I might need to ventilate more, but I would like to keep the heat in.

Some ideas:

• CO₂ sensor & humidity sensor to know when to ventilate. I must still have a breadboarded project somewhere based on Operame
• Automatic window opener, perhaps based on this project?
• Heat recovery ventilation. The room I’d like to ventilate would require 25-72 m³/h. Some ideas:
  • Something like a used Stork WHR90 (€100) unit could work, but it’s very large. I could mount the outdoor inlet and exhaust on a panel behind the wooden folding “window” already in the room. Should probably duct one of the indoors connections of the unit to the other side of the room to prevent short circuiting.
  • A smaller heat exchanger with separate fans would likely be sufficient, and would be much smaller and easier to mount. Also, perhaps can be quieter, especially if mounted in an insulated box. However, it would likely be less efficient and probably more expensive.
  • Something along the lines of this project or this one
  • A heat recovery ventilation system using a single vent, with heat recovery material inside, blowing in/out alternately. Sometimes called “recuperator ventilation”. Less efficient, but simplest solution. A new unit can be had for about €150 but should also be the easiest to DIY, because the core doesn’t have to exchange heat through space, only over time. Ventilation (in/out) regime and thermal capacity should be matched carefully for highest efficiency, which means that throttling might not be possible. Commercial units use a ceramic core and roughly 70+70 second cycles. I guess instead of throttling, I could control the amount of cycles it does per hour based on the ventilation requirement. And/or use multiple temperature probes to control cycles? Ceramic cores are available: for instance this hexagonal one, and I wonder if this ceramic catalytic converter core substrate is suitable but might clog up too fast.
  • To keep in mind: outside-to-inside noise reduction by installing a noise filter on the vents? And or acoustic insulation on the wooden window to lower fan-to-outside noise?

So far, the single fan “recuperator” seems the most viable option. Perhaps with a 12V case fan, and the whole thing encased in an insulated box mounted to the wooden window. And some filters. Or perhaps two units together in the box for a low/high flow option. Can even set it to just blow into the room at night on very hot days.

First, get a CO₂ sensor working.