Weird air filter inside the furnace


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Old 04-25-21, 10:36 PM
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Question Weird air filter inside the furnace

When I opened the cover of the furnace in the utility room at the basement level, the first time after I moved in to this house built year 2000, I was so surprised to find two air filters put right under the fan(please see the picture #1 below): one 16x25x1 old filter with dust, one old style of filter with plastic frame, both were held under a curved stick. What are they used for?? This doesn't look like a professional work.

In picture #2, the intake airflow is from the right side, through a 16x25x1 air filter(I changed it a few times after I moved in). Also, I noticed "20x25x1" was marked on the "base" of the furnace, but there is NO slot to insert the air filter like the slot on the right.

In picture #3, I lifted the filter a bit, I could see a big hole under the filters. I don't think it goes anywhere because the base sits on the concrete floor of the basement.

Should I keep it as is, or should I "fix" it? This doesn't look right to me.

Please advise. Thank you!!



Picture #1: two old air filters found *within* the furnace


Picture #2: filters within the furnace, cover the base and under the fan


Picture #3, air filter was lifted, a hole to the base

 
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Old 04-26-21, 12:14 AM
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This doesn't look like a professional work.
So what gave it away??

Yes, if the bottom sheet metal structure is just supporting the furnace then that crammed in filter(s) are not doing much. I would have expected to see a piece of sheet metal closing off the chamber. If it's open to the floor prior owner may have thought air was being sucked in and therefore needed to be filtered!
 
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Old 04-26-21, 05:30 AM
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I feel there is a high likelihood that the bottom opening doesn't go to the floor but is connected to the vertical return duct. If you remove the bottom filter and reach in to the right of the riser you could confirm this. You occasionally see this or a wrap around duct to additionally enter on the opposite side of the furnace cabinet. The current bottom filter placement looks poor, but It was probably originally installed professionally with two filters properly in place.
 
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Old 04-27-21, 09:48 PM
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Hi fastback, please see the picture below, it doesn't look a vertical return duct could exist below, but correct me if I was wrong. Thanks!


 
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Old 04-28-21, 04:35 AM
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You have a vertical return duct. It is the one with the tray attached with papers in it. The riser is the ductwork that the furnace is sitting on. If there is opening between the two it would allow return air (filtered) to enter at two locations. This is not uncommon. The poor placement and sealing of the bottom filter is what is improper. the filter should fit and be held down with that spring steel u-shaped rod in your earlier photos. It should only take about one minute and a flashlight to confirm either way.
 
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Old 04-29-21, 10:32 PM
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Hi fastback,

You are right, there is opening between
the furnace base(riser) and the bottom of the duct, I confirm they are connected:






Why the airflow needs to go two paths instead of just one? I wonder why the bottom should be connected, why the airflow can't just go through the air filter(red one, 16x25x1, slide in) but also go from the bottom?

Should I replace the two old air filters inside the furnace, with one piece of right size filter? The base was marked 20x25x1, but I measured it's about 25x28x1 for full cover.
Or should I block the airflow through the base/riser/bottom(that is, leaving only one airflow through the 16x25x1 filter), by covering the base with a 25x28 sheet metal, or blocking the hole between the base/riser and the duct bottom?

Please advise. Thanks!
 
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Old 04-30-21, 05:10 AM
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I think the "why" may become clearer if you can find or download the installation manual for your model and size of furnace as it should indicate minimum requirements on return opening(s). This won't change your set-up, it will only answer your "why" question. -Furnace installers seldom increase the amount of work and material used for no reason-. If you have confirmed that you have two functioning return openings then you will need two properly fitting filters that are securely held in place. Without investing the time to look with a flashlight it is pretty clear to me that the riser was also a source of return air by how dirty the back side to the existing bottom filter is. Clean filters will stay in place much better than dirty restrictive ones.
 
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Old 04-30-21, 05:23 AM
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The extra air flow paths might be to increase the surface area for filtering, which will reduce strain on the blower motor and could allow the use of higher grade filters.

Unless the furnace was designed for them, higher grade filters than what came with the furnace should not be used. I am guessing that you would have to double the square inches of filter to go up one grade of filter quality as manufacdtured (e.g. good to better, not Merv 5 to Merv 6)

It is definitely sloppy when filters don't fit and have to be folded. The purpose of the filter is lost and duct cleaning may be made necessary if there are gaps around the edges of filters that unfiltered air gets past.
 
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Old 05-01-21, 11:14 AM
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Thanks everyone, I will replace the filter at the bottom. I wonder what type of filter could fit this, I heard I shouldn't use high number of filters like I do for the return vents at ceiling…
It marked 20x25x1, but I found the whole size of the cabinet is about 25x28x1, not sure if the spring wire can hold down the 20x25x1 size filter in place tightly.
 
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Old 05-03-21, 05:23 AM
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Highly restrictive filters should be avoided. You can make any filter highly restrictive by not changing it. Doing so will also cause it to distort allowing return air to create a path to avoid the filter. The overall, long term effectiveness of filter(s) can usually be judged by how dirty the blower wheel blades have become. Setting a filter change reminder on your phone can help prevent forgetting to change them. If the one spring wire you have is not doing it's job then you could add a second. The spring steel/wire is often referred to as "music wire" and is available in some hardware stores or online, just overbend it into the shape you need. Obviously you could try to find a duplicate to what you have at a greater price by ordering it from a supply house. As far as the size, you only have to cover the area that return air is flowing.
 
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