the filter is for the whole system, all the air flows thru the same components regardless of whether heating, cooling, or just fan blowing.

for the el cheapo see thru filters, if the filter looks clean, it is clean. for the thick filters, change them at the suggested interval or when they start to show dirt.

mine get changed about every 4-6 weeks now. i used to have to change them every 3 weeks at the most, but then we ripped out our carpet and the dust was cut down tremendously.

if you cannot find them at a box store, either get them from a local HVAC company, order them online, or get the universal filter you cut to size. i usually go with the latter since i can wash it out and reuse it for a year or so.

"Is there a shortage of 12" x 24" filters due to an industrial fire at one of the plants, as Home Depot and Lowe's are out of them?"

I had to laugh at this one. Maybe it's just a common size filter and that's why stores are out. Or maybe the fire burned down the 12"x24" filter factory, but the other filter factories are OK?

I installed an AprilAire media filter directly under the furnace and then I use 2 big 99 cent 'pre-filters' (one 24 by 24 and the other 24 by 36) which I change every other month . . . . sooner if they look dirty

The media element has been in ther for almost a year and it still looks remarkably clean - The replacement element for it is about $25

Don

I have a Labradoodle who turned out to be a shedder so we have quite a lot of pet hair to deal with, so I've gone with a prefilter which seems to work well, i do plan at some point to get the ducts cleaned but I really want to keep the big stuff and the dander out of the system so the prefilters seem to have worked well although I still see quite a lot of the fine grey dust that is characteristic of human/dog dander/dead skin cells on the furnace filter -

I was finding a monthly change of filters rather financially challenging (I run the furnace fan 100% of the time, over the summer it with the humidifier proved a great benefit in our dry climate, no more static issues, and no mould issues (I can't see how you're going to get significant condensation etc with a system that is not cooled but just circulating air) so I went with the 'permanent' electrostatic filters, they say they don't require power.

I bought two, so I put the clean one in, vacuum then hose the other one off - seems to work pretty well.

I was kind of slack and went a few months using the same pre-filters, vacuumed the cold air return grille regularly. I did notice some leakage round the edges, so I am installing weatherstripping round them so they seal better. May be overkill but I think it'll keep the air in the house cleaner. I intend to put in a uv air steriliser too, at some point.

Virtually any size can be ordered, or made to exact specifications. There is not a shortage of filters 12 x 24.

check out: www.filterfolks.com

I see there's no shortage of spam, either.

"Virtually any size can be ordered, or made to exact specifications. There is not a shortage of filters 12 x 24.
check out: www.filterfolks.com"

- Your website needs a lot of work. You should have sizing and prices listed, just like

www.ushomefilter.com

This is where I get my filters because the size isn't common. Pricing is decent. Shipping included, and not just to Florida.