re: How often should you replace your A/C filter?

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On a side note, I've heard that if you get those really efficient 1900 and above filtrete filters, unless your A/C is very new, you are making the unit work much harder to pull air through because they weren't designed for those type filters (or something like that). Any truth to this?

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Those filters are more restrictive and will make any blower work harder. Newer ones might be designed to handle the higher external static pressures (ESP) they cause.

Here's some actual data. After using the fancy filters for years and hearing all sorts of anecdotal reports, I measured the following ESPs for my system last year (filters were new):

No filter: .4" WC
Filtrete 1250: .56
Honeywell MERV 10 4" deep filter: .45

The Honeywell is like a Filtrete, but the pleats are 4" deep to increase surface area and make it less restrictive. It has a lip so can be inserted into a standard filter opening if it's deep enough (needs to be a good bit deeper than the filter is thick for best results - my return duct is about 3 feet long, which is way more than enough). They cost about $45 but can last a year or even more. I've been using them instead of Filtretes for about seven years now, changing them once a year.

So what does all this mean?

The table in the manual for my blower gives CFM and furnace temperature rise for the various fan speed settings at ESPs from .1-.5, but .5-.8 gives only the CFMs. It stops at .8. My understanding is that this is pretty typical. It'll work at the higher ESPs, but it's outside the specs, and it's best to keep it below .5; the Honeywell does this, but the Filtrete 1250 was at .56 brand new. I measured the .45 on my Honeywell in August and .46 in June, so it's been doing well. I need to measure it again and will probably replace it this month regardless.

At a given fan speed setting, the unit moves less air as ESP increases; that is, the CFM (cubic feet per minute) decreases. The furnace temperature rise increases, but the effect on cooling is going to depend on whether you have a fixed orifice or TXV, with the TXV being better able to handle the changes in the CFM. The orifice is much more dependent on the conditions at the time the system was charged, and that includes the air flow. So, if you were to change filter types (or fan speed) on a system with an orifice, the charge might need to be adjusted.

And BTW, the primary purpose of any filter is to protect the coil from getting dirty, and the blue fiberglass ones are not great at that. As for the pleated ones, if the blower isn't running, they're not removing anything from the air, and how often does yours run on average? I'd love to see an independent study of how effective they are at cleaning the air; I don't see how to test that myself. That said, the Honeywell is great at keeping the coil clean, which you can tell just by looking at the coil. The Filtretes are good at that, too, but they are indeed much more restrictive per my testing.