# Furnace Filters



## rjordan392 (Apr 28, 2005)

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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## beenthere (Oct 11, 2008)

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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## rjordan392 (Apr 28, 2005)

<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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## beenthere (Oct 11, 2008)

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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## rjordan392 (Apr 28, 2005)

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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## beenthere (Oct 11, 2008)

Your welcome.


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## rjordan392 (Apr 28, 2005)

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?


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## beenthere (Oct 11, 2008)

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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## rjordan392 (Apr 28, 2005)

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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## beenthere (Oct 11, 2008)

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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## rjordan392 (Apr 28, 2005)

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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## beenthere (Oct 11, 2008)

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 *T*otal *E*xternal *S*tatic *P*ressure.


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## rjordan392 (Apr 28, 2005)

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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## beenthere (Oct 11, 2008)

Thank you.

And you to.


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## key1cc (Nov 21, 2008)

rjordan392 said:


> 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.

Key1


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## rjordan392 (Apr 28, 2005)

Thanks for your data. I will save it to compare to my 4-3/8 inch filter by 16 X 25 inches. I suppose that to measure properly, I would need to test two times; once with the filter in place and once without. I am glad that "beenthere" answered this thread because there's a lot of people that do not know about the differant pressure drops between OEM and third paty filters even of the same material and size.


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## rjordan392 (Apr 28, 2005)

I decided to do a performance test to check out both low and high heat for static pressure and pressure drop with a 4-3/8 X 16 X 25 inch 3M filtrete high performance filter. The filter was in use for 23 days before the test.

Low Heat, 0.29 static pressure with the filter in place.
High Heat, 0.58 static pressure with the filter in place.

Low Heat, 0.17 static pressure with the filter removed.
High Heat, 0.37 static pressure with the filter removed.

The pressure drop on Low Heat is 0.12
The pressure drop on High Heat is 0.21

This filter has a merv rating of 12 whereas the OEM has a rating of 8. Right away, this tells me that with a merv of 8, I should see a lower static pressure and a lower pressure drop. 
And with a high heat static pressure of 0.58, I am under the recommended range of 0.8 per message #10. 

I'll be running this test monthly to see how the numbers change. It does appear that the present filter is ok for now.


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## beenthere (Oct 11, 2008)

When its time to change out that filter.
Try an OEM MERV 13 and see what your PD is.


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## rjordan392 (Apr 28, 2005)

I will do that but it will take a year or more before I use up my filters. I have two Bryant filters on order. I looked over the installation manuals and owners manual and could not find any mention of what type of motor runs the blower. You mentioned ECM and I did not see this term. Do I need to remove the blower cover door to check the motor label? I have the model 315AAV/JAV Bryant plus 80 furnace.


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## beenthere (Oct 11, 2008)

The Bryant 315AAV is an 80% AFUE VS/ECM blower furnace.


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## rjordan392 (Apr 28, 2005)

Thanks.
While I was testing the performance, I also checked to see the pressure drop when I closed off the two returns in the basement with the filter in place and it went up to o.66. A big jump from 0.58 when they are left opened.


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## beenthere (Oct 11, 2008)

If you redo that test.

Listen carefully, and you will hear the blower speed up, to maintain set air flow CFM.


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## rjordan392 (Apr 28, 2005)

This has turned out to be an interesting thread for those interested in improving the performance and economic capabilities of their furnaces. Now it also has given me another thing to check in the cooling season. I have both bottom and top returns installed and I been closing the bottoms when using air conditioning. Now I wonder if that is raising the static pressure and causing the blower to be less efficient economically.


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## beenthere (Oct 11, 2008)

Do you have the high ones closed off now.
If so, open them and see if your static drops.

I tell most people to leave both open weather heating or cooling.


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## rjordan392 (Apr 28, 2005)

Thats interesting.
I thought the idea was to maintain efficiency by using upper returns during the cooling season and lower returns during the heating season. I never tried leaving both opened during the heating season. I will open all upper returns, after I get back from shopping to see effect on sp. The basement returns are in a high location. I did not install low returns there. The rest of my two story home has both upper and lower returns and discharges. I know that if I opened all the upper and lower discharges for cooling on the first floor, it does affect the cfm output to the second floor. So I think doing this for the heating season may affect it just as well.


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## beenthere (Oct 11, 2008)

If you only open the lower returns in heating.
The the warmest air will stay at the ceiling, and the coldest at the floor.

Open both the high and low returns. You don't really need your ceiling to be 2 to 6° warmer then your floor do you.


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## rjordan392 (Apr 28, 2005)

Thats right.
I never thought of it that way. I'll open them up now and go shopping. When I get back, I'll test again to see if sp was improved.


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## rjordan392 (Apr 28, 2005)

With both top and bottom returns opened and the furnace heat on low, sp was the same as before, 0.29.
with the furnace heat on high, sp moved up a bit to 0.60. I did notice that the rpm's did increase slightly also.
So I guess I can dismiss this test and see what result I get when the cooling season starts.


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## beenthere (Oct 11, 2008)

Strange that it went up.


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## rjordan392 (Apr 28, 2005)

I believe it may have something to do with moisture in the air and the physics of air movement under changing conditions. The only thing I did was to leave the blower on low speed for over two hours before I tested. I normally leave it on auto during the heating season because I do feel cool air coming out of the discharge long before the heat kicks on. Both low and high heat tests last 5 minutes and I take my readings at midstream.


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## beenthere (Oct 11, 2008)

May have been caused by turbulance of the air meeting at the lower return grille.


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## rjordan392 (Apr 28, 2005)

I have three bottom return grilles on the first floor and the one closest to the furnace on this floor was 1/2 way closed. This may be why there was a slight increase in static pressure on my last report as per message #28.
However, I'll explain something as to why my static pressure is higher then normal. Many years ago, when I replaced my gravity furnace to a blower model, there was no duct leading to the gravity furnace. All the return air dropped through wall and ceiling openings into the basement. When the blower furnace was installed by me, I also installed a 10 inch by 13-1/2 od drywall return duct in the finished basement where it goes into the next room and connected to a 8 inch by 20 inch metal return duct. 
I used 2 inch by 3 inch wood framing for the drywall duct. I also have a return grille in the finished basement and one in the furnace room also. 

Also there is another return coming to the furnace from the opposite side and is a 3-1/2 by 12 inch metal duct. This is the return air from the rear of the house, including second floor rear bedrooms. 
Now I am wondering whether the drywall duct is of insufficient size or is the interior wood framing causing too much drag on the return air flow or both. Whats your opinion?
Now that furnace has been replaced with the latest model from Bryant and the installing dealer did not mention anything about the 10 inch by 13-1/2 inch drywall duct. 

Also when I received my new Bryant 4 inch X 16 X 25 inch filters, I tested right away.
Static pressure on low heat was 0.23 from the previous 0.29. Rpm were 687 and cfm was 712.
Static pressure on high heat was 0.48 from the previous 0.58. Rpm were 989 and cfm was 1033.
I left the filter in because I want to see what the pressure is after 30 days because the other filter was in use for 30 days when I originally tested. What do you think? Do I need to improve the drywall duct by lining it on the interior with 1/8 or 1/4 inch smooth material or remove it altogether and make it bigger?


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## rjordan392 (Apr 28, 2005)

This is an update on my OEM Bryant 4 inch furnace filter.

After 3 months of use on the heating cycle, these are the results:

Static Pressure on low heat is 0.23
RPM is 680
CFM 712

Static Pressure on high heat is 0.47
RPM 984
CFM 1033

The filter also was checked and not much dirt was visable. The numbers are basically the same as when the filter was new. I do expect the SP to increase to perhaps to 0.60 on high speed before I turn on the cooling system sometime in June.


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## beenthere (Oct 11, 2008)

I think your blower appreciates the OEM air filters.

After 3 months. They still have less PD then the other one did after 1 month.

With your low Static. I doubt you really need to do anything with your returns. Unless you see the static increase a lot in cooling mode.


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## rjordan392 (Apr 28, 2005)

I suppose I could get a little more life out of these OEM's if I keep a eye on ther SP and wait for it to climb 0.60 to 0.70 before changing to a new one. I believe a while back you mentioned that the blower cost more to run when the SP reaches 0.80 on high heat or high cooling. Is that correct? I want to be economical but I do not want the blower to work too hard. What do you think?


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## key1cc (Nov 21, 2008)

rjordan392 said:


> I suppose I could get a little more life out of these OEM's if I keep a eye on ther SP and wait for it to climb 0.60 to 0.70 before changing to a new one. I believe a while back you mentioned that the blower cost more to run when the SP reaches 0.80 on high heat or high cooling. Is that correct? I want to be economical but I do not want the blower to work too hard. What do you think?


I had similar questions about energy costs so I purchased a Kilo-watt meter that plugs into the wall outlet and the furnace plugs directly into the meter. It allow you to read wattage draw and amps easily so you can view the electrical impact on any change or adjustment you make.
For example: when I run in "fan-only" mode, my unit draws 92 watts (less than a 100 watt light bulb). When the heat is on the unit draws ~256 watts and when the AC is on the unit draws ~380 watts (not including the wattage draw from the outdoor unit). The wattage draw is directly proportional to the blower speeds which are 640 CFM for fan-only mode, 1000CFM for heat mode, and 1280 CFM for cooling mode.

Key1


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## rjordan392 (Apr 28, 2005)

That's interesting Key1 but a bit extreme don't you think? There's not anything we can do to affect the costs of energy delivery but we can do something about our usage of it. As our equipment ages, I suppose the energy usage will also be affected and we cannot do anything about that except for yearly maintenance to keep the equipment running.


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## key1cc (Nov 21, 2008)

rjordan392 said:


> That's interesting Key1 but a bit extreme don't you think? There's not anything we can do to affect the costs of energy delivery but we can do something about our usage of it. As our equipment ages, I suppose the energy usage will also be affected and we cannot do anything about that except for yearly maintenance to keep the equipment running.


 Actually there is quite a bit you can do. When My static pressure was up around 1.1, my wattage draw was between 600 and 700 meaning it was costing me twice as much. I made some adjustments by adding a supply helper duct (Beentheres reccomendation) and a new return and now my home is more comfortable.

There is a direct relationship of airflow, static pressure, temp rise, and wattage draw (energy used).

...And yes........I am quite a bit extreme :yes:

Key


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## rjordan392 (Apr 28, 2005)

Oh, I see. You did have a high static pressure and your meter helped you solve that and your energy usage and also made your home more comfortable. I don't exect to need a kilowatt meter on my setup. My system gives me a digital readout on the thermostat on sp, rpm and cfm delivery. That's enough for me to determine when to change filters.


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## beenthere (Oct 11, 2008)

Your Evolution ontrol will also tell you when you air filter is too restrictive. And needs changed.

Usually, it should be changed before the Evolution tells you though.

And yes, the lower the static, the less electric consumed by the blower.


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