A Canadian well consists of passing a proportion of fresh air through pipes buried around 1 to 2 meters in the ground, before it enters the house.
During winter, at this depth, the ground is warmer than the outside temperature: cool air is then preheated as it passes through the pipes. With this system, the air sucked in by the CMV is not directly taken from outside (via the windows air vents), hence heating savings.
In the summer, conversely, the ground is cooler than the outside temperature: this clever “well” is going to use the ground freshness to temperate the air coming in the house.