"Can anyone explain why this is happening?"
Well I will surly try miss 47. Let's do a little detective work to see if it turns somthing up. Does the filter(s) fit their holders well preventing air getting around filter instead of through it? Are you useing cheap $1 to $3 filters? If you are,is the filter coated with lint and dust enough to obscure color of filter? Those two questions are very important,please give them consideration and answer as specificly as possible because the answers go a long way in weighing the next item. While the AC is running with all windows and doors closed, bath vent fans,range hood and cloths dryer off. Raise the garage door ( alternatly,you can use any door from living area to outdoors,the idea is not having strong wind blowing on the door) slightly open the living area to garage door as if peeping in with your face close to the narrow opening. If you feel air rushing into your face and/or the door try's closing itself,there is a leak in the AC return air. Use your own judgment on trying to find it but depending on several construction facts,it can be difficult for all but the best of hvac techs to locate. If youall want to give it a shot and there are multiple returns,hold a sheet of paper near each one to see if it is pulling in air. Here's one possibility more suited to homeowner's ability to deal with. Your vacum cleaner may not filtering air before exhusting it back into living space. Ok,before going on,let's see about those items first.

This post was edited by klem1 on Thu, Jun 19, 14 at 12:37

Thanks for your reply Klem!
Our filters are 30x30 and they do have dust on them but not enough to obscure the color. Have no idea of the cost, husband orders them online. Tried the door test but did not feel strong wind blowing. Vacuum is a fairly new Shark Lift Away and I do clean the filters on it regularly. Have only one wall intake vent in the hall and did the paper test on it. Intake working well. FYI - we run the air before bed then turn it off. The house stays cool enough that with the ceiling fans running, we don't have to turn the air on till the next evening. This is a puzzlement!

Puzzlement,been a long time since I heard that. Mom used puzzlement fairly often.
Be aware that you are my eyes and ears so your observations and statments must be accurate. If you don't understand anything,please say so and we will stay on track.
Sounds like you trust your vac and leaky return air is only iffy and shouldn't be ruled out so let's go deeper on filteration. Filteration is one of if not the least understood amoung hvac professionals. I have been told many times just how wrong I am about the subject but I have researched it through tests on several buildings. The jury remains out regarding your filters based on how dirty you say the house is and filters don't catch a large amount. It's possible your husband orders filters because he can't find them nearby. We need to be sure (A) The filters fit well and (B) The filters are of suffecint quality to capture dust instead of passing it through. Unless your husband is far more free with money than the average,he isn't throwing away expensive filters 12 times a year. Once we solve this mystery,I incourage useing filters that cost upwards of $10 each. For now I would like you to ask Home Depot if they stock "True Blue #230301.4 filters. The dot 4 means a pack of 4 in case they have singles. If They don't have it in stock,they can order them for $25 per 4 pak,no shipping if picked up on store. Here is how to get maxium life from each filter without compermising performance ( will be pretty important once you start paying $10 to $30 each). The filter is pleated similar to an accordion. Dirt will first collect in the recesses and progressivly work it's way up the walls until the highest ridges collect dust. You can clearly see this as it obscures the color of filter. Optium timing to replace is when recesses and 80% of walls are covered in dust. I assume your filter holder is desighned as 75% are to take 1" thick filters. 2,4 and 5 inch thick are commonly in use. Ask hubby to confirm how thick of filter the rack takes. Is there a lot of visible dirt on ceiling vents and/or ceiling near vents? Where is your cloths dryer? Check hose to make certain it is hooked well at both ends and doesn't leak air while running. Turn dryer on and make certain air is freely discharging from hood outdoors. That's your homework assighnment for today.

Are your ducts located inside of the house envelope or outside? Leaking supply ducts outside of the house envelope depressurize the house causing air to be sucked in through leaks elsewhere. Leaks in returns that are outside of the envelope pressurize the house blowing air out through leaks elsewhere. These returns can be sucking in outside air directly.

If there are not balanced returns and supplies in each room, and there is not enough room to room air flow, closing doors can pressurize and depressurize individual rooms sucking in outside air and blowing conditioned air out. There are lots of ways to bring in outside air with its associated dust with an HVAC system with "outdoor" ducts.

Thanks again Klem1, I have accepted my assignment and we're headed to HD today to get the filters. I'll report back with hopefully positive results.

Thank you Ionized! I printed your response to show my husband since I have no idea how to responde to you. But thank you for the added information. I'll let you know what we find out.

Thank you both, you've been a great help to us!

cellulose insulation in attic?

ducts in attic and supply grills in ceilings?

return air is in ceiling or low on wall?

I'd lean towards duct supply boxes not being
sealed to attic floor & return that is not air tight.
more the former than the latter.

play along being our detective & we may figure it
out for you.

nice posts Klem!

best of luck

Thanks energy and welcome to the"Whodunnit Playhouse".
We have an intermission for costumes and lead gum shoe go's to the Depot for new props. It appears consensus is the culprit is likly return,supply or handler leakeage. It's probibly inevitable the home owner or someone gos through the attic but due to his respitory problem,I thought we might make that last resort. I want 47 to tell us what if anything she noticed while conducting the open door test. She said no "strong" flow through the opening but there may have been noticable air flowing. Any suggestion of another test for air imbalance? What they are doing while hvac is off every day may have something to do with it.
So Miss 47,did you feel any breeze while "peeping" through crack in door? Are you raising windows/opening doors for a period while hvac is'nt running? Are you or Mr 47 a woodworker,auto tinkerer,gardener or other hobbiest that deposits dust on your clothing? This might be the goofest question I could ask but do you have a friend that can compare your atic insulation with samples from your table tops and filters under a microscope? You might also ask around about city health office,Jr college,enviromental lab,industrial plant or other source of comparing the material. Point being if the dust on your funiture is danderuff and grizzly bear belly button fuzz,we need to stop looking in your atic,,,,,,,,,,,,,,,or should we.