I had this homemade during a visit with Carrie in China. I actually think that she got the recipe from someone else, but I will attribute it to her here, since it gives me warm fuzzies about our visit.
Toss into a crock-pot:
2 quarts water (8 cups)
8 bags black tea (or 8 tablespoons loose leaf)
3/4 cup sugar
16 whole cloves
16 whole cardamon seeds
5 cinnamon sticks
8 skinny slices fresh ginger
Cover. Cook on high for 2 to 2.5 hours. Strain mixture and discard solids.
Use 1 part milk to 2 parts chai. Serve hot or chilled.
The concentrate can be refrigerated for 3 days.