In a small saucepan, bring the sugar, warm water and lime zest to a boil. Remove from the heat and let steep for 30 minutes. Strain into a measuring cup.
In a large saucepan, bring the cold water and hibiscus flowers to a boil. Remove from the heat; let steep for 20 minutes. Refrigerate until chilled.
Strain the tea into a pitcher. Stir in 1/2 cup of the lime syrup, or more to taste. Serve the tea in tall glasses over ice.
Dried hibiscus is available at health food stores, Latin markets and tea shops. If you can't find hibiscus flowers, you can substitute dried rose hips or Celestial Seasonings Red Zinger tea (which contains hibiscus, rose hips and other herbs).