How Earth Tubes Work
Updated: Aug 10, 2021
In our last blog article we introduced Andrew Mathis and his four season greenhouse. He built this greenhouse himself on his homestead in rural New Brunswick, a part of Canada that often has snow on the ground for six months of the year.
The growing season in Atlantic Canada is quite short so naturally subsistence farming has given way to commercial production, not just for convenience but also out of sheer necessity. In order to feed all of Atlantic Canada a lot of food must be imported from outside the region, often from warmer climates. Our food security here is heavily dependent on imports, but it doesn’t need to be, as Andrew is proving with his model. He and his family have continued to harvest some produce from this greenhouse all the way into the winter months.They’ve also been able to save plants from previous years, winter them in the greenhouse and then have them revive in the spring and start producing again.
Many New Brunswickers really enjoy gardening and growing their own food in the short summer months, and would relish the idea of being able to grow all year round. The problem is that traditional greenhouses warm up in the daylight hours, but when the sun goes down the cold seeps in. Andrew, a Mechanical Engineering PhD student at UNB, designed a way to keep his solar passive greenhouse warm during those long dark winter months. He buried long tubes of plastic called ‘Earth Tubes,’ under the greenhouse. The Earth Tubes used the earth’s heat to pump warm air up during winter, and cool air up in the summer.
The very first thing Andrew had to do was to dig a deep hole below frost level that spanned the width and breadth of where the greenhouse would go. Once he dug the hole he insulated the perimeter of it. After the hole was insulated he dropped a network of layered earth tubes into the hole and buried them, with the two main tubes in the network sticking up out of the ground on either end of his greenhouse.
Andrew designed the network to have two main Earth Tubes on either end dropping down 6' into the ground. The main tubes both 'T' off to break into header pipes that run to the left and right of the main tubes. The header pipes break into 10 smaller pipes that run across the ground in two layers, 5 in each layer. The 10 smaller pipes then rejoin on the other side, in the same configuration, with a main tube coming up to connect with a fan. This underground Earth Tube system exchanges all the air in the greenhouse in approximately two minutes. It moves the air pretty quickly so it maintains a stable temperature and it works like a big thermal battery. The Earth Tubes effectively work to release warm air in the winter and cold air in the summer.
It took Andrew the whole first year to dig up the ground, insulate it, put the pipes in, and fill it back up. All winter he looked out the window at the work he’d done the summer before and all he had to show for it was 2 pipes sticking out of the ground. Despite the anticlimactic visual evidence of his hard work, Andrew was able to test the system that first winter. He set up a couple temperature sensors to see how much the air was able to be heated and cooled and it was pretty exciting! It could be about 10 degrees warmer than outside at night, and thanks to all the other passive design configurations he implemented, on sunny days the green house was +20 C even when it was -20 C outside. He, his wife Anna and their two little children set up hammocks out there in the green house and enjoyed reading and playing ‘outside’ like it was summer in the garden.
In our next article we’ll be getting into the other passive solar modifications that Andrew used to achieve his four-season greenhouse. See how it all comes together in our SolarShip Interactive Model!