I made a passive solar heater in an attempt to maintain indoor temps during the day while we are away from our house. It's silly to have the furnace running when no one is home, but if the temp stays higher, the furnace has to run much less to get it back up to a comfortable temp in the evenings. I originally got this idea from a video I watched online a couple years ago, but I can't find that video back to credit them. This heater can be made in many different sizes and configurations depending on where you plan to hang it.

EDIT: I happened to be browsing for another 'ible and ran across the original video that I built this from. It's actually another Instructables member OhMike. His instructable can be seen here. https://www.instructables.com/id/Foil-Solar-Panels-for-Windows-VERY-Easy/

Step 1: Gather Supplies

You will need the following

Tools:

Utility knife
straight edge
drill / drill bits
hacksaw
hair dryer

Materials

two window screen frames
aluminum flashing
aluminum tape
black spray paint
shrinkable window film
two eye hooks
two large suction cups

I purchased two used window screens from a local building supply resell shop. for ~$6. You can sometimes find these in the garbage - especially with torn screens, which is perfect because you're just needing the aluminum

Step 2: Prepare the Frames

The flashing I purchased was only 24" wide, but the screens I had were 30" wide. Wider flashing is available, but I couldn't find any locally. I removed the screen material by pulling the black rubber strip out of the frame. The screen material can then be rolled up and saved for a future project (or just kept as some spare material if one of your window screens gets torn). The frames are held together in the corners with some plastic elbows that just slip into the frame. Taking into account the length of the elbow, cut your frame pieces with a hacksaw to the length needed for your flashing width. Take care to keep each parallel frame pieces equal in length. Once the frame parts are cut to size, reassemble the frame by pushing the corner pieces back in.

Step 3: Attach the Flashing

**Aluminum flashing has sharp edges and can easily cut you. Take necessary precautions when handling the flashing**

The flashing is thin enough that it can be cut with some heavy duty scissors, but I found that using a straight edge and scoring the flashing with a utility knife resulted in a cleaner cut. Mark you flashing length, hold your straight edge firmly on your marks, and score the flashing lightly making several passes. You can then bend the flashing at the score line and it should break cleanly. You will need two identically sized pieces of flashing.

The back panel of the heater needs some holes in the top and bottom. These holes should be made prior to attaching the flashing to the frames. Place some scrap wood under the flashing and drill the holes I used a 1/4" drill bit and drilled 14 holes spaced evenly on each end of the back piece.

A piece of flashing will be attached to each frame. I used aluminum tape to attach the flashing to the frame. The two frames will eventually be sandwiched together, so take your time to make sure that the aluminum tape is free of wrinkles. Prepare the frame/flashing that doesn't have holes in it and rough it up using some sandpaper. This will give the paint something to adhere to.

Step 4: Paint the Front Panel

With the front panel scuffed and wiped clean, paint it black. I applied several thin coats to get even coverage. I use a trigger handle when painting large pieces. Once the paint is dry, you're ready for the window film.

Step 5: Join the Frames and Attach Film

Lay the back panel flat with the flashing side down. Now lay the front panel on top of it with the flashing side touching the back frame. Join the two frames with the aluminum tape. Be sure to leave the front face of the frame smooth.

Open the window insulation kit and use the provided double-sided tape around the face of the frame. Lay the window film over the frame, being careful to avoid as many wrinkles as possible. A helper comes in handy if you're dealing with a large frame. Once the plastic film has been stuck down all around the edge, use a hair dryer to shrink and tighten the film. Once the film has been shrunk. I put a "finish" layer of aluminum tape just to tidy up the edges. You should end up with a sleek slim heater.

Step 6: Install Hooks and Hang It Up

Determine where your eye hooks should be positioned depending on the size and orientation of your heater. Drill a small pilot hole in the top edge of the frame. Screw the eye hooks into the frame. Hook the suction cups into the eye hooks and position the heater on the window, pressing the suction cups firmly so they stick.

So, does it work? Watch the video and find out.

Step 7: Final Thoughts

If you watched the video, you saw that the difference in temperature between the lower (intake) temp and the upper (exhaust) temp was a 6-7 degree difference. I think that number would have been higher if there was a layer of insulation over the back of the heater where the heat would stay travel to the top. In its current form, the back of the panel gets very warm to the touch, so some of the heat is being transferred through the flashing into the air. Since this is mounted to the inside of the window, the heat is not actually being lost, it's just harder to measure the effectiveness since all the heat isn't centralized to one location.

For what it is, I think it does a pretty good job. Unfortunately, the door that I have it mounted on only gets about an hour of direct sunlight a day due to surrounding trees, houses, etc.

Thanks for reading to the end. Remember, even if you don't vote for me, vote for somebody in the contests.