A second HVAC air filter is not recommended. In fact, it is one too many.
Some furnace/AC systems have filters beside the unit. Others, when the system is in the attic, have return grills in the ceiling to conveniently be able to replace the filter without having to climb into the attic each month.
Those ceiling return grills are specially made with clips and hinges to be able to lower the grill, replace the filter, and clip the grill back into place.
The fixed return grills on the wall are not intended to be removed so often, and are screwed in place.
This homeowner decided he was really going to filter that air in the house! So he installed a second filter in the return grill just outside the kitchen.
This is not recommended for two reasons:
1. It puts a strain on the system as it restricts air flow, and makes the blower actually spin faster due to less air volume to try to equalize the in and out air needs. According to my buddy Charlie Buell that super spinning is called cavitation, with the blower essentially inside an air bubble, and he proved that when the return air is like this and cavitation is happening the system is actually using less electricity. Modern heat systems, especially heat pumps, utilize a lot of air flow to manage temperatures efficiently.
2. Since the grill is screwed into place it is more difficult to remove and the filter would be easier to forget.
The grill in this wall is much larger than the return duct. As a result the filter in this photo can be seen to have two colors. It was a little difficult to photograph, still the lesson can be seen.
The blue portion of the filter on the right side, much larger than is shown here, covers the section of the return where no air passes.
The left side, which is gray with collected dust, has been filtering the passing air for quite some time. It was completely clogged, blocking the passage of air. You should know that restricted air flow is a great way to kill a heat pump. This air flow was so restricted you could hear the whine of the blower as it was spinning faster and faster.
My recommendation: HVAC air filters do not handle enough air to keep the household air clean in any house. Not enough volume passes through the system each hour to be able to do that. So the real intention of the filter is to keep the system clean. A second filter is liable to do more harm than good, and is NOT recommended. An air filter is not a place and forget product. Replace or clean them regularly, and each month during seasons of high use.
Jay Markanich Real Estate Inspections, LLC 
Based in Bristow, serving all of Northern Virginia.
Office (703) 330-6388 Cell (703) 585-7560
If you are looking to further restrict air flow, we will hsip you the chipmunk family !
That always works S&D! And those raccoons in the chimney which restricts draft from the fireplace are a great addition too!
I say go for it! I was on restriction a lot as a kid, so I am with you.
Good morning Jay Markanich ,
Great post! I've been told the same thing..using a second filter is overkill!!
This is a question we are asked quite often by homeowners and having already known the answer we agree with your assessment of this situation.
Overkill indeed, Dorie. When you see it please put the homeowner in time out.
S&N - me happy. I'm okay. You're okay. I read that somewhere... ; >)
Clean air, living where it happens all the time and no pollution helps too! Great post and timely with everyone tightening up their homes to be as efficient, easy to heat and cool as possible!
Keeping the house clean and as dust free as possible it the best tip Andrew. But no second filters!
I haven't run into that but surpised I haven't. I have run into filters that look like they have been there for years. In our second home we removed the popcorn ceilings and in a matter of days the filter went from blue to brown. Check them often.
Good morning Jay. This is a good reminder for the purpose and intent of filters.
It never ocurred to me that was even a plausible option, so it's good to know it's not!
Whoa, not a good idea, we have the ceiling vent filters and yes they are easy to replace.
Any suggestions for cleaning the grilles themselves, they always seem to get build up on them of dust and fluff however clean the house is kept.
Thaks for sharing!!!
HVAC air filters do not handle enough air to keep the household air clean in any house. Not enough volume passes through the system each hour to be able to do that. So the real intention of the filter is to keep the system clean. A second filter is liable to do more harm than good, and is NOT recommended
At first, this appears to be a good thing to do but your post explains otherwise
Never even occurred to me. Just clean those filters, folks!
Jay, I just saw the craziest filter system yet (for me). The filter was inside the unit beside the blower motor. Of course, the blower was perpetually sucking the filter into the fan motor. The company had designed two small clips to hold the filter in place. They were thin metal that probably broke off the the first year of install. The system basically had no filtering system.
Sometimes the do-it-yourselfer does far more harm than good. It's pretty easy to see how this is NTO a good thing!
The same goes for those super small micron filters... too small. You want to filter the air, but not choke down the unit! Sheeesh!
I use the minimum # filters and also the ones that allow maximum airflow. No need to restrict the airflow whatsoever. If I need to clean the air I would just vacuum often and use a separate air cleaner.
Interesting read. On a related note, I always ponder the effects (positive or negative) as I shop for filters. Is MERV 8 good enough, should I get the MERV 11 or 13? If I go with the higher rating, what negative impact will it have on my HVAC system? Ended up going with the Filtrete 1500. Just change it every 3 months.
DIY will always be another word for "collateral damage" if unchecked!
They must be checked often Joe, especially when dusty work is being done in the house.
Sheila - and hopefully new information for some.
Ron - definitely not. One good filter is plenty.
N&T - we have ceiling filters also and I hook that brush attachment to the vacuum to keep the grills clean.
Anytime Sham. I'll be here all week. And next week...
Richie - it does! Nobody likes to work harder than they need for very long, including the blower in the HVAC!
That's good Hella. And yes, clean regularly.
Mike - unseated or loose filters are pretty common sights in my line of work. I have seen those useless clips too. Like John Wilkes Booth's last word - "Useless."
Congratulations on your featured article. We are looking forward to the next one.
When you show people the result the idea's value comes right to mind, Jeff.
And they can choke the system, Fred. Especially when not kept clean.
Athina - there are two philosophies on air filters. I wrote a blog about it five years ago. Read it here.
Stephen - which filter to buy is a never-ending story. See the blog link to Athina above.
Gary - I have said for years that the DIY TV shows are damaging! Everyone thinks they can put on an addition in an hour.
Thanks Harry. More will come, for sure.
Jay Markanich - well, learned from expert! One filter is enough!
One clean filter is enough, Praful!
A second filter could also harm the blower motor by Not allowing enough cool air to keep the motor from over heating
One home I lived in the air return itself was smaller than the register cover & you could see the stud thru the cover. It did whistle sometimes which was very annoying.
That's an aspect I had not thought of William.
Lyn - not enough air will cause the whistle! And poor design.
And why not go out the door with two pairs of underwear on just for safety sake.
I do that every day Joe. Really.