There are purifiers that create ozone, avoid those as ozone is a lung irritant.

https://www.epa.gov/indoor-air-quality-iaq/guide-air-cleaners-home#ozone-gen

http://learn.allergyandair.com/ozone-generators/

Otherwise UV light has been proven to disinfect by killing bacteria, mold, and viruses. Yearly cost is approx $100 (bulb and electricity).

UV-C, or short wave ultraviolet radiation, is used to destroy bacteria, mold, viruses and other biological contaminants in the air, in liquids, or on surfaces. UV-V is produced by ultraviolet lamps which produce ozone.

http://www.lumalier.com/why-uv-works

http://www.generalfilters.com/support/faqs/uv-light-faqs.html

http://housesogreen.com/2014/07/hvac-uv-lights-for-ac-systems-do-they-work/

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC93076/

https://www.sciencedaily.com/releases/2012/10/121018130923.htm

Best way to manage air quality is a low restrictive pleated filter. Most HVAC pros dont recommend more than your standard pleated filter and to replace it every month. High MERV filters are very restrictive and can put a lot of stress on the blower motor and heat exchanger due to reduced airflow.

Dag nap-it,Vith was on the right track then had to go and repeat so called hvac pros. If filters are proper size to begain with (many are too small but that just prove's many hvac pros aren't actually pros) high merv filters are not restrictive. I have proven this numerous times with tools that measure flow from vents,temp-pressure readings and load on motor. Do you see the irony in listening to hvac pros when it was hvac pros (either installers,maintainance or both) that caused mold in system to begain with? Use stand-a-long purification then have it serviced by purification experts if you must install one. Before doing anything else,you need a competent hvac technician to get your system right.

If filters are proper size to begain with

Yes, but this is not true for most cases. I would love to throw a 4" thick filter in my return, but it would require major modification of the return plenum. Majority of houses have a 2 or 1 inch thick filter (whatever size). With high MERV filters of those thicknesses they do get restrictive. Also, the higher the MERV the faster it get gets plugged and more restrictive. A happy balance should be found. A fiberglass filter doesnt do squat for pollen, pet dander, allergens. A high MERV filter can be restrictive. A standard MERV filter is less restrictive and blocks allergens, pet dander, and pollen.

I'm incouraged simply by the fact you are buying into pleat filters of any type Vith. Just a liitle more time I'm optomistic you will come all the way over. (: Stay tuned,in a few days I might start a thread dedicated only to pros and cons of hvac filters in general.

When we did an addition to our 1925 tudor almost 5 years ago, the remodeling company recommend a new HVAC unit. In the interest of (their) time, the remodelers obtained a quote from only one HVAC company for us. At that time, the filter location was changed from directly behind the return air grill to a unit that allows a 4" filter. I've been using 4" high merv filters ever since. My mold issues seem to be in my flex ducts, on the ceiling diffusers/vents and in a duct board plenum, thankfully not in the unit itself. I have to agree with klem1, that my HVAC company (who has been performing twice a year service checks) should have suggested we replace the duct board plenum and old flex lines-or at least assured the flex was well attached-when they installed the new unit. In fact, I did not call this HVAC company out to give me a bid on this new job. I'm a little frustrated to see that the insulated flex that they installed in my attic for the addition was only a R 4.2. Needless to say, I'm replacing that too, for I live in the midwest with great temperature swings.

I'll admit I didn't look for you back hzz2016 after no pat answer with a bow tied around it. I was wrong,you are in this for the duration so good on you for not giveing up. Have no doupt,ducts are an intergal and important component of hvac system. Now that you gave info about ductwork I look for energy-rater to wiegh in with helful suggestions on bringing ducts up to standards and sealing things up. Meanwhile I wouldn't be overly concurned about mold remidiation until seeing how it looks after getting ducts up to standards. Not stuff we want in our air supply but the majority of mold/mildew(there are thousands) has little if any effect on healthy individuals. It concurns me that mold has enough organic material inside ducts to survive. If properly installed,4" high merv filters should catch material that mold needs to grow. Here's a simple action I'd like you to do. Remove a vent cover to look carfully inside duct for dust or lint. If you are really handy,pull blower cover or otherwise access blower to look for dust or lint. If you can pinch up a wad of material from either area,100% guaranteed,air is by-passing filter,return is leaking or both.

klem1-My old vent covers are a mess. Over time, they have been painted to the ceiling so they are hard for me to remove. Looking up through them, I see some light dust, but mainly mold in the duct. Our addition's duct work looks clean. I pulled the cover off the blower and pulled the filter out to look at the blower and that area is looks great. I peaked into the duct board plenum and it looks covered with mold. I've read that a too large AC unit could be part of the problem since it would cool and shut off before the it had a chance to lower the humidity level, but other HVAC companies have told me its sized correctly. I'm hoping that by removing all of the old insulation in the attic, getting the attic floor, etc., sealed up, replacing the duct board with sheet metal (and covering that with closed cell foam), replacing the vent covers, vent boots, flex and adding new insulation (blown in for an R-50 value) will solve the problem. So you can see why I would want to check into the air purifiers just to be sure.

Sound's like the filter is doing it's job,that's one less thing to be concurned with. Your plan will save energy if nothing else but should help with condensation as well. Air purifier will not hurt anything, might help keep you healthy and sound's like it will give you peace of mind. I reccomend stand alone so you can have it serviced by a specialist and not rely on hvac techs. Regarding the evidence moisture has been collecting in ducts,has Mr I Heared or Mr They Said mentioned improper condensate drain as a possible source? If you want to investigate that possability and need guidance just let us know.

My condensate drain goes from the west of the attic to the east side of my basement. I'm not sure how it gets there, but it must go between walls, therefore, I do keep an eye on it to make sure it is draining. Other than that, how else would I know if there's a problem with that?

Well starting with a simple but most often overlooked "other than that",a proper trap. Within 6" to 1' of leaving drip pan the 3/4tr primary line often has an inverted T open at top to alow adding algacide or blowing out line. That access opening is better capped. The primary line is often not trapped and run's horizonally to dump into a sewer line or dedicated condensate line. It's best to have line trapped as near pan as pratical or line to emmediatly drop 6+ inches then turn horizonally into a running trap. Secoundary/emergency line is often untrapped,although it's best equally trapped as primary plus have means of being routinly charged. Alternatly,to reduce installation cost and ongoing maintainance,secoundary can be of sufficent leingth to dampen suction through it. Regardless how condensate drainage is handled,preventing reverse air flow through lines is desiriable. In a nutshell,air drawn in through line can distrurb water surface causing "spitting"droplets to be picked up and blown into discharge ducts. The condition is exacerbated if return is restrictive ( a condition Mr Heard and Mr Said both agree is very common). Competent techs will check out this"other than that"along with others to insure condensate drain's rather than blowing into discharge ducts.