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Link goes to this month's power bill.

I added black corrugated pipe (plastic) over the clear vinyl air hoses giving me what seems to be a 2 to 5 degree increase in the amount of warmth coming out (temp on hand/skin estimate - it is warmer but I didn't measure it). Further, corrugated pipe is stiffer so this straightens out the pipe layout so it's not snaking all over the place (no ups and downs in a passive system which means smoother air flow overall). The insulation effect of the pipe combined with the black heating effect in sunlight means that I'm not spending heat trying to heat the pipes (and perhaps getting some kind of help from the corrugated pipe as well). I'm still not getting passive air movement without the solar panels/CPU fans but it hardly matters since if there's not enough sun to drive the panels and fans, there's usually not enough sun to make as much of a difference in heat generation. I only half-assed the installation since I didn't really want to make a full-on production of mounting the pipes so if I did it right, I might get another 1 to 2 degree increase in temperature (the ends are lose and not sealed on the corrugated pipe). Further, expansion and contraction from heat and cold cycles has cracked the acrylic in most of the mounting points leading to undesired heat transfer in unhelpful ways. I suspect I should have drilled the mounting holes larger to account for expansion. For now, I just covered the cracks with electrical tape and will replace the acrylic next spring when I remove the unit from the window. When I do this, I will replace the acrylic with either a thicker piece or a double pane of the same current thickness (not sure yet which).

I also added foam weatherstripping along the mounting bracket and lined the inside with two layers of cardboard to help insulate the weakest part of this and prevent cold from leaking in.

With all the changes, I can now walk up to the "vent" and stick my hand in night or day and it is room temperature instead of being colder and somewhere between inside and outside temps.

Regarding power:

It's not been warm enough to use the AC in either month and average temperature is only 6 degrees higher in December (huh??).

Power usage is 21% of last month. That's a significant amount of change and not representative of an average increase of 6 degrees between the two months. Total amount for power usage is $27.05. For reference, the main part of my house is heated with natural gas (forced air electric fan) and usage has gone from $10.70 (just the cost to keep the gas on) to $47.35. The amount billed this year is barely more than last year but there's already been a rate hike since last year and with current economic predictions, there will be more soon. So what we're seeing is a massive decrease in electrical usage with an expected corresponding drop in the usage of natural gas even if the amount paid for natural gas is higher.

With the addition of the corrugated pipe, I've been able to keep my heat set between 60 and 64 instead of 64 to 66 without (and 70 to 72 without the passive heater at all). In fact, I am now keeping it off most of the day.