When Mark Spector’s central air-conditioning system stopped cooling his Trumbull, Conn., home this summer, he sent an S O S to his repairman. What happened next illustrates the myriad challenges the United States faces as it tries to phase out the popular but environmentally devastating cooling gas that was in Mr. Spector’s unit.

The Environmental Protection Agency has tried to reduce use of this gas, HCFC-22, which depletes the ozone layer and contributes to global warming, by imposing strict quotas on its production. Since 2010, it has also banned the sale of new air-conditioning units containing the compound, and has promoted recycling of the gas from old machines so it will not be released.