Recommendations for Ventilating a 60-Square-Foot Dwelling
A friend has asked for suggestions to improve ventilation in the Conestoga-style homeless sleeping unit being built in Madison, WI. Currently they have ventilation dampers on each end of the building that can be opened and closed manually. If occupants leave them open, they feel drafty, especially if it’s windy. If occupants close them, then the humidity is so high they have condensation everywhere (and horrible air quality, I’d presume). I thought an HRV made the most sense, perhaps with carbon-dioxide controlled ventilation (which has worked well at my home and business), but they don’t want to spend any money upfront. Next thought was the smallest, most quiet exhaust fan one could find on a timer. I’m guessing, given how small these are, you would need to couple it with a passive air inlet, maybe locating the vent behind/right-next-to the electric heater. Here is a link to an article about the shelters. https://madison.com/wsj/news/local/govt-and-politics/nonprofit-looks-to-build-more-and-tinier-tiny-houses-at-new-village-on-madisons-north/article_464fefd0-3bc8-5379-b398-c14ddaa59aa7.htmlabout. Also attaching photo of the inside. Looking for suggestions. Thank you!
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That is definitely not an easy problem. Especially if you want to do it on the cheap with no/minimal power.
One item I can think of is a DIY heat exchanger out of coroplast (lawn sign material) like here:
https://ecorenovator.org/forum/showthread.php?t=891&page=4
Cut a slot through one of the walls near the top and stick it half way through (two ports outside the hut, two ports inside the hut). Connect a duct to the interior port that would be the fresh air supply and run it down to ground level.
Stack effect should create some pressure through each side. The rising hot air inside would flow through the core and out, the cold air from outside would flow through the other side and drop through the duct to ground level.
I don't know how much flow you can get, would be fairly cheap and would require no power.
Another low cost option but requires a bit of power is this:
https://web.archive.org/web/20200925191628/https://www.loudawson.com/17884/how-to-build-air-cross-flow-heat-exchanger/
Thanks for the suggestions!
It's a bit of a rock and a hard place situation--the better the heat exchange, the less stack effect you'll get. I think to make it work without a fan you'd need really big channels, more like the pipe in the second example, with the heat exchange efficiency deliberately limited to less than maybe 50%. Probably still need a small fan to make sure it doesn't stall, and once you've done that, I'd be inclined to just use the fan and not stack effect, so you get consistent flow independent of temperature.
I'd be interested in seeing ongoing VOC and PM2.5 measurements from your CO2 controlled systems.
There are a few really small and cheap HRV systems on ebay, but they look like they are made for the european market, running on 220 V.
$246 https://www.ebay.com/itm/ENERGY-SAVING-VENTILATION-DECENTRALISED-UNIT-HEAT-RECOVERY-CLIMTEC-RD-100-Base/233814688502
$200 https://www.ebay.com/itm/Single-Room-Heat-Recovery-Kit-Extractor-Fan-Extract-Supply-Fresh-Air-HRU-HRV100/322424498584
There used to be a Bionaire window-mount unit that cost even less, maybe $150, but the only used one I found is listing at $325 https://www.ebay.com/itm/Bionaire-BAP336M-U-EverFresh-Air-Exchange-Air-Purifier-System/184576755625
I haven't seen these for sale in the US yet - but they do claim to be 110 or 12 V (DC fans) and come in at $150. Oddly the cores and other parts are available but I haven't seen the units themselves.
https://www.alibaba.com/product-detail/VTRONIC-Through-the-Wall-Ventilation-Fan_62430888244.html?spm=a2700.galleryofferlist.normal_offer.d_image.b33a3de2af2shC