Kettle Deep Clean

For yall that work in bagel shops/bakeries or have a commercial bagel kettle, how do you deep clean it? Specifically there’s what I think is a layer of caked (boiled) on malt at the bottom of the boiler that I can not get off unless I scrape the heck out of it. I’ve tried baking soda, bar keepers, oven cleaner, dawn dish soap, and most recently Five Star PBW cleaner to no avail. Any tips?