# Thread: Air Filter Life Expectancy

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I have been trying to determine how long to let the filter go before changing it. With new 'stats that measure run time, I have an answer that some might find useful. What follows involves some anecdotal data.

MERV 8 Pleated Filter life Calculations.

Based on watching a filter get dirty to where the flow was noticeably restricted (too much dirt)

Household family of 4 without pets

Filter was a MERV 8 pleated filter.

Only one filter grille with a 32X16 filter

120 hours of use based on digital thermostat timer

Blower set to 800 cfm

Total cfm of air that passed through filter was--

120 Hrs X 60 Min/Hr X 800cuft/min = 5,760,000 cu ft of air filtered.

Filter area= 16 X 32=512 sq in.

Volume of filtered air/Sq In is 57600000/512 = 11,250 cu ft/sq in

Since the filter was too dirty, I'm going to use 10,000 cu ft/sq in as my ideal volume per sq in.

Now, the ideal filter is sized for 2 cfm/sq in of filter area.

Therefore, the expected life of a filter on a properly designed system would be--

10,000cu ft/sq in divided by 2 cfm/sq in= 5,000 minutes or 83 hours.

A more messy household would shorten this value. A retired couple could lengthen it.

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you have way too much spare time.

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Originally posted by madmark
you have way too much spare time.
Maybe he should use it to vacuum a couple times a week.

4. ## How to know when to change

I can see you are trying hard to analyze this problem, and taking lots of measurements and providing lots of facts. However it seems to me you still are not talking the language that the pros speak, sorry to say.

It is my understanding that a filter needs to be changed when its pressure drop exceeds a certain amount due to loading up, no sooner and no later. All the things people say, be it 3 weeks or once a year, are at best attempts to estimate this pressure drop without knowing any measurements. At worst they are marketing mis-truths designed to sell a product (usually their filter).

If there is any way to measure the ESP ("External Static Pressure") of your system, that would give you numbers you could use to answer your question. Many AC techs have the tools to measure this, I suspect they could be used more often to good advantage. If your tech would measure ESP before and after your filter change, that would give you some useful info.

The proper way to measure is a manometer or gauge with one tube leading to before the filter (i.e. in the house air) and the other tube leading to after. I have done this with a Dwyer red-oil manometer bought on Ebay -- but I am a nut about this stuff and don't expect you to do the same. Dwyer professionally documents this application for commercial installations, where there is more money at stake.
http://www.terrauniversal.com/produc...iltergauge.php

There is also a little "G-99" gauge intended to measure pressure drop changes across the filter, specifically intended to tell you when filter change is needed.
http://www.filters-now.com/products/dar.html

It is cheap and simple, and looked good to me -- but somebody on the board told me they disapproved of it, I forgot what they said but it sounded like a professional opinion at the time. Still, how could this be a waste of \$15?

Hope this helps -- P.Student

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if it looks gray, change it -- with family of 8 + 2 dogs, I used to change ea week -- el' cheap o -- Yankee country = COLD

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## Filter It Requirement is HIGHLY SUBJECTive

Originally posted by wendel
Filter was a MERV 8 pleated filter.
.. life of a filter on a properly designed system would be--
A more messy household would shorten this value. A retired couple could lengthen it.
------------------------------
NICE calc absolutely requires validation and modification:

Is 83 hours or changing a MERV 7 ( or 8 ) filter ~ every 10 days realistic?

What FILTER dP and AIR FLOW did you measure
with Clean Filter? Dirty?
------------------------------

http://www.dwyer-inst.com/htdocs/pre...s2000Price.cfm

........
Use Merv 7 ... Measure and Track over an extended period.

Seems like you would have to decrease air flow > 20% to make significant, noticeable difference in A/C performance.

------------
Actually, retired couple may be MORE SENSITIVE & at HOME 3 times more than others, so MORE frequent filter change may be required.
------------

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Refreshing to see an engineering mind at work!

Your calculations (maybe a bit overdone) seem to be acceptable. However, I would think they are ONLY useful within your specific house AND assume that you don't do anything drastically different, like have the windows open during pollen season.

One thing that I've learned from the Pro's around here is to CHANGE THE FILTER. So, I use once a year, but check quarterly for abnormal changes method (big 20x20x5 filters). Others use the monthly, or the "does it look dirty method". If this works for you, I think you are ahead of the game.

8. So people with PSC blowers that run the fan 24/7 should change their air filter twice a week, and VS once a week.

The location of the house, and amount of air infiltration will greatly vary the life of the air filter.

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Thank you for your replies. The intent was to devise a simple way to calculate a reasonable filter life expectancy for a homeowner to use. Of course each house is different and you'd make adjustments for pets, open windows etc. When you're in a house I'm sure you have a sense of its air quality. I determined a (my) MERV 8 filter can handle 10,000 cu ft of air per Sq In in a relatively clean air home before needing replacement. Since I knew the (my) blower air flow, the time was simple division. If a house has great filter area relative to air flow, the calculations would extend the life of the filter. If the filter is small, then the filter life would shorten. These are calculations that can be done easily and programmed into the thermostat's "reminder" feature. The best part of these new thermostats is they keep track of 'run time'. We homeowners don't have fancy pressure equipment nor do we want to be bothered checking the filter all the time--we're lazy. So a simple calculation from data you have at your finger tips, specific to each home you're working in, would be of great value to a homeowner, IMHO.

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## Scatter

Originally posted by wendel
Based on watching a filter get dirty to where the flow was noticeably restricted (too much dirt)

Since the filter was too dirty, I'm going to use 10,000 cu ft/sq in as my ideal volume per sq in.
It can't be KNOWN to be "Too Dirty" unless there are definite symptoms of significantly decreased flow or __ you MEASURE it!

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That's true, Dan. In my case the evidence was the formation of condensation on a supply register. After the filter change, no more condensation.

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