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I'd suggest not adding one in series with the IQAir filter, but buying a separate unit. I've thought a long time about this problem, and here's a quick summary:
1. Active carbon filters use several mechanisms: adsorption, catalytic decomposition of ozone and related oxidizers, and absorption. Adsorption means that whatever the filter captures is only captured temporarily and can get desorbed later on. Absorption uses up the carbon. The catalytic mode remains functional even if the carbon is saturated with VOCs, but its degree of activity declines slowly with time. To be effective in absorbing VOCs, you need *pounds* of activated carbon. The little carbon meshes don't come close -- I'd guess that they'll absorb a few puffs of Lysol at most, and the air isn't in contact with enough carbon to catalytically reduce ozone much. It's better than nothing but don't count on them for much.
2. The carbon filter meshes restrict air just like a regular filter, so the total pressure drop becomes too much for the furnace to handle when you use both a regular filter and a carbon filter.
3. Carbon alone doesn't deal well with all VOCs.
4. Residential photocatalytic oxidation (PCO) units are being touted as a solution but their safety is controversial, they are new (not time-tested, see recent recall), require expensive UV bulbs to be changed yearly, those bulbs contain a lot of mercury that requires special disposal, and hope you don't break one in your house. They'd be safer if they were followed by a substantial carbon filter but then you're back to square one -- why bother with PCOs then. PCOs have their place in some industrial situations but I'm not convinced about their appropriateness in most residential applications.
Separate units can use mixes of carbon and zeolite or other compounds to be effective against a broad range of VOCs and pollutants. In my house I use a Carbonaire filter (40 lbs of carbon, I found it very effective) with a Fantech fan running continuously, and ducted to the furnace's air return. I also have boxes of sodium bicarbonate close to the return vents, but not blocking them, that act like an acid trap because bicarbonate is alkaline -- that takes care of the smelly sulfur (acid) emissions by the high fructose corn syrup plant nearby.
You may already know this, but just in case... Molds will become inactive if you measure, reduce and control the humidity.
Ventilation can reduce VOCs too, but obviously can't reduce it below the outside level of pollution... With filters you can attain lower VOC levels than with just ventilation.
Originally Posted by genesis
