What size furnace filter do I need?

Old 01-20-16, 07:38 AM
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What size furnace filter do I need?

To me, replacing furnace filters was easy: remove the old and put the equivalent in. When I bought my house in 2014, the furnace filter was 20X25X1 and I've been replacing it with the same size ever since although it seemed unusually difficult to get in. See the photo.

Yesterday I noticed a old oil company sticker on my furnace that said the filter they installed was 24X24. I've used this filter in the past and know 24X24X1 is hard to find so my predecessor may have used a different size.

I'm going to change to a 24" filter but am wondering about total size and exact placement, so here's my questions:

1. The actual opening to the furnace (air flows right to left) is approx 15" in height. So, the filter now covers a lot of sheet metal. Can I use a 20X24 or even 16X24?

2. The place to install the filter (see photo, please) is approx 4" and I've placed the filter against the furnace opening. However, you can see the filter takes up only a small portion. As you can hopefully see in the photo, that location is split with a little over an inch on the right side and the balance to the left. It seems to me the 1" filter should go on the right side (where I've placed an arrow) but that leaves a space between the furnace opening and the filter. That space has a cover, but not tight. Where does the filter go: far left where it is now, or far right?

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I take it I need to seal the area if I put the filter to the right.

Thanks in advance.
Old 01-20-16, 09:43 AM
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Hi, hard to tell from that pic,but if the filter is installed correctly it should fit tight against the open return duct. 24x24x1 filters should not be that hard to find.
Old 01-20-16, 12:43 PM
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If the furnace is on the left of the photo, the filter placement is correct as shown. From what you say about the opening size on the furnace, you could use a 16 or 20 x 24. I would prefer the 20, 22, or 24 x 24 for a bit more over lap thus a bit more rigidity.

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