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Old 12-24-2008, 10:09 AM   #1
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Furnace Filters


Hello,
Are the wide four inch filters getting a bad rap. I have seen comments from some on this and other forums that these filters do or can put too much resistance to air flow due to their thickness. Well that is true if the filter is not inspected on a regular basis. Well, I like to get this clearified. These filters are not actually four inches thick as they say. Only the outer demensions are four inches. If one looks closely at the construction of the media, one can see that the surface area is increased to allow more dirt to be collected. This allows increasing the interval between filter changes. From guessing at the surface area, I would say my four inch filter has about four to five more times surface area then a one inch filter. Your thoughts?

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Old 12-24-2008, 02:45 PM   #2
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Not all 4" media filters are made equal.

Aprilaire has a different resistance then Honeywell.

OEM replacements have the least resistance.
HD, and Lowes sell third party refills that have twice as much resistance new as the OEM.

So it depends on the filter.

A 1" blue fiberglass may oly have .05" PD.
A 4" may have .14" PD

If the return duct is already running at .3", then adding a 4" media can raise it to .44", which creates a velocity problem with air handler coils.
And lowers air flow on any system.

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Old 12-24-2008, 04:00 PM   #3
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<OEM replacements have the least resistance.
HD, and Lowes sell third party refills that have twice as much resistance new as the OEM.>

Now that, I did not know. I just started using third party refills from Lowe's. I was buying them from the internet using the Bryant part number. Now I'll re-order them through the internet. Do you happen to know an internet address where all of the specifications of each manufacturers filter is shown. I want to compare.
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Old 12-24-2008, 04:31 PM   #4
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No, there aren't any sites per say.
The third party filter makers won't tell you, because they would loose business. Some of third party manufacturers will give a PD for a low air flow, but not for the higher air flow they say their filter can be used for.

I found out the hard way that third party filters are more restrictive when customers complained about poor performance from there units.

Test of OEM to third party showed those customers that they need to use the OEM, or else change out the third party filter every 3 months, which then makes third party more expensive to use.
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Old 12-24-2008, 10:04 PM   #5
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I sent my order in to purchase OEM along with one of those gadgets that indicates when the filter should be changed. Thanks for the information.
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Old 12-25-2008, 04:50 AM   #6
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Your welcome.
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Old 12-25-2008, 05:20 AM   #7
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I might do a little experimentation on air flows when the cooling season starts to check airflows with both a 4 inch and 1 inch filter to see what the differance is in airflows. I might decide to use a 1 inch filter during the cooling season because of the weight of the air being harder to push through the system or do I have it backwards and use a 1 inch during the heating season and a 4 inch during the cooling season?

Last edited by rjordan392; 12-25-2008 at 05:23 AM.
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Old 12-25-2008, 05:37 AM   #8
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In the summer, the A/C coil being wet, makes it a great air filter.
Which is exactly what you don't want the coil to do.
So a good filter is important in the summer.

In the winter, most areas use heat more often then cooling, so good air filtration is just as important since the heating season moves a great total volume of air for the season.

Also, some of those 1" pleated air filters when new are more restrictive then a dirty 4" filter.

The best thing to do, is make it easy for the blower to get return air, by improving the return system.
Can be done by adding turning vanes to ells and or tee's, or adding another return to the house.
Or increasing the return drop size.
EG: a 10 x 24, moves almost 30% more air at the same pressure as a 8 x 24.

Or, it will allow teh same air flow of the 8 x 24 with only half the resistance. Giving you a longer life span for the 4" air filter.
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Old 12-25-2008, 06:08 AM   #9
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Well that settles that. I'll stick with the four inch OEM. I believe my returns are sufficient as I added two more in the basement a long time ago. However, my thermostat allows me to test the system performance such as cfm, static pressure, etc. So maybe this can help me determine if my returns are sufficient? Are there standard numbers that I can compare my results with?
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Old 12-25-2008, 06:16 AM   #10
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Not really any standard numbers.

Most furnaces and air handlers are rated at .5"wc.
Very very few of them operate at that static presure in high heat or cool mode.

With a 4" media, its best it your total static is under .8"wc.
At the highest CFM your system uses, weather thats the heating CFM or cooling CFM.

At .8"wc with a new filter. The filter only has to get dirty enough to increase its pressure drop by .2" to increase the cost of the blower running.

At .7", you get an extra .1" of filter life, before the blower starts to cost more to run.

This is refering to ECM blowers only, as far as blower operating cost increasing with static pressure.
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Old 12-25-2008, 06:20 AM   #11
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Thanks again. I'll test my system after the holidays to see what the static pressure is. Is static pressure, the same as water column?
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Old 12-25-2008, 06:27 AM   #12
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Static pressure for furnaces and air handlers is measured in inches of water column.

So if your stat says .9, its .9"wc.

And that will be your Total External Static Pressure.
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Old 12-25-2008, 06:37 AM   #13
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Sounds good, thanks. I am going to sign off now and enjoy the holiday. So Happy Holiday or Merry Christmas; whichever is your preferance. From an old do it yourselfer.
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Old 12-25-2008, 06:47 AM   #14
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Thank you.

And you to.
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Old 12-25-2008, 09:19 AM   #15
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Quote:
Originally Posted by rjordan392 View Post
I might do a little experimentation on air flows when the cooling season starts to check airflows with both a 4 inch and 1 inch filter to see what the differance is in airflows. I might decide to use a 1 inch filter during the cooling season because of the weight of the air being harder to push through the system or do I have it backwards and use a 1 inch during the heating season and a 4 inch during the cooling season?
I ran some filter experiments myself back in August during the cooling season to compare my 5 inch media filter versus some 1 inch types.

The blower was set on 400CFM/ton (1600 CFM) but was likely delivering only 1400 CFM at my high static pressure according to the servicts facts section in the installation manual. All filters tested were new and 20x25.


1. The 1 inch blue cheapo see-thru filter had a pressure drop (PD) of only 0.05.
2. The 5" Merv-10 media filter that came with my GeneralAire 5" filter-cabinet had a PD of 0.14.
3. The 1" Merv-7 pleated filter (Ace Hardware brand) that looked identical to a Merve-8 Natralaire pleated filter, had a very high PD of 0.27
4. A 1" thick Merv-4 Naturalaire washable Fiber filter had a PD of 0.07 .
5. The 1" 3M brand Filtrete-300 "High Air Flow" pleated filter had a PD of 0.18
6. The 5" Glassfloss Z-line 500AB (Merv-8) had a PD of 0.18

So far of the 5" versions.... the Merv-10 Generalaire filter has the lowest PD.
Of the 1" versions.... the thin blue fiberglass version has the lowest PD.

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