Anyways, I’ll also plug my own little part here. If you carry a CO2 meter around, I’d love it if you share your data with my crowdsourced co2 tracking project, CO2 trackers!
https://www.co2trackers.com/home

Dr. Kashif Pirzada runs a similar and slicker project, The Raven Clean Air app, but mine was first! 😉🤣

The Honeywell filters are about 3 to 4 inches deep, but they have frames that mount in conventional 1-inch deep holders, as long as there is sufficient room behind the filter holder to allow for the depth of the filter plus several inches for air transport. In our case, we have two returns, one that was built into a space adjacent to a closet, so the behind-frame space is about 1 foot; the other was built with a deep box metal inlet going to a 12-inch duct, which is 8-10 inches deep. Both provide adequate space for air flow with no significant restrictions. In addition to Honeywell, there are knock-off brands that offer similar deep pleated and low-pressure-drop pleated filters, but I cannot speak for their quality or effectiveness. The ones that we got fit our particular filter sizes. Looking at the actual filters, I see that I was mistaken about the MERV level, which is MERV 10. The ones we got are the current 3-inch deep filters rather than 4-inches deep (which the original products were), but they are described as having the same MERV value and pressure drop as the previous versions (likely by changing the details of the filter fabric, so that the more-shallow installation would fit more situations). The filters that I purchased do not display the model number, but you should be able to find these or equivalent products being sold. We bought our genuine Honeywell filters through an on-line HVAC supply company called SupplyHouse.com. The 24×24 version went by the SKU FC40R1078 on their web site, where the “40” is actually a carry-over from Honeywell’s original product that was 4″ deep; the ones we got, and the ones you are likely to get, are actually 3″ deep.

Could you share more info on this Honeywell filter that mounts are inch filter to a 1 inch opening? My search didn’t turn one up.

As Allison has pointed out in several other posts in the past, simply installing a MERV-13 filter into an existing system or in a new HVAC or ERV installation is likely to create pressure-drop issues at the filter. We bought our 1987 home in 2013 and immediately installed a new HVAC system to replace the existing failed unit, but the new system uses the existing return and supply ductwork, etc. The return filters are at the entry points for the main (low-level) return and smaller high-level return (rather than at the air handler, which is attic mounted). I jacked up the MERV rating using what were thought to be good-quality 1-inch pleated filters, confident that I would be improving everything. The filters were sucked into the returns by about 1.5 inches, but they showed the expected collection of dirt and dust on them. Then I read Allison’s article describing the effect of pressure drop at the returns on overall system flow, which is something I had been concerned about since I could not get enough supply flow to evenly heat and cool certain rooms in the house. These 1-inch thick filters were providing very high pressure drops because they were trying to stop small particles (the high MERV), so even with the support network of the frame they still were being sucked into the openings due to the resistance. A few months ago I switched to Honeywell 4-inch deep filters that mount in the normal 1-inch frames (and stick out the back, where there has always been plenty of space in our case), which retain the high MERV ratings (in the 13-14 range) but the deep pleats allow the air to flow with very low pressure drop, close to a low MERV pleated filter. The air flow through the returns and out of the supply ducts immediately increased. Voila, the supply issue had disappeared. So, be sure to take the pressure drop on your filter choice(s) into account, and keep it as low as possible to maintain your system supply sufficiency.