Now that's a good question. I have to admit that my cover has not fallen off yet. So, you can feel free to question my 'crummy design' comment. I made it because in normal maintenance/handling of the filter cover, I noted to myself that it most likely was going to be trouble some day. That's because the locking tabs hardly make enough contact to stay in place, and retainer 'rings' look very fragile. The way the filter assembly stands out from the engine, you have to keep it in mind when cutting under bushes/obstructions on the carburetor side, because one good hit on the filter housing might break a retainer ring. At that point, you could rig a makeshift keeper for it but the filter housing should be replaced. This opens another can of worms because the housing screws are self-tapping in the aluminum, and might strip out easily. Does it sound like I have been there with this? Indeed I have. My LB 22271 I got from Wise Equipment was poorly packed (they had set it up on display and repacked it for shipment) and the filter cover got knocked off, breaking a retainer ring. I had to get them to send a replacement along with some other parts. There were no more 22271s, and I didn't want to deal with shipping it back anyway. So, that's when I found out about the housing screws which brought lots of aluminum shavings out with them. They tightened back up, but I was holding my breath. Next time they probably will strip. If that happens, I hope to be able to tap a slightly larger size in those holes and use conventional screws.

I don't have any statistically valid numbers, but I hear about as many anecdotal reports of premature failure with TCP products as with any other brand. TCP imply on their website (or at least used to) that they operate their own factories. (They are made in China, according to an emailed reply when I inquired.) I'm not sure whether they actually have full control of those plants, or whether they just contract with the facilities that produce the vast majority of helical CFs now sold here. FWIW, TCP supply the "Commercial Electric" CFs sold at Home Depot. Some of their CFs were subject to a safety recall a while back. I recommend Panasonic and Philips CFs. They are not cheap. You'll pay around $15-20 each, but you'll get at least some of it back in longer life. However, I'd avoid the Philips helical CFs usually sold in multi-packs, as they appear to be rebranded Chinese generics. Their tri-tube CFs should still be good.

You will not find a 20x40 filters in stock,it must be special ordered$$$$$$. As others said,don't use less than Merv 8. I would use 2 filters and install a "mullion" between them at earlerest oppertunity. Contact a local sheetmetal shop and tell them what you have. They will fabericate a thin metal TEE that you must take home and pop rivet in place. The reason for it is so two unsupported filter edges don't flex and alow air to bi-pass between filters. I'll bet those are like juggling because one filter require's quick hands to hold in place while door is closed.

@opaone, In an earlier post, you quoted your "buddy" at ASHRAE who provided this position on behalf of ASHRAE: "Negative health effects arise from exposure to ozone and its reaction products. Consequently, devices that use the reactivity of ozone for cleaning the air (emphasis added) should not be used in occupied spaces." I don't know if ASHRAE states that as their position, but I would fully support it. There are cleaning and disinfecting devices which intentionally produce ozone as a disinfecting agent. These devices are advertised to remove chemical contaminants, odors, kill mold spores, etc. which, if they do it at all, is by chemical reaction with ozone. They aren't advertised as particulate filters. By contrast, electronic air cleaners, such as Honeywell's F300, are electrostatic precipitators. They remove particles from air by electrically charging the particles which are then collected on a collector plate which has the opposite electrical charge. Although not intentional, electrical arcing can sometimes occur in electrostatic precipitators and ozone can be produced as a consequence. To be abundantly clear, electrostatic precipitators don't use the reactivity of ozone for cleaning the air . Ozone is naturally occurring in outdoor air. Indoors it can be produced at trace levels by motors, hair dryers, laser printers, and photocopiers to name a few sources. If you want to eliminate ozone from your home, you'll need to start by getting rid of all of those. Then you'll need to add a device to filter out naturally occurring ozone that enters via infiltration. I realize that houzz.com threads are dominated by homeowners and DIYers. If you want to argue a technical point with an experienced and credentialed professional, please bring some data, a link to a research report or something other than your opinion, a quote from an unnamed source or a general reference to your "extensive research." If you'd like to invite Allison Bailes to weigh in on this, please invite him. I'm getting tired.