Cut the kielbasa into 1 inch slices. (not at an angle, straight down cuts).
Add to the slow cooker.
Add the ketchup, horseradish and brown sugar.
Stir.
Cook on HIGH for 3 hours, or LOW for 5 hours stirring occasionally. You want to cook these longer than just heating for kielbasa will start soaking the sauce up the longer it cooks.
Notes
Add any leftover sausage to an airtight container and keep it in the fridge for up to 5 days. You can reheat them in a pan on the stovetop over medium-low heat.
To keep warm at a party, use the "Keep Warm" setting, so the sausage and sauce don't continue cooking. These will stay good for about 2-3 hours if you stir occasionally.