When Monica Spalding got the renewal letter from her health insurance company with premium details for the upcoming year, she couldn't believe her eyes. The insurer estimated that the share of the monthly premium that she and her husband would owe for their marketplace silver plan would go up from the current $28 a month to $545.

She immediately called Sharon Barker, a health insurance navigator with Family and Children's Service in Nashville, Tenn., with whom she and her husband had worked this year to buy a marketplace plan after relocating from California to take care of her husband's ailing parents. They sat down with Barker at their local library and showed her the letter.

Barker helped them run the numbers to determine that their premium tax credit next year will be much higher than the insurance company's estimate, more than making up for the premium increase. The couple's share of the monthly premium for 2018 will actually go down, Barker showed them: from $28 to $0.