Today I put my passive solar heater back into the window.
At the basic level it is a 24″ piece of foil bubble wrap folded into a U shape and thumb tacked into the bottom of my window. It is getting fancier but that was my test pilot.
How it works is that an inch or so opening at the bottom and 4 inches or so opening at the top allows the movement of cold air on the floor to siphon through, get heated between the foil and glass window, then quickly exit out the top and into the roon.
The natural rise of heated air pulls the coldest air from the floor into the heating area and as it gets heated rises out the top, sucking more cold air up from the bottom. It is between the warmth you feel from a sunny window and the heat from a solar oven. Passive solar with a boost that is economical. Since it does not include a heat storage unit like a trombe wall or direct sun on concrete, after sundown the heat stops and there is no residual night heat. Depending on your insulation, the heat dissipates slowly.
When I thought about trying it, I worried it would heat too much, but with 4 inches open at the top, the air rises before getting dangerously hot. It is obviously warm to your hand but moving like vented air and heats a larger volume of air than just sunlight through windows.
It looks like this from the top: [| with the [ being bubble foil and the | being the glass window. It is 24 inches tall and as wide as the inside of the window frame. Mine is only 24 tall by 30 wide and warms the house pretty quickly.
I get a lot of sun here so it is very effective. Mine is in a west window because I don’t have a south window available. I haven’t used my east window for morning heat because that is my dog’s lookout window, not to mention the mountain shades my house until nearly 10 am.
Just one passive heater last year lowered my electric bill over $100 a month. I skipped adding a second one because the first one got pretty warm in the afternoons. Not like it has a thermostat but not like it costs anything either. I will put one in the east window, it will turn “on” mid morning and “off” before the west window turns “on.”
This is a good circular economy use in New Mexico because we have about 300 days of sunshine. It uses what I have (free sun) and amplifies it into a reliable, low cost, no moving parts, source of warmth. All thanks to the market economy manufacture of foil bubble wrap. Foil board would be effective if you have it lying around. Cardboard with aluminum foil folded around it. Or go fancy with a miniature foil backed shoji screen. The summer got away from me or I would already have the shoji screen version.
This simple heater can reduce fossil fuel consumption without sacrificing warmth.