Apolo water heater system

I have an Apolo water heat system. 18 years old. Never any problems. Now the cold inlet is as hot as the hot outlet. What could be wrong

Welcome to the Straight Dope Message Boards, Harleyjoe, we’re glad you found us. I’ve moved your question to a forum where you’re more likely to get some responses (from “Chicago-related topics” to “General Questions.”)