Hi Adam and welcome to the forum. This will seem like a stupid question but are you sure that the 2 hoses running from the engine to the tank are connected ? Sounds like your engine coolant is not running through the water heater. I suspect PO may have had a problem with a leaking hose and installed a bypass not to loose any coolant when running engine. This would allow the engine to run normally but the water heater would not work without engine coolant going through it. If it's the case, you need to reconnect the hoses and find out if it works normally, i.e. no leaks and hot water. If there is a leak then you need to replace the hoses in order to get your water heater operational again. Good luck