Hvac filter advice

[Joel Goodman]

I have had the oil furnace and central air replaced as the compressor was shot and the furnace was old. The filter the installer (don't get me started) put in the machine was minimal to say the least. I was thinking that a pleated job MERV 11 or 12 would be better. There is plenty of height to put a thick filter in, although the size is odd - 19X21 (actual). Do the higher MERV #s have more back pressure? Is that an issue? The furnace is a Patriot 125,000 BTU (output) paired with a 4 ton Carrier AC. Is there any reason not to use a 4" filter? I assume that more material means better filtering, less back pressure, and/or longer life. But what do I know?
Amazon sells an "AFB" brand in that size -- or various online places also sell filters. 3m and Honeywell don't seem to make that size in any thickness. Any thoughts recommendations on thickness, MERV #, brand, supplier? BTW house is on Long Island, NY. Thanks.

[Lee Schierer]

I buy custom made 29=1/4 x 31-3/4 x 2 filters for our heat system from Filters Now. They end up bout half the price of getting them from the dealer. We get a Merv 11 and it seems to do a good job. The higher the Merv the more back pressure there would be, but I think the higher Merv filters get more folds to create more surface area to keep the pressure down. I'm sure you could ask the people at Filters Now that question.

[Chuck Wintle]

not an expert but wouldn't the depth of the pleats provide less back pressure since there is more surface area? Do you want a HEPA filter for the furnace? I would imagine the installer used the cheapest filter available?

[Jerome Stanek]

How much dust is in your house. I have put in a hepa filter and didn't seem to do anything except low the air flow I now just a cheap filter and it works better.

[Joel Goodman]

Thanks for the input. What I gather is that the higher the MERV rating the more back pressure (if that's the word) . The folks at Filters Now suggested a 2" rather than a 4" and I am trying the MERV 6 (least effective pleated filter) as it will have the least back pressure. They say that the 4" lasts longer but implied it might have more back pressure -- maybe because you change it less often? -- seems illogical! In terms of dust in the house I am not sure that these really help indoor air quality unless the HVAC fan is kept on all the time. It's the noisiest part of the system (maybe there should be a flexible section to isolate the furnace from the return air ductwork???) so I don't want to run the fan constantly. For me I just want to keep the AC coils clean. BTW looked at the coils of the old AC (which always had the nonpleated least effective filter) and they didn't look too bad. Interesting to hear about the HEPA lowering airflow -- supports my theory that better filtering = more back pressure. It's hard to get accurate information especially when the installer doesn't see to be on top of things. Any why can't they standardize filters to a few sizes.....

[Andrew Pitonyak]

MERV rating may, or may not, affect pressure drop depending on brand (and a few other factors).

In 2009, a comparison was made (http://www.homeenergy.org/show/article/id/667) . Now, understand that this data is at least five years out of date and there have been changes made to the filters, but, in this test, the Filtrete 1000 reduced air flow more than the 1550 or the 1700. I believe that all of these are roughly MERV 11 with the 1000 on the lower end and 1700 on the higher end. All of these soundly beat some other that had a much lower MERV rating.

A higher MERV value probably also means that you need to change the filters more often because they are more inclined to clog (for obvious reasons) and, as the filter becomes dirty, I expect that it will block air flow to a greater extent.