Calgary fire crews responded to one call on Christmas Eve and a second call on Christmas morning.
While the gas readings in the homes were high, none of the people in the homes were injured.
Investigators say a blocked vent due to ice and some possible design issues contributed to the build up of gas in one of the houses.
Neither of the homes were equipped with carbon monoxide detectors.
Carbon monoxide is a colourless, tasteless and potentially lethal gas. (CHQR-The Canadian Press)