The thread Mr Bus Guy linked to has some information, and so as to not repost all of that I will simply say that it is a good idea to install a good quality pleated filter and change it regularly. Post #2 in that thread will give you some excellent manufacturers of filters.
To add to that information I would point out that most people do not leave their blower on all the time—that is to say that the blower is only on when there is a demand for heating or cooling. However, that means the filter is only working when there is heating or cooling going on. If you live in an area where you would benefit from a humidifier, it would be an excellent choice to have a humidifier installed and then run your blower continuously. (24hrs/day) Then you are constantly filtering the air, and adding humidity as needed. (in the wintertime)
You specifically asked about “add on” systems if you’re already using a high quality filter. IMO, the add on sytems are outstanding—but not needed for everyone. Currently, your filter is in series with the furnace blower—meaning that all the air is passing through your filter. The add on systems that are designed to be installed with your central heating/cooling system (as opposed to those that are designed to sit in the living room) are designed to piped in parallel—meaning that a ‘cross section’ of air is taken from the [return] air stream, filtered, and then put back into the [return] air stream, downstream of where it was taken. So, at any given time only a portion of the air is being filtered with the add on system. However, assuming your blower is left on continuously all of the air in your home is filtered in this fashion over a 24 hour period.
They are fantastic systems. Why doesn’t everyone have them? Two reasons: Not everyone needs one—generally a high quality pleated filter will do the job adequately. The add on systems excel where there are allergies due to cat dander and things like that----and while I think that whole allergy/mold angle is oversold there is no doubt that the HEPA filter filters out very small particulate matter and help immensely. I recently sold one of these systems in a high end house and she simply didn’t like the amount of dust in the house. (she already had a high quality box filter)
The second reason is cost. Here in the midwest they will cost between $1400-$2000 to install. What doesn’t get mentioned is the ongoing cost of filters; there is a pre-filter and a final filter and depending how often you change them you will spend between $100-200 per year to change them. Here is a link to a Honeywell system that is excellent. I would typically charge around $1400-$1600 to install it in most homes. (prices vary by cntractor and region)
I’m not sure what the sales person was suggesting for $6000—but for $6K I would be able to install Honeywell’s top of the line filtering system, (linked above) provide 3 years of filters, a top of the line programmable thermostat, install a new humidifier and 3 years of filters, a new high quality box filter system and 3 years of filters, re work the duct around the furnace, provide a 3 year maintenance program for both your furnace and A/C, and probably have money left to sub out the duct cleaning (a different topic and one I’m dubious about), wash your dog and detail your car.
Do I think you need one? Maybe. If you have the cash, have allergies or simply like the idea of having superior indoor air quality, go for it. But I’d make sure I would get competing bids and make them detail what they’re providing. Make sure you get references and check the BBB. It’s a great system—make sure you have the right people installing it.
