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stephen_smith 01-11-2009 08:53 PM

Furnace air filtration advice
Greetings again!
To restate some info from my previous post, I live in southern New Mexico and I was interested in upgrading the air filtration of an old furnace with an external or internal air cleaner such as the Aprilaire 2210, 2410, or 2250.
I have an older PBKM-LD12N080C furnace manufactured circa 1994 that is installed in a small closet in the garage. This is an upflow furnace with a bottom return. Instead of having a conventional cold air return from the attic, indoor air is drawn into the blower from a central location in an adjacent room up through the wooden base/pedestal which supports the furnace.
Manual: (
The previous owner of the house haphazardly placed a 16x25x1 filtrete filter over top of the junction between the hole cut in the particle board base/pedestal and the blower compartment. The filter is actually too small to completely cover the opening and so an additional length of filter media was used to plug this area. Both are held down, loosely, in place by a piece of wood. I would've replaced this right away because the furnace manual actually recommends a 16x27x1 "high velocity" filter, but I've been unable to locate one.
As far as I can see, there is no proper filter rack inside or outside of the furnace. According to the furnace manual, an "internal mount bottom return filter accessory" 1BF0301-4 was available at one time, but I've had no luck finding these. I've even tried contacting the manufacturer for help with this, but I've received no response to my emails.
Because of this, I was considering an upgrade of the furnace's air filtration with the Aprilaire furnace air cleaners, though these, and similar cleaners, appear to be installed between the cold air return and the side of the furnace. This old furnace doesn't have cold air return ducting, so I'm not sure where to install an air cleaner.
BTW, there are two narrow, parallel ducts in the upper left corner of the furnace closet which connect to nothing either in the furnace closet or the attic. Any idea what this pair of slim ducts was intended for?
Once again, any help would be appreciated!


JohnH1 01-11-2009 09:20 PM

The open ducts are for combustion air. They will eather lead to the outdoors or possably the attic depending upon your situation. The filtration unit you are concidering will set under the furnace. order a unit with the same width of your furnace. The gas, electric, and supply duct etc. will need to be disconected the furnace lifted and everything reconnected. You sead this goes to 1 main return. There may be another choice depending upon the grill you have. You may be able to install a filter grill or if deep enough Honeywell makes a filter high effiency similar to the April Air that can inseret into a filter grill.

Home Air Direct 01-11-2009 10:05 PM

+1 what JohnH Said.:thumbsup:

Stephen, Can you take a picture or your return in the other room. That may help us give some advice. Might be easier to make this a carpentry project (Return air filter grille) instead of an HVAC project.

Raising the furnace to add a media filter will cause a domino effect of additional "to do's". Relocate or alter gas, elec, vent, duct.

Good Luck,


stephen_smith 01-12-2009 12:28 AM

From what I can see, there is no filter or filter mount behind the return grille. Unfortunately, there appears to be a wall stud just behind the grille too.

There isn't a return duct leading back to the furnace, just a hollow chamber lined with sheetrock on the sides and particle board on top to support the furnace. The sheetrock lining is demolished in several places revealing the insulation beneath, so I assume that would all need to be patched and sealed. The grille makes up a portion of one facet of the chamber and faces the indoor pathway to the front door of the house.

I inserted the camera into the chamber and titled it upward to snap a few photos of the filtrete filter from below.


Given these images, would it be more labor intensive to raise the furnace and adjust the flue/hot air duct to install a cleaner below the furnace, or to deal with the wall stud and seal off the chamber underneath the furnace to install a grille mounted/space guard cleaner?

Thanks for your input!

Home Air Direct 01-12-2009 08:11 AM

I love pictures. I also am no longer surprised at some of the shortcuts I see made in the name of profit. (not you, the original installation)

From here, I think it would be about as easy to raise the furnace. The gas is on a flex connector and electric is hanging loose so those items will be no problem. You can probably (push) extend the flue up through the roof flashing (may have to re-seal). The larger part of the project will be cutting off the plenum to match the new height and resealing to the furnace.

Just my $02. Good Luck


Daltex 01-12-2009 07:01 PM

I agree that raising the unit is the best bet. Before doing this you may consider the fact that the unit is old and may crap out at any time. If the burner has rust or is cracked it may not be worth the effort. I'd have the HX checked and if you do add the new filtration system test the temp rise. Is the blower wheel clean? I bet it's eaten alot of dust with the current setup.

donaldpeter28 12-24-2010 12:14 AM

Forced air heater filter
1 Attachment(s)
Hello, Your heater reminds me of mine in my cabin in Big bear Lake Ca. Mine is a Night and Day forced air heater from the 60's. The filter installs and hooks under a metal protrusion at the far end of the inside of the unit. The front panel has a protrusion the holds the other side of the filter when the panel is installed. I also have the two combustion inlets about 6 inches in diameter, one about a foot from the ceiling and the other about a foot from the floor both coming from the attic. The combustion air intakes seem over kill and lets in cold air but since my forced air heater is off the living room with a unsealed closet door and I have a fire place with a front screen the air inlets keep the fireplace from sucking combustion air from the heater in the house. My next heater will be sealed and I will plug the two air intake. For now I close the cabin for the winter.

hvaclover 12-24-2010 02:24 AM

If you want my 30 years of experience, I would cut the plenum up about 6" to 7" and push a media filter under the furnace. You use a 5" box filter that just slides in and out for replacement.

Depending on the conditions in the in the home you change the box filter once or twice a year.

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