AC filter inspection

I know I need to inspect the filters in my heating and ac system every three months. OK, I just inspected them. What am I looking for? On a visual inspection the filters look just about like the ones I have standing by to use to replace them. So how do I tell when they are due to be replaced?
Thanks a lot for any answers.
Bill Gill
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On a visual inspection

Are you removing them and looking at the backside? Usually the topside looks fine even when the are clogged completely. I personally don't think once a month is too often to change the common type filter, they only cost about a buck, and with soaring energy costs, how much fuel do you want to waste on a dirty filter?
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If you just have the replacable filters, replace them every month . Use the ones that are about one dollar or so each and not the more expensive kinds that say they are good for 3 or more months. Some months they will not have hardly anything on them as you may not run the unit very much, but it is just good to pick a day, like the first or last day of the month and change them by habit.
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The dollar filters are next to worthless. They'll get the hair and huge particles, but do next to nothing for the fine dust and pollen.
Bob
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I suspect there is a catch 22 here. The really expensive filters that fit in the stock location are so restrictive that they are like using a partly clogged filter even when new, and will cause loss of efficiency and waste of fuel. If you really want to filter the air well and not lose efficiency you need an accessory filter with much more surface area so the 11 micron filter (or whatever micron size) can pass enough volume of air to allow the HVAC to run efficiently. Otherwise the "trash" filters are the better bet.
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Bob wrote:

Those filters aren't supposed to remove fine particles - they're supposed to protect the HVAC equipment, which can be damaged by larger particles. If you want to improve the air quality of your home, get a dedicated air filter appliance.
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I use the puralator fiber filters (cheap ones). I buy them by the case and spray a new case full with WD40(used to use Filter Kote) and let them bake in the sun for a day to remove volatile compounds. This makes them really sticky. When I use these treated filters dust usually only penetrates just the surface of the filter. My filter holder will hold 2 inch thick filters so I use two 1 inch ones. When I change filters I move the bottom filter to the top and put the new filter on the bottom. Ive been doing this for about 15 years and will testify that this is probably the best use for WD40 I have found and I am no fan of WD40.
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