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View Full Version : Home furnace filter - Is this right?



Prashun Patel
02-16-2018, 8:29 AM
I recently had my furnace replaced at home. I am used to having to open some kind of door to install the filter.

The HVAC company - who otherwise I've had really good experience with - told me THIS is a common way of doing it: slice the return and just stick the filter into the duct.

Is this legit?

super convenient to change, but a) the filter cannot be inserted completely into the duct, which means it can't be completely efficient, and b) the 'lip' of the duct catches the paper facing of the filter when I remove it.

Go ahead, let me have it.

Rick Moyer
02-16-2018, 8:42 AM
That's how mine is, but mine does go all the way in flush (20x20). If yours is sticking out as shown you may need a special size; or the HVAC company messed up....

Lee Schierer
02-16-2018, 8:43 AM
No it is not normal IMO. There should be guides or channels inside the duct to hold the filter in place and a door/panel to seal the air duct. It looks like a low cost short cut. Did they also not install a separate air intake pipe for the combustion air?

Pat Barry
02-16-2018, 8:50 AM
There must be some sort of brackets inside for the 3 sides of the filter to rest upon, right?

Chris Damm
02-16-2018, 9:37 AM
That's the cheap way to do it. My mother's 1990 condo was done that way with a non standard size duct so I had to make custom filters to fit. The developer also put $2000 window walls but spent very little on the skylights and $20 toilets. He later (after all the lawsuits) went out business.

glenn bradley
02-16-2018, 9:41 AM
That seems like a total hack but, it could be that Iíve just never experienced a system using that method. My filter compartment is obviously integrated into the pathway with a spring loaded door with a magnetic catch and a sealer gasket.

Jim Becker
02-16-2018, 9:50 AM
One of my systems is setup the way your photo shows, (with a flapper door to cover the edge of the filter...that door holds shut from air flow). The other system requires me to physically open the furnace to change the filter. I prefer the former, but the installer of the later wouldn't put it in that way. I guess it's a different philosophy. I think that making filter changes really easy would encourage folks to actually change the filter more often than most people do. ;)

Brad Adams
02-16-2018, 11:13 AM
That's what you get when you take the cheapest price. The contractor has no idea of how to size a furnace filter. That filter is way too small for your furnace. Those filters are rated at 400 fpm air velocity. The actual surface area being filtered is only 10"x24" sincle the duct is 10" wide. Thats equal to 240 sq. inches of surface area. The formula to size a filter is cfm/rated fpm=sq. ft. of filter. That gas furnace requires at least 1000 cfm during the heating season (If not more, I don't know the size of the furnace), could be more during the cooling season, depending on the size of your air conditioner. Do the math. You need a minimum of 360 sq. inches of air filter, if you only need 1000 cfm of air. It WILL shorten the life of the blower motor, it is running higher static than it should be. That's the sign of a installer that doesn't know how to size things properly. Did they provide you with a load calculation on what size the house requires?

Prashun Patel
02-16-2018, 12:13 PM
Thanks for the thoughts, Guys.

Bill Jobe
02-16-2018, 12:29 PM
Deleted.
Found demensions of the filter

Bill Jobe
02-16-2018, 12:31 PM
I would demand they make it right.
The furnace is brand new.

Myk Rian
02-16-2018, 12:33 PM
I don't see a problem with it. As long as it's in the return air path, it'll work.
Make a cover to seal the opening.

Prashun Patel
02-16-2018, 12:41 PM
Bill, I did, but they maintain that it's right. They contend that because it's in the return path, it does not require to be sealed; all it means is that some return air may come in from the furnace room. The argument seemed logical.

They were not, by the way, the cheapest option out there. I've used them for other work and they have been professional.

Jerome Stanek
02-16-2018, 2:45 PM
Every house I have had and all the houses that I have worked on has a similar set up but the filter was sized to fit in without sticking out.

Brad Adams
02-16-2018, 2:46 PM
The issue is that it is too small a filter. I gaurantee that the total external static pressure on the furnace is above what it is rated at. Does the furnace whistle or change air flow sounds with the filter in? I size, design, and install these things for a living. IT IS TOO SMALL.

Andrew Joiner
02-16-2018, 3:00 PM
Bill, I did, but they maintain that it's right. They contend that because it's in the return path, it does not require to be sealed; all it means is that some return air may come in from the furnace room. The argument seemed logical.

They were not, by the way, the cheapest option out there. I've used them for other work and they have been professional.

It seems logical, but-- . The dirty unfiltered air from your furnace room is drawn in on the sides and bottom thus unfiltered. I'd want it sealed. I'm not an hvac expert, but I'm not selling anything either.

I have to unscrew a cover that has a rubber gasket on mine. It's a hassle but it's airtight. I like that because my furnace room is my shop!

Furnace filter info gets so complicated. From thick filters that must slow down air flow (just like a dirty thin filter). That's hard on the system, but picks up more dust. For me a thin cheap filter changed or vacuumed every month or two works well.

Prashun Patel
02-16-2018, 3:30 PM
Thank you, Brad. I will get this resized properly...

Jerome Stanek
02-16-2018, 5:49 PM
The issue is that it is too small a filter. I gaurantee that the total external static pressure on the furnace is above what it is rated at. Does the furnace whistle or change air flow sounds with the filter in? I size, design, and install these things for a living. IT IS TOO SMALL.

Where do you see that the filter is to small. It goes from one side to the other and if it is pushed in all the way then it is the size of the return duct. Are you saying the duct is to small. Mine is like the picture and it has rails that you can't see when the filter is in place. No whistling and it fits tight

Jim Becker
02-16-2018, 5:54 PM
It doesn't really have to be "sealed"--the contractor is correct about that, but that's a darn small filter/air return...I didn't notice the dimensions previously.

Dan Friedrichs
02-16-2018, 6:09 PM
Where do you see that the filter is to small. It goes from one side to the other and if it is pushed in all the way then it is the size of the return duct. Are you saying the duct is to small. Mine is like the picture and it has rails that you can't see when the filter is in place. No whistling and it fits tight

I'm guessing Brad is saying the size of the filter is too small relative to the size of the duct in terms of static pressure vs. volume (not just that it physically doesn't fit the space). A 4" thick filter, for instance, would "fit" in the same cross-sectional area but have much greater filter surface area.

Brad Adams
02-16-2018, 6:13 PM
Where do you see that the filter is to small.
The filter is too small in the sense that it is not the correct size to allow full airflow through the furnace. Not that it fits in the space. A filter can only flow a certain cfm at a given fpm. From experience I can tell you that the pressure drop across a small pleated filter like that is at least .8" of static pressure. A furnace is normally rated at .5" static total external pressure drop. This will cause issues with a high temp rise across the heat exchanger and much decreased motor life. The duct is fine.

Brad Adams
02-16-2018, 6:31 PM
I'm guessing Brad is saying the size of the filter is too small relative to the size of the duct in terms of static pressure vs. volume (not just that it physically doesn't fit the space). A 4" thick filter, for instance, would "fit" in the same cross-sectional area but have much greater filter surface area.

+1 ^^^^ and give a lot less pressure drop across it

Bruce Wrenn
02-16-2018, 9:46 PM
Because it's a gas furnace, unless it has a sealed combustion system, then filter area should be sealed to prevent exhaust gases from being drawn back down flue pipe. Since I can't see filter size, I making an assumption on my recommendation. Use and "A" filter rack in return duct. This will double filter area, and get you to what Brad recommends. You can't fit a 10" filter into a 10" duct that has interior insulation. If this is the case, then installer should have replaced this section of duct with some with exterior insulation.

Jerome Stanek
02-17-2018, 6:19 AM
To get a bigger filter wouldn't you need a larger return. There are many unknowns such as how big is the house and is the return the correct size. just from a picture you can't tell if it is to small you would need to check air flow and balance of the system. If everything works as it should then it is the correct size. I can not tell if that filter is a mesh one or a pleated one it looks to be mesh

Phil Mueller
02-17-2018, 6:54 AM
Iím no HVAC expert, but I jusr had our furnace replaced as well. They installed a box filter, which they highly recommended for the size of our furnace. Iíd look into having something like this installed.

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Jason Roehl
02-17-2018, 8:43 AM
My setup is similar to Phil's, except instead of a single filter, I have 4 filters stacked--a washable/reusable filter is hit first by the incoming dirty air, then two somewhat inexpensive disposables, then a premium allergen filter. My (and my family's) sinuses are thankful...

As for the filter in the OP, I've seen plenty of that type of setup, but I would agree that it should probably be moved to the section of duct that is horizontal--the larger cross-section means the air velocity is much lower and more particles will drop out of the air and cling to the filter there, and the filter will be larger to boot (less static pressure drop across the larger, higher-flowing filter).

Pat Barry
02-17-2018, 9:42 AM
The filter is too small in the sense that it is not the correct size to allow full airflow through the furnace. Not that it fits in the space. A filter can only flow a certain cfm at a given fpm. From experience I can tell you that the pressure drop across a small pleated filter like that is at least .8" of static pressure. A furnace is normally rated at .5" static total external pressure drop. This will cause issues with a high temp rise across the heat exchanger and much decreased motor life. The duct is fine.
It's not clear what you are recommending be done here. Could you please elaborate? Are you saying they should have a thicker filter, or bigger ductwork, or What? My own furnace setup is similar except the filter drops in vertically at the bottom of the duct, right where it enters the furnace.

Dan Friedrichs
02-17-2018, 9:53 AM
To get a bigger filter wouldn't you need a larger return.

No, it could merely be a thicker filter (like the one Phil pictures).

Think about it: If you had a flat piece of filter material that was 10"x15", you could fold it up ("pleat" it) to be, say, 1" in thickness (measured between the peaks of the pleats), but fit in a 10x10x1 frame. You have 10*15=150sq in of filter material. If you start with a piece of flat filter material that's 10"x60", you could fold it up to be 4" thickness, and have it fit a 10x10x4 frame. This filter has 4x as much surface area, so 4x LESS pressure drop for a given air velocity.

Pat Barry
02-17-2018, 11:30 AM
No, it could merely be a thicker filter (like the one Phil pictures).

Think about it: If you had a flat piece of filter material that was 10"x15", you could fold it up ("pleat" it) to be, say, 1" in thickness (measured between the peaks of the pleats), but fit in a 10x10x1 frame. You have 10*15=150sq in of filter material. If you start with a piece of flat filter material that's 10"x60", you could fold it up to be 4" thickness, and have it fit a 10x10x4 frame. This filter has 4x as much surface area, so 4x LESS pressure drop for a given air velocity.

I would see this as 4x as much restriction.

Dan Friedrichs
02-17-2018, 1:11 PM
I would see this as 4x as much restriction.

Then by your logic, shouldn't every filter be as small as possible?

If you have the same amount of air moving through a larger surface area, the volume of air per unit area is less.

See this chart:
http://www.homeenergy.org/UserFiles/file/26-6_p36bbig.jpg

Brad Adams
02-17-2018, 1:23 PM
The bigger filters have more surface area. The pressure drop across them is less. A 20x25x5 air filter typically has about a .15” pressure drop across it. I’ve seen 16x25x1 pleated filters with as much as .8” or pressure drop across them. Systems just aren’t efficient with a small restrictive filter.

Alan Rutherford
02-17-2018, 10:04 PM
IMO the vertical portion of the return duct is too small. It should have the same effective area as the opening on the air handler, or more. Putting the filter in it adds some restriction to the air flow and makes the problem worse. They could have put a larger filter in at an angle and make a decent-looking job while giving you less restriction to the air flow. Sticking the filter into the duct like that would be OK if it fit and you could close it up, but if the return is choked by under-sized ducts, you have a bigger problem than the filter installation.

Bill Jobe
02-17-2018, 11:25 PM
It looks as though your filter is hanging up when you reach the innermost channel.
I'd try pulling it out and measuring back to the farthest surface of the channels.
If that ain't it, I would consider a box filter on the bottom between the down section of duct and the furnace. That's where mine is and I have a Skuttle. It has a door that closes and latches.
I simply would not accept the way they set it up if the filter is, in fact, too large.

Kev Williams
02-18-2018, 12:23 AM
This house I live and work in is on its 3rd furnace, and they've all had similar filter setups,
the exception being the filter installed in the wider, bottom section of the duct-
(excuse the quickie cheesy photoshop'n) ;)
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this will give you a more sizeable 24x20 or so filter (mine's 25x20).

However, from experience, I can say that the smaller filter shown will work just fine, depending on the filter. 25-something years ago my parents had to replace the first furnace and had central air installed too--finally. The HVAC guys gave them a couple of electronic air filters, which I still have, they insisted they were necessary, just gotta keep that condenser clean. First thing I noticed after the install, was even tho the furnace was larger, there was less than half the air coming from the heater ducts than before. So I pulled the electronic filter out and replaced it with a cheap 4 for a buck 30 day filter, and I could fly a kite above the ducts now. I probably vacuumed those filters out a couple times a year, changed them maybe once a year. About 10 years ago that furnace (the 2nd one) cracked a heat exchanger, and I had a Goodman installed. I asked the guy who installed it, how's the AC condenser, covered in guck? Nope, clean as a whistle. And in 2016 the old AC quit, and I had it replaced-- and after still using 25c filters, I asked the installer about the condenser... Nary a plugged fin or dust bunny...

Now, 25c filters might not trap viruses & such, but they did keep my AC condenser clean for over 20 years, and the air in your house will actually move around.

Pat Barry
02-18-2018, 8:22 AM
Then by your logic, shouldn't every filter be as small as possible?

If you have the same amount of air moving through a larger surface area, the volume of air per unit area is less.

See this chart:
http://www.homeenergy.org/UserFiles/file/26-6_p36bbig.jpg

The surface area of the input and output is the same. There is more material inside, therefore, everything being equal inside, there would be 4x the restriction. Yes, less filter. Ie no filter, is less restriction in this universe. What I don't know is what the internal differences are. Maybe that 4 in filter has lots more low density material that is inherently less restrictive.

Pat Barry
02-18-2018, 8:56 AM
Also, note that the chart refers to face area, thus doubling the face area of a 12x24 filter would be using a 24x24 filter. Of course that is typically not possible to do since it means making the duct work bigger also. Bigger duct work means lower air velocity (faster flow in a smaller pipe).

Brad Adams
02-18-2018, 10:17 AM
but they did keep my AC condenser clean for over 20 years, and the air in your house will actually move around.

That must be one awesome filter. It kept the condenser clean all the way outside!

Bill Jobe
02-18-2018, 2:47 PM
Directly above your filter is what appears to be a section of track used to hold filters. What is it doing there if it is not a door that closes after installing the filter?

Kev Williams
02-18-2018, 6:32 PM
That must be one awesome filter. It kept the condenser clean all the way outside!
Ok, sorry, the EVAPORATOR... :D

Mark Paavola
02-19-2018, 12:08 AM
After 40 years of servicing HVAC equipment and also doing a lot of airflow testing Brad is correct. Although without knowing the size of the furnace this is still a best guess. Also that is not a good way to install a filter, there should be some type of door or cover. Did the install company provide any type of commissioning report? That would include airflow and providing static pressure tests at various points of the system. I have seen thousands of units with filters that are too small or too restrictive for the system. Unfortunately the homeowner for the most part has never had a system that works like the manufacturer designs them. High static pressure drop across that filter is a given. As an example I have a 4 ton heat pump system that runs at 1500 cfm. I have four return air filters that equal approx. 7 sq ft. This allows a little over 200 FPM flow across the filters. Ideally you should always have a flow of less than 300 fpm in a residential application.

Alan Rutherford
02-19-2018, 8:54 AM
...Did the install company provide any type of commissioning report? That would include airflow...

I can't speak for the OP but won't be surprised if his experience was like mine. I had made a crude airflow tester with a computer fan in a square of cardboard attached to a meter. It was too sophisticated for our installer. He told me to feel the airflow with my hands but I was doing it wrong. He said "Use 2 hands". Now that I know I should use both hands to measure airflow, I culd be a prufeshnul.

No disrespect to the true professionals out there.