Electrical engineer and keen DIY-er Terry Johnson wanted a budget-friendly way to heat his freezing Kaikohe home.

"It was one of those houses which was pleasantly cool in the summer but freezing in the winter," he said. "We needed something to make it thermally efficient but didn't have a big budget."

Partner Mel McMinn found a design for a solar heat collector made from empty aluminium cans and Johnson thought he'd give it a crack.

MEL MCMINN The panel used 272 coke and beer cans, the majority of which were donated by friends or scrounged.

It took two or three weekends to construct and install the large panel, which ended up warming their large living area by between four and seven degrees and cost an estimated $1.50 per year to run.

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They sold the home with the panel attached four years ago but say the experiment was well worth it.

"The heat from our panel, spread across our great room, didn't immediately feel like a huge boost when it came on," McMinn explained on her blog The Frugal Kiwi. "What did happen was that the thermal mass of the whole large space was lifted by several degrees."

It worked by sucking in cool air, heating it up in the empty cans and sending it back inside through a simple system of fans, vents and valves.

MEL MCMINN The air came of the vents at over 50 degrees.

"The panel got up to about 60 or 70 degrees (celsius), and the air came out of it in the 50s," Johnson said.

He began the project by cutting the bottom of the cans out and snipping baffles in the top to direct the air, before gluing them together in sets of three.

MEL MCMINN Baffles were cut in the top of the cans.

They were then spray painted matte black to absorb as much of the sun's warmth as possible, and fitted into a plywood box frame with a sheet of polycarbonate laid over the top.

Two 20cm diameter holes were then cut into the eastern wall of the family's house, a spot chosen because it captured the warmth of the sun first thing in the morning.

"It was at its heat peak output at about 10am," he said. "My only regret is that I didn't move it to the side a little bit so I could have put another one up."

Cutting into the walls of the home took "a fair amount of intestinal fortitude," but for Johnson the risk wasn't so big considering they sold the Kaikohe house for just $150,000.

MEL MCMINN The cans were fitted into a plywood box which a sheet of polycarbonate on top.

"If you had a million-dollar house in Auckland you might think twice," he said. "There's nothing like getting a run-down house in a cheap area to embolden you to DIY."

He does caution that anyone attempting a similar project should either be skilled themselves, or have someone "reasonably handy" helping them.

"You need to make sure you maintain weather-tightness and have adequate flashings to prevent rot and degradation. And if you put one in the roof and didn't have good forced convection with fans, it could get really hot and create a fire risk."

Peter Gallagher, national fire risk management advisor for Fire and Emergency New Zealand didn't have any concerns about Johnson's panel causing fires but said "as with any solar device which can get very hot, we would advise people to be wary of burning themselves on it."

MEL MCMINN Ed the Devon Rex investigates one of the two 20cm diameter holes that were cut into the side of the house.

Johnson also added two low-power quiet fans on the inside of the house so the air would circulate efficiently, and cut a sheet of plastic from a plastic bag and installed it over the top inlet grill.

The sheet was easily blown open when heat entered the house, but prevented cold air from streaming into the house by stopping the backflow.

Ashton Partridge, associate professor at Auckland University's department of chemical and materials engineering was enthusiastic about Johnson's panel, but said: "There's a limited amount of air you can pull out of a panel that size, but it's a good way of offsetting your power bill over the winter."

Dr Al Subiantoro from the university's department of mechanical engineering agreed saying, "it's a very simple and cheap solution to heating. The only downside is that it doesn't work in the evenings and you'd need direct sunlight to make it work."

"In summer you might get too much heat," he added. "But it is an excellent idea."

MEL MCMINN Terry Johnson's solar panel heater cost $500 to make and an estimated $1.50 per year to run.

Johnson and McMinn have now moved so aren't sure whether the panel, built six years ago, is still going or not.

Their new house is insulated and has double glazing and polished concrete floors that retain warmth so they haven't needed to make another one. "The new set up basically does the same job."

Johnson said the idea might also be good for people who worked in large sheds or warehouses, but if it had a commercial application it'd need a solution that would fit the average house and "it depends on so many things."

"Pre-made solar panels are the other option of course, and the price has come down a long way and could come down a lot more," he said. "There is some very interesting technology out there now."