The question is about the life of a filter in an air handler. I have been watching some of the 5 inch filters in our group homes, and while they can look gray in a matter of 2 or 3 months, the static pressure drop across the filter hasn't changed much. I can start in the range of 0.25" to 0.35" WC and I have been replacing them yearly at the spring PM. When I replace them I might get a measurement of 0.75" or so, but have never seen over 1" WC. In the name of science I ran one until I got to that 0.75" reading and it lasted almost a year and a half. It made me wonder, what is a good way to tell when a filter is shot? What would be a good number to use to determine filter drop? I know, air speed changes those numbers, but then a new filter has a higher initial number, so it might have to be a percentage. (0.25" to 0.75" = 300%) Any ideas, or better, any hard science? I can't find anything in the instruction manuals.

Manometer reading are generally the best way to determine how restricted a filter is. Everything else is variable.

I would think the time spent doing intensive testing for filter clarity would cost more than just replacing the filters on a regular schedule. Just a thought. Filters are cheap(comparitably) to purchase and change. Why overthink a simple process. However, you do raise an interesting question, and I look forward to hearing all advice.

SP between 0.5 to 0.75 at the highest fan speed measured on the discharge side. Definitely no more than 1.

You can turn the reading across the filter into a % filter used, but not directly. 0.5 to 0.75 may just be 100%.

As the SP increases, your CFM decreases. AC requires a higher CFM than heating.

You may then be able to get a higher filter lifetime over the heating season.

The length of time a filter is in service is not always the best indication of need for replacement. System usage, pets, remodeling, housekeeping, etc. etc. all factor into how long a filter will last. I provide the high-MERV air filters for many of my service customers, and filter usage varies a great deal from the most to least frequent replacement. Some are nearly clogged in just 6 months, while others show very little usage after 1 year. There is no one-size-fits-all answer for the question of filter replacement.

MissouriBound,

I am using a UEI 2 port electronic manometer. My measurement method is one port in the return air duct 2 ft or so ahead of the filter, and the other between the filter and fan, at least a foot or more away from the filter if possible. My measurements therefore are DIFFERENTIAL pressure. The pressure drop ACROSS the filter.

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I would think the time spent doing intensive testing for filter clarity would cost more than just replacing the filters on a regular schedule. <snip>

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MyGirlsDad,

Yes, I agree, but this is research, not just blind mechanics. I work for a non-profit, and at $20 or more a throw, if I can save them some filters, that's as good as a donation. If I feel bad about the time researching things, I can donate that. I am just curious by nature and want to do the best job possible. Note: all this AMHD stuff is on MY time! :p

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SP between 0.5 to 0.75 at the highest fan speed measured on the discharge side. Definitely no more than 1.

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KISS,

Clarify discharge side. Of the fan? Of the filter? Of the complete air handler?

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<snip> As the SP increases, your CFM decreases. AC requires a higher CFM than heating.

You may then be able to get a higher filter lifetime over the heating season. <snip>

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Yes, I know this. However, I do not have a pitot/static tube to probe the ducts and actually measure CFM so I can't actually correlate pressure drop numbers to CFM numbers. (Yet... :D Tools are better than candy.) And yes, I usually like to change filters in the spring if possible. My biggest concern is to catch them in time BEFORE they cause problems without just using LAR service techniques. What would be a realistic percentage of CFM drop that would indicate time to change the filter? 95%? 90%? 80%? And of course that begs the question of correct CFM numbers for the equipment at hand. We have systems that have good duct systems and some LOUSY ones that probably don't even come up to air flow spec WITHOUT a filter. And almost NONE of them give actual CFM number requirements in the manuals that are laying with the systems. Establishing baselines might be fun!

KC13,

Agreed wholeheartedly. Hence my question.

I really like the MERV 11 filters that we are using wherever possible. I actually think I can tell a difference in the cleanliness of the houses since we got rid of the hogs hair filters and changed to these, but I want to be certain that there aren't some hidden gotchas that I might miss.

Thanks any and all for the input. Any more information or links to something that can help me apply science to my job?

EPM

Note: LAR = Looks About Right. Unfortunately a very common method of engineering.

Use your manometer and look at the SP after the blower.

I could find out the hard way, but it would take a while and I'd have to start now.

My tstat at home has a filter monitor built in and can measure RPM, static pressure and CFM.

Right now I'm at:
30% filter used
1450 CFM
1004 RPM
0.59 Static pressure

Remind me in a month and I'll do the readings again. It might be more useful, to do the calc with a new filter.

I could put in a new filter and measure the parameters and assume that the new one was identical as the old one when it was new.

The tstat does a dirty filter test once a day by running the fan at max speed for about 5 minutes.