You're right. I doubt the system was designed to handle two filters stacked. One filter is PLENTY. I recommend the standard pleated type (not ultra-restrictive ones like you can find at your hardware store). Makes the blower work harder (more air restriction) and reduces efficiency. Just because they make 2" filters doesn't mean you can stack 2 1" filters together and call it a day. 1" filters are more restrictive than 2" filters.

2 - 1" filters?

that's no different than using 1-2" filter which is done frequently although more often commercially.

2" filters capture more and last longer.

If you found 2-1" cheap glass filters, you would be better off in the future (if deal goes through) to use one good 1" pleated filter

Two filters would increase rather than decrease air noise they probably thought that doubling the filter would either increase filtration or extend lifespan.
You need to ask yourself what is most important to you if you are concerned with indoor air quality and you want to increase filtration then do it right and get a media filter that is properly sized for your equipment. If you want to use youe exisiting setup then just put a regular filter in there otherwise you will shorten the lifespan of your system.

Here is a link that might be useful: Air Conditioning Repair made easy

I use 2 filters . . . . actually 3 filters, if you count them all

I have a pair of the standard 1 inch filters on adjacent walls which lead into the air inlet chamber where there is an AprilAire whole house filter attached directly to the bottom of the furnace

The throw away filters catch the large junk and I change them every 3 or 4 months (more frequently in the A/C season) and that way the AprilAire lasts lots longer - 18 to 24 months in my case

Don

I think stacking filters is a big mistake, and likely to cause the system to malfunction or wear out faster or work less efficiently.

It's best to use just ONE filter of the right size. Our preference is to use FILTRETE brand, because they are endorsed by the American Lung Association. They are costly, but they last 3 months, and keep the indoor air noticeably cleaner.

And they also reduce a lot of airflow, thus reduce efficiency and put more strain on most systems (the Filtrete filters). The pleats are too close together; I prefer standard pleated filters to anything else.

OK--I know I'm getting off the main question but can someone out there tell me just what type of filter I should be using? What MERV # should it be? Is MERV even a meaningful measurement? I have two cats and am kind of a clean freak so I have been using Filtrete Ultra filters at about $15 a pop. This is getting costly! Is this overkill? If so, suggestions please?

Gisquared,

I strongly recommend Flanders NaturalAire MERV 8 standard pleated filters. You can purchase them at www.ushomefilter.com in packages of 6. This is what I have been using, and I'm pleased with the results. They don't seem to be too restrictive, and the filtration is good. The Filtrete Ultra filters do seem overkill and costly. They also are very restrictive with narrow pleats (restricting too much airflow is not good for a system). The type of filter that your system can use highly depends on the ductwork system, but if you've been using the Filtrete filters without issues, then I see no issue with using the NaturalAire filters.

Thanks Ryan Hughes! I'll look into those filters!

To answer your MERV question-

MERV is an industry standard rating, so it can be used to compare filters made by different companies.

Most filters are labeled with a MERV (Minimum Efficiency Reporting Value) rating number, which measures a filter's ability to trap particles ranging in size from 3.0 microns to 10.0 microns.

 A MERV rating of 6 means the filter is 35% to 50% minimum efficient at capturing particles, sized 3.0-10.0 microns.
 A MERV rating of 7 means the filter is 50% to 70% minimum efficient at capturing particles, sized 3.0-10.0 microns.
 A MERV rating of 8 means the filter is 70% minimum efficient at capturing particles, sized 3.0-10.0 microns.
 A MERV rating of 11 means the filter is 85% minimum efficient at capturing particles, sized 3.0-10.0 microns.

MERV ratings go right up to 15+, not seen on residential systems though as these are enhanced HEPA, that won't work on standard blower systems.