Solar box-type window heater – to be rewritten soon about an upgraded build.

Window box heater from scrap
The heater, made from scrap

I’ve been fixing the window box heater. It’s been working OK over winter but not to its maximum.

What is it?

It a box made from scrap wood, perspex and and has coir flute as its backing and thin Styrofoam as insulation. Inside is a doubled up length of 100 mm aluminium duct and a re-purposed computer fan.

I’ll rebuild it over Summer and add a temperature control so that it only comes on when the air temperature in the box is over 20°C. I’ll use thick glass as the cover instead of thin perspex as glass traps more heat. I’ll also make the whole box fit the front windows much more closely. That’ll mean making it longer, potentially doubling it’s length.

This box was originally made for the chook run to warm that over Winter but it was too effective so I had to take it down.


Does it work? You betcha!

It’s 24°C outside and 18°C inside the house (more about that later)
The air coming out of the heater is a toasty 31°C. That’s a boost of 7°C

I’d like to get an air flow meter from somewhere so that I can measure the flow of the air coming out. Then I can work out how much heat and energy this little system is producing.

This sort of system doesn’t produce constantly. Being run off a single 5W solar panel it only blows when the Sun is shining so its no good on dull, overcast days and it’s output drops when a cloud passes over. I’ve thought of adding a battery but then it would blow when the box isn’t trapping heat.

I know how well it works and I’ve got much better materials now so it’ll be a real furnace next Winter.

I also learned how to improve its heating ability. First, I repositioned the inlet so that it takes air in from brickwork that has been in the Sun. That’s added heat to that end of things.

Solar heater inlet
I moved the inlet so it takes air in from brickwork already warmed by the Sun

Re-positioning the whole box helped a little. Moving it only 15 cm made a measurable difference to the output earlier in the day.


This sort of passive device can trim enormous amounts off of a house’s energy use but I reckon you’d still need some kind of heating in the depths of Winter. It can however, extend the range of comfortable days greatly at no expense beyond the cost of the initial build.