I don't like the idea of double stacking filters without having an explanation or reason. Install a 20x20x1 pleated filter model, MERV 8-10 should be fine. Just curious, what size zone and size system does this return air serve?

IMO

So, you're saying that the opening is 20x40" and there are two 20x20" filters side by side?

Air will get through in between unfiltered if side by side. Depending on the answer to weedmeister's question you need either a 20x40x1 or a 20x20x2 filter.

Sorry I wasn't clear. The filters are not stacked, they're side by side.

Thank you for the quick replies

@Tigerdunes, I don't know the size but it's a dual zone - first and second level.

My bad...misunderstood....you need a 20x40x1 pleated media filter...MERV 8-10...I would like to know the size of your zone and system...that is a large return air vent.

IMO

Depends on the system. Its 800 sq inch which would be fine for a 4 ton. Some people go 100 sq inch per ton, some go 200 sq in per ton. As far as CFM, I believe its 2 cfm per sq inch so that would put you at 1600 CFM. And bigger is quieter.

I went back and looked at it again. Earlier I just opened the grill and looked at the size of the filters, but this time I opened the whole grill (and the filters fell out), and saw that there are 2 big ducts, side by side. So I guess it's one 20x20 filter for each one? Maybe that's all we need, 2 20x20's. Sorry guys, this is all foreign to me. Usually my husband does this, but he's been out of town almost every week lately, so I just took it upon myself to do it.

You will not find a 20x40 filters in stock,it must be special ordered$$$$$$. As others said,don't use less than Merv 8. I would use 2 filters and install a "mullion" between them at earlerest oppertunity. Contact a local sheetmetal shop and tell them what you have. They will fabericate a thin metal TEE that you must take home and pop rivet in place. The reason for it is so two unsupported filter edges don't flex and alow air to bi-pass between filters. I'll bet those are like juggling because one filter require's quick hands to hold in place while door is closed.

Hmm, since there are two ducts maybe there is already support in the middle?

I don't know what the support in the middle is, but I took a picture. The filters fit perfectly on those slots, and there's a wood in the middle. It's just really hard to put the filters back in. A third hand would be nice .

Judging by that picture,why was there ever a question about filter size? The divder was never mentioned until I said that one should be used. Presto ,,, there it is.

I apologize. I didn't see the divider until I took the picture last night. When I first asked the question, I opened the grill to check on the size. And I appreciate you all taking time out to answer my question.

I assume this is overhead? There may be little clips on the sides that hold the filters in place while the screen is open.

Nope, no clips. Would be nice if there were clips. The filters will just fall on to the screen if I open it slowly.