Filter grill size vs filter thickness
Basic info, ClimateMaster 27 gethermal 4 ton unit.
Currently comes with a slot for a 30" x 31" x 2" pleated filter. Which I don't use.
I have a return grill of 24"x30" in the house in which I use a 1"
Antimicrobial treated; dry tactified polyester media. Its AEGIS microbe shield not only eliminates the filter as a potential breeding ground for micro-organisms, it also controls airborne germs. replace every 30 days . Arrestance : 87% @ .50" w.g. Airflow: .09" w.g. @ 300 fpm
I have come to find out that a 4" or 5" thick pleated filter can fit into my current grill (as I have about 18" of space behind the current filter) (backs into a closet).
My thinking is that a 4" pleated filter is equivalent to a flat filter 'about' 3 to 4 times the same grill size as far as air flow is concerned?
Based on similar materials used in each filter.
If so...the filter that came with the unit (29x30x2) is effectively equivalent (air flow wise) to a flat (29x60x1) aprox?
If thats true then the square inch requirement for that 2" filter is aprox 1740sq in?
And my using a flat 1" filter 24x30x1 has 720 square inches. almost 2.5 times to small?
If I alter the size of my current grill to a 24x24 and use a 4" filter will I gain air flow? Aprox 2304 Sq inches?
I'm interested in doing the sheet metal alteration (for a common filter size) I just want to be sure I'm not choking (but hopefully improving) the appropriate air flow.
Greater filtration area=lower pressure drop=slower face velocity=longer filter change intervals. All of that is good. Antimicrobials are available in 4" pleated designs.
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2 Tim 3:16-17
RSES CMS, HVAC Electrical Specialist
AOP Forum Rules:
the face of the grill will only give a certain amount of air check out the Hart and Cooley website engineering data on filter grills. You also need to know what your manufacturer specs are for your equipment and duct sizing.
Hmmmm, Based on the info on the Hart and Cooley site...
Originally Posted by Danimal535
My current size return grill is meant for a 3 ton unit size wise.
Original package unit 30yrs ago was a 4 ton.
24"x30", 720 sq inches, 3 (1200) Ton (cfm)
Apparently undersized then?
In first stage the unit is a 3.2 ton aprox, then in second stage is aprox 4 ton.
As the Grill is in the wall and I don't wish to enlarge the wall cavity and redo the ducting in this 30yr old home, I guess I should at least 'Keep' the filter size I have the same and suck up the additional cost of replacing that size.
So at least replacing the 1" flat filter with a 4" or 5" pleated filter would be an improvement over what I already have.
ever wonder why the filter is so big at the unit? you just answered that question but you need to realize that a cheap blue filter has less friction loss than a pleated type filter and with undersize duct and grill better filters may make your unit work improperly or worse
That size may have been fine for a conventional system.
A water-to-air geothermal heat pump outputs air at about 95 – 100° F, meaning it is more efficient, but requires more airflow for proper operation. Your air ducting must be able to provide this higher airflow, efficiently, and without being noisy. Additionally, the duct design should allow the proper amount of air to reach each area of your home.
The total air flow through the geothermal heat pump/air ducting must be at the geothermal manufacturer’s rated CFM (cubic feet per minute), and at the rated static pressure. If the air ducting is too small, the air will move too slowly, causing inefficient geothermal operation. Simply increasing blower speed will not solve the problem, since the extra blower energy use will lower the system's efficiency. The only way to get the correct amount of air flow, with acceptable energy use, is to have large enough air ducting.
Thanks for the informative discussion!
I have some thinking to do about what to do with the air return size issue then.
Either I have to gut the existing duct/grill/wall and replace it with a larger unit, or add an additional return line with the appropriate additional grill size sq in from the entry side of the 'unit's' filter slot to another place in the house, and then use the 2" filter slot in the unit for my filter.
I was thinking that a 4" pleated filter would lower the air flow resistance given the additional surface area vs a 1" non pleated filter, but apparently not?
Call your installer or go on the website of your manufacturer to get a new one get a pro to inspect your systems and see were your shortfalls lye and go from there.
A medium efficiency Merv 7 Economy Pleat, mfg'er Flanders Precisionaire 25X29X4 (725-sf) yields 24-sq.ft of surface face area. Pleated Face loading filters
Originally Posted by SteveNet
That 25X29X4 yields 1500-cfm@ 300-fpm 0.13"-pd; 2515-cfm @ 500-FPM 0.22"-pd
Most 4" deep pleated Merv 7 filters yield .10" @ 300-fpm; .13" @ 400-fpm; .14" @ 500fpm.
In the tests, they used 24X24 filters.
Last edited by udarrell; 07-31-2011 at 04:35 PM.
Reason: Data on 25X29X4...
Originally Posted by udarrell
From what I have gathered I should have aprox 400cfm per ton.
4 ton unit two stage compressor operating mostly in stage 1 which is aprox 3.2 tons.
My current tap settings are set as follows
Max ESP (in. wg) .75
Stage 1 1300 cfm...stage 2 1550 cfm
Stage 1 1400 cfm...stage 2 1650 cfm
So from what I can see by the numbers you provided I'm not that far off system wise even with a filter that size.
I am shy aprox 50 cfm in stage 2 cooling mode.
And shy about 150 cfm in stage 2 heating mode.
There are other speed setting available to me in my TAP settings one set higher and one 2 sets lower. and I have a 15% reduction/addition setting in all 4 sets of settings as well.
As this area (Tennessee) is mostly a A/C area, with a smaller not so coldish winter, perhaps I'm not so bad after all?
I'll have to look for that 4" filter you provided to see if it's available as a insert for a 1" system. Or are all merv 7 filters basically the same?
4" pleated filter
Ok, so after doing some more reading I'm considering using a Honeywell
24" X 30" RETURN GRILL FILTER FC40R1177 4" pleated.
Box of 5 = about $ per filter.
Specs are as follows.
Airflow Max.-Capacity (cfm) 2050 cfm
Airflow Max.-Capacity (m³/hr) 3480 m³/hr
Fractional Efficiency E1=31%, E2=61%, E3=86%
Minimum Efficiency Reporting Value (Media Filters Only) MERV 10
Static Pressure Drop (in. w.c.) 0.12 at 300FPM
It's close (I believe) to the merv 7 ratings of 0.10 at 300FPM
As well as the current biosponge pad filter .09" w.g. @ 300 fpm (edit addition)
They make a merv 8 but it seems not to be too available.
So the merv 10 seems a reasonable choice of not being too low or too high merv wise.
I'm also thinking of getting the 'General Aire 4002 - G99' just to see what the difference is/was between... no filter, 1" biosponge pad, and the 4" pleated filter. And to maintain proper filter changing times.
I appreciate ALL the input by everyone
As well as any comments suggestions to this post.
Last edited by beenthere; 08-07-2011 at 11:37 AM.
So you have a filter grille that is ok for a 2.5 to maybe a 3 ton A/C. And prefer to use that for your 4 ton unit instead of the larger filter slot that the unit has. Why?
Hmmm I gather from the question that there is more to this than what I'm understanding.
Originally Posted by beenthere
The return air grill is already existent 24x30, the units filter slot is a 29x30x2 aprox.
I had thought that using a 4" in the 24x30 would be an increase of filter media over the units 2" pleated (even though it's a 29x30).
So the air flow restriction would be less with the 4" in the 24x30 return than a 2" in the 29x30.
Plus ease of changing, and ease of finding a size filter.
That and my compressor is a 2 stage, 3.2 ton/4.18 ton aprox.
It always runs only in 1st stage even when it's 102deg outside.
Second stage kicks in when "I" (or a programed setback) adjust the t-stat a couple of deg lower or higher A/C-Heat, so it kicks in to compensate at that time till it reaches the new setting then hums along in stage one 3.2 ton.
The 24x30 return is already there and unless I rip it out and alter the wall and the ductwork it remains the bottleneck so to speak. (when in second stage)
At least that was my thinking on the issue.
Just noticed the price for the filter I had posted in my last post was edited out, I guess it is not permitted, sorry about that won't happen again. (edit)
Last edited by SteveNet; 08-07-2011 at 12:38 PM.