[jimcol]

12vdc does control some functions on your water heater. That is how it operates when boon docking. 12vdc is required even when running on electric. If you trace everything out I'll bet you will find the 15 amp fuse you pulled supplies the thermostat, water heater controls, & tank indicators. Once the thermostat went dead the furnace turned off. Despite how they are labeled you probably have another 12vdc fuse for just the furnace. I think on my much older unit the fuse supplying 12volts to the furnace is a 20amp, but my unit is not here to verify that. Jim