I have a burham oil furnace not hot heater water heats by a coil new furnace was but in 1999 over the years I notice sometimes water doesn't stay hot for the shower only just recently experience 1 time no water in shower had furance cleaned yesterday was told coil not producing not sure if that because I could never fill tub up to take a hot bath ever so not sure if coil or maybe something to do with piping or fixture. I need help before I spend 1,000 for a new coil

Your coil probably needs to be acid cleaned by a professional. Lime and scale form on the inside of the coil thus not much hot water transferr.