Inspecting for Proper Installation of HVAC Filters

by Nick Gromicko, CMI®, Katie McBride and Kate Tarasenko

Air filters are an important component of forced-air HVAC systems because they're vital to the efficient operation of the unit, and they greatly improve indoor air quality. Good indoor air quality can help homeowners increase their home's energy score, reduce their carbon footprint, and improve the health of the home and its occupants. Home inspectors should learn about the different components and correct installation of air filters in order to better assist their clients in maintaining their HVAC system properly.

Filters are installed on the return side of the HVAC air handler to clean the air to protect the HVAC motor and to improve indoor air quality. Filters should also be installed in fresh-air intakes to clean the outside air that is brought into the home. Filters with higher minimum efficiency reporting values or MERVs (6 or above) can trap pollutants like pollen, dust mites, and mold spores. However, regardless of the MERV rating, a dirty or clogged filter can dramatically reduce air flow, which increases the furnace's run time, which, in turn, increases wear on the motor and energy consumption. For optimum operation of both the filter and the HVAC equipment, filters should be cleaned frequently or replaced regularly.

If the furnace air handler is in an accessible location (such as a utility room, a basement, or an attic with a pull-down staircase), the filter can be installed in the air handler at the return plenum. The air handler box should be equipped with a filter media box that has a removable access panel cover, which has a gasket for an airtight seal when closed. The filter box might be prefabricated by the manufacturer or could be fabricated on site; this is something the home inspector can check for. The filter media frame should be appropriate for the size and type of filter desired. The filter dimensions and filter depth both influence filter capacity and air flow velocity. Filter sizing is taken into account when professionals design the HVAC system to ensure that it can handle the associated pressure drop, especially with the use of high MERV filters. Increasing the filter surface area will decrease pressure drop; options for increasing surface area could include using more deeply pleated filters or larger-dimension filters.

If the furnace is in the attic, in addition to a staircase or pull-down stairs, there should be a permanently installed walkway for the homeowner and any service technicians to access the furnace. This access is an important feature that home inspectors should inspect for when conducting the HVAC portion of a home inspection. 

If the home is equipped with a fresh-air intake that is ducted to the air handler, a filter should also be installed at the fresh-air intake.  Additionally, the fresh-air intake should be located where it is accessible so the filter can be cleaned or replaced as needed. Home inspectors can also check for this.

Figure 1. If the furnace is easy to access (in a utility room or closet), the filter can be located in a filter media frame between the return-air plenum and the air handler box (image courtesy of CalcsPlus).

If the furnace air handler is located in an inaccessible location (in a crawlspace, or an attic without a staircase), then the home inspector should make sure that the furnace filters are located in the return grilles, with a filter located in each return grille.

Figure 2. If the furnace is hard to access, the filters should be located at each return-air grille. If there is a fresh air intake, a filter should also be located there (image courtesy of CalcsPlus).

One Option Professionals Use to Increase the Filter's Surface Area

If the furnace is in a location where it can be accessed by the homeowner for filter replacement, and where the furnace's location permits an increase in the width of the return duct, the following option may enable the installation of higher MERV filters.

Calculate the pressure drop and air flow requirements for the desired filter MERV and dimensions according to ANSI/ACCA 2009 Manual D. Construct a box to hold two filters side by side in the return duct, above the 90-degree turn, and into the return-air plenum. Increase the duct's width to accommodate the filter dimensions above and below the filter box. If possible, install the filter box at shoulder height for easier servicing (see Figure 3).


Figure 3. To increase the surface area and reduce the pressure drop for high-rated MERV filters, the return duct can be constructed to permit the installation of two furnace filters side by side, if space at the HVAC installation permits, and if the HVAC location makes filter replacement practical (image courtesy of CalcsPlus). 

How Professionals Install a Filter in a Furnace Air Handler

  1. Fabricate and install a filter media box on site. Install the filter media box between the return-air plenum and the air handler box. Or, purchase an air handler that includes a pre-fabricated filter box. Ensure that the box is the right size for the filter to be installed. Filters must be sized using ANSI/ACCA 2009 Manual D. Improperly sized filters can cause a mechanical failure.

Figure 4. Install the filter media box between the return-air plenum and the air handler box (image courtesy of CalcsPlus).

  1. Slide the filter into place and fit the access panel cover over the opening. The panel cover should have a gasket to ensure an airtight seal. The cover can be duct-taped at the edges, which provides additional air sealing and can be easily removed when the filter is checked for cleaning or replacement. If the filter cover is left off or unsealed, the air handler can pull unconditioned and/or unwanted air into the home. Leakage at the filter cover can cause problems with indoor air quality if the air handler is located in a garage or other area where the air quality is not optimum.

Figure 5. Size the filter box for the appropriate filter according to ACCA Manual D, taking into account pressure drop across the system (image courtesy of CalcsPlus).

Figure 6. If the system is designed to use a thinner filter, the filter box should be sized appropriately. Install an access panel cover with a gasket, and duct-tape the edges (image courtesy of CalcsPlus).

Figure 7. Leaks at the air filter cover panel can draw in unconditioned or undesirable air (image courtesy of CalcsPlus).

      3. Complete the air handler installation by installing wiring and condensate piping.            Ensure that the piping and wiring do not block access to the filter.


Figure 8. When installing piping and wiring, do not block access to the filter (image courtesy of CalcsPlus). 

How Professionals Install a Filter at the Return Grille

  1. If the furnace is in a location that is difficult for the homeowner to access (such as a crawlspace, or in an attic that does not have a stairway or drop-down ladder), then install filters inside the return grilles.

  2. Ensure that the grilles can be opened from within the home. Install grilles that can be removed safely, such as grilles that are attached by a hinge along one side that minimize the risk that they will accidentally fall down while being opened.

Figure 9. If the furnace is hard to access, locate filters at return registers covered by hinged grilles that are easy to open from inside the home (image courtesy of InterNACHI®). 

3.  Install MERV filters that are rated 6 or higher, and match the sizing designation specified by the HVAC designer in accord with ANSI/ACCA 2009 Manual D. Do not install filters with MERV ratings that exceed the maximum design of the HVAC system. Installing higher-than-specified MERV filters will increase air resistance, raise normal 
normal energy usage, and possibly contribute to premature equipment failure.

How Professionals Install a Filter at the Fresh-Air Intake

  1. Choose a location to install the outside air intake (such as a porch ceiling or eave) that is away from polluting sources and easily accessible to the homeowner to facilitate filter replacement (see Figures 2 and 9).
  2. Cut a hole for the air intake with dimensions similar to the grille and the filter to be installed. When selecting a filter size and MERV rating, consider air-flow resistance because the pressure drop of this outdoor air intake duct and filter must be included in the Manual D calculations when designing the entire HVAC duct system. A MERV 6 filter should be adequate.
  3. Install a register box (boot) to which the duct will be attached. Most register boxes are at least 4 inches deep. This space will keep the duct terminus from touching the filter. Allowing several inches of distance between the filter and the duct terminus helps to decrease the velocity of the air flow through the filter, and also ensures that the full surface area of the filter is used, which will improve the filter's performance and increase its service life.
  4. Install a hinged grille at the exterior of the fresh-air intake opening (see Figure 8).
  5. Cover the grille with a wire mesh insect-bird screen.
  6. Place the filter on the interior side of the grille and close the grille.
  7. Inform the homeowner to check this filter periodically.  Replacement should be made with a similar MERV-rated filter.
  8. If the outside air intake must be located in an inaccessible location, construct and install a filter media frame in a filter box that is positioned in an accessible place at the duct connection to the air handler's return-air plenum, or somewhere along the fresh-air intake duct in an area that's accessible.


Proper installation of filters is extremely important for the overall functionality of the HVAC system in terms of protecting the HVAC motor and improving indoor air quality. While conducting an inspection, home inspectors should check to make sure that the filters are located in a place that is easily accessible to homeowners, such a utility room, a basement, or an attic with a pull-down staircase. If they are not in an accessible location, such as a crawlspace or attic without a pull-down staircase, home inspectors should check to make sure the filters are installed inside the return grills. Lastly, home inspectors should advise homeowners to clean or replace their filters frequently for optimum operation of both the filter and HVAC system.


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