Not nessarly. The question is,after the filter is inplace and both doors are in place, Can any air get around the filter instead of flowing through it?

Thats the thing, there are no "doors" or covers...??

Hu-oh,that's bad. Yes,both sides should be sealed while operating. Sorry I didn't realize that. Need some more info before suggesting what you need to do. Is the furnace gas,oil or electric? Do you have central cooling coils in combo with the furnace ducts? Is the filter in the attic,basement,wall of living area,ceiling of living area or other? Does the filter fill the cavity top to bottom and side to side? Is the filter 1",2" ? Are you useing 99 cent fiberglass or $5 pleated filters? How well are equiped with tools? Tin snips? 2" duct tape? Drill/driver and bits? Know how to use the tools?

I used to have a similar set up with my old furnace. There was a one inch slot cut in the return to allow a filter to be slide in from one side on the top of the duct. About an inch of the filter extended above the duct. The slot opening was sized well to allow a fairly tight fit with the filter. This set up was installed by the previous owner of my house. It worked like this for almost 25 years without any problems.

The only issue I see if air can bypass the filter. Seal one side if possible. If the filter is not a tight fit in the opening then try adding something like weather stripping. I suppose you could made some type of cover out of sheet metal. You will have to figure out a way to attach it and make it easy to remove for a filter change.

This post was edited by mike_home on Fri, Sep 13, 13 at 10:39

"Hu-oh, that's bad."

Why do you think this is bad?

95% gas furnace, direct vent with AC coil above. 1" Furance filter on side of unit. Using basic pleated filters.

I cut a sheet metal cover and sealed the back opening if for nothing else so that the filter lines up exactly when quickly replacing from front.

Manufacture said the installation instructions doesnt call for a cover but to consult the installer before installing one as to not cause restrictions.

Installer said that they dont add sheet metal covers for 1" filters but it wouldnt hurt anything to cover it but its not neccesary since filter pulls tight when operating.

Of course I can find contrary info about pulling air, dust pollutants from mechaincal room, negative pressure issues, CAZ's and all sorts of other stuff online....

RIght now I just have the back opening sealed, should I keep it this way?

Had you said up front it is direct vent and how much research you have already done and was still confused by differing advice,maybe I would have said " IMO it's probibly bad ". And went on to say rather than add any additional contrary information,I will offer what I believe is a way for you to decide for yourself based on common sense and observation. As long as the vent works properly and there is no gas supply leak,there is no risk of pulling combustible gas or carbon monoxide into the liveing area. How confident are you that there will never be a mal-function on either? Use a source of smoke ( I use smoke candles but you be the judge on what you can handle safly) placed near the opening in question and observe wherther smoke is drawn in while the unit is running. If no smoke is drawn in,ask yourself if covering the opening will change anything. If smoke is drawn into the opening,ask yourself if air being drawn in is a problem. I can't say because you didn't give me information I asked for regarding location of furnace. The possible problems you might think about is if the furnace is in the garage,will hot air drawn from garage help or hinder indoor cooling. Can air drawn from garage possibly contain fumes from cars,lawn equipment,gasoline, pesticides,or othe items stored in garage? Afterwards,ask yourself what possible harm might result from covering the opening? On a personal note,I would like to hear what undesirable restrictions the manufacture thinks might be caused and why the installer is better qulified than manufacture's engineers to make the call. Since this is a discussion forum , I have some questions. Right,wrong or otherwise,what harm do you think could possibly result from covering that hole up? Some advice about asking questions. Quility of answers will greatly depend on quility of information you furnish in your questions. Good luck to you and I hope this alows making a decision you are comfortable with.