Solar water heater

This page is a translated version of the page Chauffage solaire and the translation is 100% complete.

Tutorial de avatarLe Tour des Possibles | Categories : Housing, Energy

This low-tech solar heating system was discovered in Mongolia. Place the heater against a window, and it will circulate air to warm up it by 10 to 15°C!

License : Attribution (CC BY)


Overall function

The air heater is positioned against a window. It is a box made of wood, except for the side facing the window, which is made of a transparent plastic. There is a thin black plastic layer behind this transparent plastic . The black plastic will be exposed to the sun, and will heat up. The absorbed solar energy will then be transmitted to the air in the box. When this air exceeds a temperature selected on a thermostat, a fan starts. This allows the air to circulate between the room (cold air) and the inside of the box (hot air). For this, the side facing the window has holes at the top and bottom. The air is drawn through the bottom, and released from the top.

The difference in temperature between the incoming and outgoing air naturally depends on the amount of sunshine and room temperature, but the estimate is usually between 10 ° C and 15 ° C.

The power of the incident sunlight is approximately 1000W/m². It's estimated that after losses, especially through transparent walls, the power absorbed by the black plastic is 500W/m². For an area of about 0.7m ², we can roughly compare this heating to 350W heating.


The disadvantage of this heating system is that it obstructs the light coming through a window. One interesting option is to place the heater against your window before going to work in the morning. It will heat up all day, and in the evening when you come back you can remove it to make the most of the natural light while also enjoying your heated room. If you have big enough windows, you can also choose to block part of them.

Still sceptical?

Some might say that this heating is useless, as the solar energy entering the room would come in anyway, with or without this heating. But this system's efficiency is due to the fact that the heated plastic is black (so absorbs heat very well), and that this plastic is very thin. Almost all the thermal energy absorbed is therefore transmitted to the air, which would not have been the case if the sunlight had shone on the wooden furniture at the back of your room.

Video overview


- a wooden panel and wooden planks

- a 12V fan

- a 220V thermostat (with a 220V switch)

- a thin, opaque layer of black plastic (0.08mm), like the type used in gardening to cover strawberries

- a clear plastic sheet

- a basic car filter (a Toyota one for example)

-staples, screws, nails

- 220V - 12V transformer


- hammer, screwdriver

- saw

- basic electrical equipment

Step 1 - Step 1 - Bottom of the box

- Measure the dimensions of the window or wherever the heater will be placed

- cut out a rectangular wooden panel in the correct size, this will be the bottom of the box (roughly 120x50cm)

- at the top of the panel, in the centre, drill a circular hole the size of the fan (or just a little wider, you can use foam to fix the fan in place)

- at the bottom, drill a few holes that the car filters can be placed in front of (see photo with filters to get an idea)

Step 2 - Step 2 - Structure of the box

- cut the boards for the edges so that the box is 9cm deep. Use screws or nails to fix them to the edges of the wood panel

- nail two small wooden bars 10cm from the top and bottom of the box and half way down (about 5cm from the bottom)

Step 3 - Step 3 - Fan

- install the fan: screw it inside the box at the level of the previously drilled hole so that the air comes out of the box. Remember to pull the fan wires out through the same hole.

"In this picture, the bottom of the box was painted black. It was a test, but it turns out it's not necessary. "

Step 4 - Step 4 - Black plastic

- cut a black plastic strip to fit the inner width of the box and a little longer than the distance between the two wooden bars (~ 15cm longer)

- lay the plastic on the wooden bars and fold the plastic up and down around the bars. Use staples to attach the plastic by stretching it out firmly

Step 5 - Step 5 - Clear plastic

- close the box with the clear sheet (small nails can easily attach it to the edges)

Step 6 - Step 6 - Filter and valve

- position the filters in front of the holes at the bottom of the box. They can be wedged in place with a simple wooden structure, or nailed into place

- install a small angled cardboard valve at the fan to close the box when the fan is not in use

Step 7 - Step 7 - Thermostat and transformer

- fix the thermostat to the outside of the box, beside the hole for the fan.

- two options: connect a 220V/12V transformer (like a cellphone transformer, but 220V /12V, not a portable 5V one ) to the plug of the

thermostat, then connect it to the fan; or dismantle the transformer and connect the fan directly to the thermostat output, as in the picture (the thermostat itself uses 12V, so if you directly connect the fan you don't need a transformer). We strongly recommend you try the first option, as it is safer and has less risk of bad connections.

Step 8 - Step 8 - Start-up!

Place the heater against a window. Think of a way to hold it in place against the window.

Now simply set the thermostat to the desired temperature. Once the air in the box exceeds this temperature, the thermostat will turn on and hot air will circulate in the room!

Step 9 - Alternatives

The thermostat can be replaced by a solar panel inside the box, facing the window. Connected to the fan, it will only power it when it is lit up (and therefore when the air in the box will be heated).

If you decide to block off a window, you can build the heater directly on the window frame, instead of using clear plastic. In this case you still need to provide hinges to open the box or case.

Notes and references

Many thanks to Froit Vanderharst for his contribution to this tutorial! He is the inventor of this project, which was used to warm his workshop in Ulaanbaatar. A passionate handyman, he continues to work on improving his solar heaters. Follow him on Facebook!