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Old 05-05-2020, 05:41 PM   #1
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Best size for new HVAC filter


I recently had a routine performance checkup on my HVAC system. The AC condenser is a 3.5T installed in 2010 to a house with older ductwork. The existing 16x25x1 filter is installed in the heater/blower cabinet. The filter pressure drop is 0.19 across the filter and 0.61 total external. No problems with system performance or comfort.

I have been thinking of upgrading to a larger and/or thicker return filter, by installing it into the return plenum just before the current filter slot. . The return plenum footprint is 14x20", so a 20x20 would fit and just cover the blower cabinet opening. My AC tech suggested a 20x25x5" with complete rebuild of the return plenum to accommodate it. Or with some metalwork I could slide it in vertically but the top 8" would overlap the side of the blower cabinet assembly, not the return opening.

I opened up the return plenum for inspection and cleaning. It looks like a 20x25x1 was once mounted inside on a diagonal. For DIY, that would be a last resort b/c installation and sealing channels looks more difficult.

So I want to channel in a 20X20X4" just ahead of the existing filter, on the bottom right of the return plenum. Is that adequate size for a 3.5T system?
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Best size for new HVAC filter-filter-opening-16x25-med_7071.jpg   Best size for new HVAC filter-fpm-calc-100pc-400fpm-20x20.jpg  
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Old 05-05-2020, 07:15 PM   #2
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Re: Best size for new HVAC filter


Here's a photo of the return plenum exterior showing available clearance. A 25x20x4 vertical installation looks like a clearance challenge.

The heater/blower unit is rated for 80000 BTU/H two stage, variable speed blower.
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Last edited by steves7839; 05-05-2020 at 07:35 PM. Reason: Add photo and info
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Old 05-06-2020, 07:44 PM   #3
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Re: Best size for new HVAC filter


At what CFM did you check the static pressures.

A pic further a way would show us better what you have to work with.
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Old 05-07-2020, 09:29 AM   #4
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Re: Best size for new HVAC filter


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Originally Posted by beenthere View Post
At what CFM did you check the static pressures.

A pic further a way would show us better what you have to work with.
Hi and thanks for the question!

I understand the rated CFM for this 3.5T system should be 1400cf/m. I can only guess it is set up that way, but will try to determine that.

From previous research the ductwork appears to be properly sized. (The tech doing my AC P check seemed to agree).
The original 16x25x1 filter lives in the lower left side of the blower cabinet. I plan to mount the replacement inside the return plenum just to the left (just below the gas supply pipe). My current thinking is that a 20x20x4 filter will fit easily there; either a Honeywell MERV8 or a Filtrete 1550 Merv 15.
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Last edited by steves7839; 05-07-2020 at 09:41 AM. Reason: add detail
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Old 05-07-2020, 12:05 PM   #5
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Re: Best size for new HVAC filter


If you go both larger AND thicker, just make sure that the pressure drop @ CFM remains the same as what you currently have, or gets lower. A lower pressure drop will equal improved filtration and no additional load on your fan and cooling capacity.

The real number to be worried about is "total open area" for your new vs old filters. If the larger and thicker element has a "tighter" or "more efficient" filtration, which is to be expected with that kind of upgrade, it will generally have a lower "% open area" per square inch of surface area. You then have to convert the % open area to actual square inches of open area for each filter size. If you get equal or more total square inches of open area with the new filter (compared to the old one), then you're golden.

Alternatively, if the total square inches of open area for the new/larger filter is less than that for your existing filter, then you're going to experience a higher pressure drop and reduced air flow performance with the new (larger) element.
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Old 05-07-2020, 12:13 PM   #6
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OK, thanks! I'll research and compare total open area for the filters I'm considering.
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Old 05-07-2020, 02:58 PM   #7
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Re: Best size for new HVAC filter


Thats more of a right angle ell than a plenum.


While a 4" filter may fit in there. It will restrict air flow due to no room/area for the air to settle down from the turbulence the hard right angle turn its expected to make.


I you have the manual for your furnace. See what CFM the blower is suppose to be able to move, at the speed tap your blower is set to.
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Old 05-07-2020, 04:44 PM   #8
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Re: Best size for new HVAC filter


Looking at your pictures you have at best a 16x20 opening in the side of the furnace. This is not enough for a 3.5 ton ac. I would have the return redone with a proper boot, turning vanes and a media filter cabinet.


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Old 05-07-2020, 08:16 PM   #9
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Re: Best size for new HVAC filter


Quote:
Originally Posted by beenthere View Post
See what CFM the blower is suppose to be able to move, at the speed tap your blower is set to.
I just checked the install manual and the furnace control switches. It is set to 1400cfm after a ten-minute Stage delay, stage 1 is 1000cfM. In my OP I mentioned the measured ESP is .61 and .19 across the internal cabinet filter.
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Old 05-07-2020, 08:23 PM   #10
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Re: Best size for new HVAC filter


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Looking at your pictures you have at best a 16x20 opening in the side of the furnace.
The cutout marks allow 15x24" maximum cutout. Only 14x20" is used currently.

Last edited by steves7839; 05-07-2020 at 08:25 PM. Reason: typo
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Old 05-08-2020, 03:16 AM   #11
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Re: Best size for new HVAC filter


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Originally Posted by steves7839 View Post
I just checked the install manual and the furnace control switches. It is set to 1400cfm after a ten-minute Stage delay, stage 1 is 1000cfM. In my OP I mentioned the measured ESP is .61 and .19 across the internal cabinet filter.

And in the manual, it will tell you if the blower can really move 1400 CFM at .61" static.
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Old 05-08-2020, 11:57 AM   #12
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I just ran a test of the heating and cooling airflow. The heater cycle moves 1000 cfm according to the “CFM' count on the handler control board. Per the manual and switch settings, heat airflow will go to “Stage two' with 1400 cfm at ten minutes run time. Individual heat on cycles usually don't stay on that long.
The AC cycle runs at 1400 cfm per the “CFM' LED.
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Old 05-08-2020, 03:32 PM   #13
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Re: Best size for new HVAC filter


Okay, so what was the pressure drop across the A/C evaporator.
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Old 05-09-2020, 05:02 PM   #14
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Re: Best size for new HVAC filter


Quote:
Originally Posted by beenthere View Post
Thats more of a right angle ell than a plenum.


While a 4" filter may fit in there. It will restrict air flow due to no room/area for the air to settle down from the turbulence the hard right angle turn its expected to make.
I long ago decided to accept the inefficiencies in the original 1979 duct design except for improvements I could do on my own. (...after hiring some ineffective professional work). While replacing, sealing, and insulating (R6) round metal duct, I straightened a few unnecessary bends and closed up a lot of gaps. So the system is tighter and a lot cleaner than it used to be. That said, the HVAC handler/heater/AC equipment were new in 2010, and the climate is North Georgia, USA, zone 4, not particularly severe.

Yesterday I had all the filthy return ducts professionally cleaned. The business owner recommended inserting a 16x25x4 filter and cabinet (estimate $550) right between the air handler and the "L" return plenum. If I replace the bottom portion of the return plenum with a curved rectangular 90 degree elbow, the downward air flow into the filter cabinet might be less turbulent. My question is: is the energy savings from this mod (part cost $100-200) likely to be recouped over 5 years?
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Old 05-09-2020, 05:15 PM   #15
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Re: Best size for new HVAC filter


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Okay, so what was the pressure drop across the A/C evaporator.
IDK but the AC tech did not see anything problematic. A couple of weeks after that PM, I opened the evap case and cleaned the coil with brushes and coil cleaner foam.

The system manual filter table specifies cooling airflow of 1334 cfm at ESP .6 at normal setting of 400cfm/ton. And heating airflow of 1000 cfm at ESP 0.6.
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