure the return ducts and filter grill is sealed as well to ensure that all return air is filtered.

    1. They make 2″ filters that fit a return grille that is designed for 1″ filters. I learned that after I special ordered a return grille designed for 2″ filters. Hope this saves you a couple bucks.

    2. @Robert, most filter grilles can only accommodate 1″ filters. Here are a couple of options for thicker filters: TruAire’s 192 Series can accommodate 2″ filters (stamped metal, 30 degree blade pitch) and the 192RF series has a removable face for ease of cleaning. Shoemaker makes an aluminum lattice filter grille for both 2″ filters (620FG2) and 4″ filters (620FG4).

      In addition to the 3″ MERV 13 FC313R series that Allison linked, Honeywell also makes the FC20R series (2″ MERV 8) and FC40R series (4″ MERV 10) that fit most 1″ grilles. These are sold through HVAC wholesalers such as Fergusons and Johnstone Supply, or through online reseller supplyhouse.com. A few sizes are available on amazon.com.

  2. You can also get filter grilles that will accomodate 4 inch filters as well. 4 inch = less pressure drop.

  3. One disadvantage with filter grilles is, if you introduce ventilation air into the return air plenum at the air handler, it must be filtered seperately. This may mean the owner or someone still has to make a trip to the crawl, basement or attic.

    1. Danny: Yes, thanks for mentioning that. It’s a good idea for the supply-only ventilation system that’s popular here in the Southeast (central fan integrated supply) to have its own filtration anyway because it can bring in a lot of stuff – wasps, pollen, gnats…

  4. Adding filters and therefore pressure drops across every return grille could accentuate return duct problems. Many existing return ducts are already somewhat leaky. Adding a filter will cause them to suck more. Plus the added confusion of seasonally opening and closing return ducts along with changing filters (half at a time) may leave customers asking “is there an ap for that?”

    1. Bob: Putting filters at the grilles should reduce pressure drops because it’s easier to add more filter area there than at the air handler. That’s advantage number 1 above.

      Yes, if you try to retrofit this onto an existing duct system without changing anything else, you could have problems with leakage. That’s why I wrote, “You’ve got to prevent bypass with tape (as shown below) or gaskets, and the return ducts have to be as airtight as possible.”

      I don’t understand your point about “seasonally opening and closing return ducts.” That’s not something I recommend.

      1. I’m assuming he’s referring to installations where the return air duct in rooms has a high and low grill so you pull the hot air off the ceiling in Summer and the cold air off the floor in Winter. I had my house in Maryland retrofitted like this. When they built the house they put the sheet metal supply ducts for the upstairs in the outside walls in lieu of insulation. The return for the upstairs was a 20″ x 20″ grill going to two 2″ x 3″ (1.5″ x 2.5″) wall cavities with the sole plate cut open about 12″ in each cavity which ran through the floor joist to the same thing on the first floor. I gave up my pantry and some hall space to have supply and return ducts run to the upstairs with high and low returns in each bedroom. Finally had cool bedrooms in the Summer and warm bedrooms in the Winter. The house was built in 1982 and the old 3 ton York heat pump was still working in 2007 when I had everything redone with an Infinity 5 ton system with zoning. Also had a SunPure SP-200 air purifier (4″ MERV 13 filter, catalytic converter and UV lamps) installed and a Skuttle humidifier. When we sold the house in 2014, the air handler still looked like new inside.

  5. Allison, since your article is about improving air flow efficiency, can you comment on changing the standard stamped steel return grilles to Metalaire 3/4 inch aluminum fixed grilles? My home is sealed, my ducts are sealed and airflow rate has been adjusted for each register. However, it is an 80’s 2-story home and has 4 returns throughout, each a different size. Before totally revamping those, I’m considering changing out the return grilles to the more efficient fixed grilles and adding returns in the doors. I currently make sure doors stay cracked to ensure airflow. Is it worth the expense of aluminum fixed grilles for any efficiency gained?

    1. @Paul, since Allison didn’t reply, I’ll bite. Checking the specs for lattice and regular stamped filter grilles (re: Hart & Cooley catalog), the pressure drops, even at 400 FPM face velocity, are on the order of 0.02 IWC for stamped and 0.01 IWC for lattice. At the preferred 200 FPM face velocity, pressure drops are off the chart, but extrapolating, they’d be on the order of 0.005 and 0.003, respectively. So no, it would be silly to spend extra money on aluminum lattice grilles chasing efficiency! OTOH, you might prefer the look of lattice grilles. I personally don’t care for that look.

  6. We should also take into account that most 1″ filters from a given manufacturer are priced on efficiency rating only and not the filter face area size. In other words, you can usually buy a 24″x24″ filter for the same price as 12″x12″ filter. Thus, when installing new ductwork and return air grills, you should install the largest face area possible since a larger face area will result in lower pressure drops and longer filter life for the same filter price. The larger return filter grill might cost more, and the installation cost will be higher if the framing needs to be modified, but I am guessing the payback on lower lifetime filter costs would be quite good.

    1. @Roy wrote: “I am guessing the payback on lower lifetime filter costs would be quite good.”

      Absolutely! Not to mention reduced blower energy, assuming proper setup for as-built external static. In new construction, the additional first-cost to size filter grilles to achieve < 200 FPM face velocity is minimal.

  7. I really like this approach and wish my current home had it.

    Grill sizing and or tapping the edge of the filter is important. My mother had return grill filters which were not taped. Every time the system turned on it would suck/pull the filter up against the frame and then dropped it once the air handler turned off. The drop would make a “dong” sound and wake me up. Annoying as hell.

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