NOTE: I also posted this in the heating forum. Not sure which is most appropriate.
New homeowner here. We have a 20+ year-old oil furnace with no separate water tank. Steam radiators throughout the house. I'm guessing there's a coil or something inside that heats the water. It fires (in theory) whenever we need hot water. Recently the call for hot water from our shower is no longer triggering the furnace to fire up. I've found a work-around when we need to shower, and that is to kick my thermostat up a couple degrees, which quickly causes the furnace to fire up, and then we enjoy our scorching hot water in the shower. Why won't our shower trigger the furnace? Who should we contact (the oil man? plumber?) to look into this?