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Stacking Multiple Filters In Your HVAC Equipment Can Cause Damage

Stacking Multiple Filters In Your HVAC Equipment Can Cause Damage
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Bryan asked:

Hi. This website is great; there is really good information. My question is regarding large filters. My furnace requires a 5-inch pleated filter, but because these are hard to find I have placed 5 1-inch pleated filters from 3M. Is this going to restrict too much airflow? Should I buy a 5-inch or cut back to 3 or 4 1-inch filters? Thank you so much.

Hi Bryan,
You should remove 4 of those filters immediately because they will have a negative impact on air flow and operating efficiency will be reduced. Stacking filters in that way will have the same effect on your system as a dirty air filter. Dirty air filters and poor duct design are two of the main causes of poor air flow.

Residential heating and air conditioning systems are not designed to accommodate more than one filter and five pleated filters stacked could cause major problems for you.

Potential problems with low air flow in an air conditioning system

Air conditioning systems are designed to have 400 CFM of air flow to operate at maximum efficiency. 450 CFM is required for heat pumps.  There are a number of potential problems that can result from reduced air flow when stacking air filters in an air conditioning system.

  • Reduced air flow which can cause poor air distribution throughout you home.
  • On an air conditioning system the evaporator coil can ice up.
  • Operating efficiency will be reduced which means longer run times and higher operating costs.

Potential problems with low air flow in gas or oil furnaces

A 5 inch filter can be left in the system much longer than a 1 inch filter, 6 months or longer depending on a number of factors such as pets, traffic in and out, kids and so on. Each home would be different. Also a 5 inch filter would have less impact on air flow than a 1 inch filter.



P.S. I’m not sure what size 5″ filter you require but the links below will take you to 3 of the most common sizes.
16 x 25 x 5
20 x 20 x 5
20 x 25 x 5

About the author: Don has been working in the HVAC industry for more than 25 years and shares his experience and knowledge on this site. If you find the information presented here helpful please share with your friends.

{ 3 comments… add one }

  • Greg Hawkin

    Don, Thank you for the article. I am an HVAC technician at Titanic Controls. It is very important, as you stated, to keep proper air flow in your furnace. The problems with a lack of good air flow can be dangerous and expensive.

  • JWalls

    If a heat-pump system (with 2 speed motor) is designed to handle a MERV 11 filter, are there adverse affects if-
    I install 2 MERV 8 filters, one at the AHU and one at the ceiling return grille.
    One MERV 8 at the AHU and a MERV 2 at the ceiling return grill.

    Why would I want to do this? To keep dust out of the ducts during some construction work.

    • Don Munn

      Hey John,

      I’m not sure how the math works but 2 MERV 8 filters would probably be a problem. A MERV 8 and MERV 2 would probably be ok but the MERV 2 is going to be a fiberglass filter of some sort and won’t keep much dust out of the duct system. I would keep the MERV 11 and install it in the ceiling return grill. If you have it in the return grill there is no need to have it in the air handler unless air is also being returned from another return grill. If that’s the case put the MERV 8 where the dust is going to be created and put the MERV 2 in the air handler. It will be fine for short term use.

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