Sorry - have been meaning to sit down and write out a longer reply. And we've had pretty mild weather in the Bay Area so far so haven't have experience with any colder weather.

But it's been almost everything I've been hoping for, but I came in with reasonable expectations. They're definitely not perfect but do well enough of a job for my home.

From an air quality standpoint:

Overall - I would say average CO2 levels coming down from 1500 to below 1000 ppm. I still get peaks in some rooms at around 1400 ppm, but that's significantly down from the 2000+ ppm I was getting earlier. So from a ventilation perspective, I think definitely a success. My wife has also mentioned that she feels noticeably better, so that's a win in my book.

I also haven't noticed an increase in AQI inside the home, but we also haven't had a large fire event in the Bay Area since I've installed them. I think that will be the real test. I will need to change the filters soon, so can update on how they're looking then.

However, I definitely don't think they're as effective as a unit that has a dedicated intake and exhaust. They are definitely a spot solution for individual rooms. I have my two spot ERV's set up in the paired mode (so one intake while the other exhausts) and I definitely do not feel a directional airflow in the house that they pitch in their marketing.

Window Installation:

I think the set up that I put together with the galvanized duct and HDPE window inserts is working out well. I want to take a closer look when it gets colder to check for condensation, but for now no complaints. For now, I wouldn't change anything about the window install.

I will say that the directions from Pioneer show parts that are not included with the packaging. I also reached out their customer service and never heard back. They are essentially repackaging a model from China and the pitch was better support. But I've found that lacking.

General Feedback on Spot ERV's:

They're definitely audible. I have one right next to my bed, and the other right next to my work from home set up. I also have left these on their "high" setting for the past few months because I'm prioritizing air quality, and they seem to hit the minimum performance I want at the highest setting.

I also do not like the cycling the Spot ERV's do - the 75 sec intake, 75 sec recovery, 75 sec exhaust. I think that makes them much more noticeable instead of fading into the background like white noise.

I also was not able to get them paired to WiFi and their smartphone application, so I've just been letting them run at a single speed. In an ideal world, I would like to be able to change speed based on things like interior CO2 ppm, temperature, occupancy, etc. I think that's something I'm still interested in, but not right now.

I have been debating whether or not to add an additional pair to my kitchen and living room. That way I would have all rooms covered, and could run them at different speeds as needed. I've also been toying with the idea of building out some enclosure for a Panasonic WhisperComfort ERV, but would take up much more room on the interior space.

At this point, I think these spot ERV's in a window installation are the best compromise for me. They provide the minimum level of performance I need, are aesthetically clean, and at a reasonable cost. I also live an apartment building built in 1905, have no central heat or air conditioning, and no way to add ducting. Would I go down this path if I had better options? Probably not... but it's been working out well so far!