@jenniferhunter
Thanks for the info. I looked at the product page for the 400 series purifier (https://foustco.com/all-products/air-purifiers/air-purifier-units/series-400-air-purifier/) and found that the HEPA and carbon tray are separate assemblies. In the last paragraph of the description they say that the HEPA will “last for years” but the Media Tray only lasts for 6-9 months. You have been able to go for 4 years without changing so I am wondering if your home is very low in VOC’s or if there is something special about their carbon mix . Their ‘Standard’ mix is 40% potassium permanganate and it may be that the permanganate is absorbing any odors that are coming off the carbon. I use a 50/50 mix of AllerAir Vocarb and Exec charcoals in mine and am pretty sure that neither one has permanganate. I may give Foust a call in the next few days to learn why they added permanganate to their charcoal in the ‘Standard’ mix. If they are willing to talk openly about their product I will also see what I can find out about their lifetime specs.

I am familiar with tube-style filters because I ran them in my lab and apartment in the early 90’s’s. The ones I used didn’t have a HEPA but it didn’t matter because I had terminal HEPA’s in the lab AC and a free standing HEPA in my apartment. In those days you could go down to the local filter supplier and buy a 25 lb bag of virgin coconut shell carbon for a few dollars a pound. It was easy to change out the carbon in the tube and I always took it outside so that the dust wasn’t a problem. You could also get refillable flat panels in those days and I had another unit with two squirrel cage motors and a panel on the front. I usually taped a prefilter over the flat panel to keep it from getting dusty.

I built a house on an Atlantic barrier island (< 1/8 mi from the ocean) in 1992 and put a 2” refillable charcoal panel in the fan/coil unit. That system had the same terminal HEPA filters as my lab, a reheat controller that kept the humidity near 60%, and a MERV 11 V-bank pre-filter on the outlet of the fan/coil unit.

I changed the charcoal in the lab and house every few months but didn’t know enough in those days to check it more often. I believe that the apartment charcoal got changed more often but don’t remember how the period between changes.
I am not sure why we are getting different lifetimes for our media but am going to spend some time trying to find out. For the past 10 years I have been spending over $2500/year on carbon and would love to find a cheaper way to keep the VOC’s out of my work area and bedroom.

I am extraordinarily sensitive to mold and control my symptoms by practicing strict avoidance. (I talk about the reasons for this in another post at https://connect.mayoclinic.org/comment/1172607/.) I have found that stand alone units with < 400 cfm help with my symptoms but don’t reduce them to the level that I need to stay healthy. I currently use 2’x4’ HEPA’s (https://www.camfil.com/en-us/products/housings-frames--louvers/terminal-housing/clean-room-ceiling-modules/slimline-dcm-ducted-hepa-filter-module-_-47742) in both of my ‘clean areas’. The work area has 3 filters and the flow rate with the motors turned all the way up (2100 cfm) should give me two air changes per minute. I’m not sure that the returns are positioned to reach the theoretical max but I am pretty sure that I get >1.5 changes per minute.

I have also created microenvironments around my desk and bed by attaching shrouds (thick cotton sheets) to the outside edges of the filters. The flow inside the shrouds is similar to that in a filtered production workstation and the number of air changes per minute is significantly higher than in the entire room. I usually feel a lot better when the shrouds are in place.

Thanks again for your response. If I come up with a plausible explanation for the differences that we are seeing in activated carbon lifetime I’ll let you know. If I can’t come up with an explanation I’ll probably buy one of the 400 series filters with the 0.3 micron HEPA and to see how the charcoal performs in my work area.