Business owners who leave their doors open while an air-conditioner is running are about to face a fine for the offense.

The City Council is poised to approve new legislation on Thursday that would bar stores from keeping their doors open when air-conditioners or central cooling systems are in use. Any store or restaurant in violation of the new rule will first be issued a written warning and charged fines for subsequent violations.

The second time within an 18-month period that a business is found to be violating the law, a fine of $200 would be charged by the city for every open door.

That fine would increase to $400 a door for any subsequent violations during the same period.

Council Member Gale Brewer, who represents the Upper West Side, said she introduced the bill after receiving a steady stream of complaints from constituents who were frustrated with stores that kept their doors propped open in the summer, allowing cold air to flow out of the building onto the sidewalk.

When Ms. Brewer first introduced the measure, critics argued that businesses should be asked to comply with the rule voluntarily and said it would create another layer of bureaucracy for businesses.

The Long Island Power Authority found that by leaving doors open, retail shops waste as much as 20% to 25% of the electricity they consume.